Sandrine Grasteau

Utlwanang Moreri

Agnès Narcy

Xavière Rousseau

T B Rodenburg

Michèle Tixier-Boichard

Tatiana Zerjal

S Mignon-Grasteau

Robustness to chronic heat stress in laying hens

Keywords: heat stress, layer, meta-analysis, egg production, egg quality

INTRODUCTION

The economic losses in poultry production due to heat stress are important both for tropical areas, where mean ambient temperatures frequently exceed 30 • C (Al-Saffar and Rose, 2002; Tan et al., 2010), and for temperate countries exposed to recurrent summer heat waves (COPA/COGECA report, 2003). For example, in the USA, heat has been estimated to increase layers mortality by 0.03 to 0.96% and to decrease egg production by 0.5 to 7.2%, leading to a global yearly economic loss of $98.1 million [START_REF] Saint Pierre | Economic losses from heat stress by US livestock industries[END_REF]. Layers are particularly vulnerable to heat stress because they have to maintain a long production cycle (50 to 70 weeks). Besides the effect on egg quantity, heat stress also decreases egg quality [START_REF] Balnave | Improving eggshell quality at high temperatures with dietary sodium bicarbonate[END_REF][START_REF] Al-Saffar | Ambient temperature and the egg laying characteristics of the laying fowl[END_REF], reproductive efficiency [START_REF] Novero | Plasma progesterone, luteinizing hormone concentrations, and granulosa cell responsiveness in heat-stressed hens[END_REF], and efficiency of the immune response [START_REF] Bollengier-Lee | Influence of high dietary vitamin E supplementation on egg production and plasma characteristics in hens subjected to heat stress[END_REF][START_REF] Al-Saffar | Ambient temperature and the egg laying characteristics of the laying fowl[END_REF].

C 2015 Poultry Science Association Inc. Received June 19, 2014. Accepted October 29, 2014. 1 Corresponding author: sandrine.grasteau@tours.inra.fr Heat stress is caused by a combination of high environmental temperature, high humidity and low air velocity [START_REF] Yahav | Ventilation, sensible heat loss, broiler energy, and water balance under harsh environmental conditions[END_REF][START_REF] Balnave | Nutrition and management of heat stressed pullets and laying hens[END_REF]. The chicken's thermoregulatory mechanisms to avoid heat stress are normally activated above 24 • C, i.e., above the 18 to 24 • C thermoneutral zone [START_REF] Celik | Effect of supplemental L-carnitine in drinking water on performance and egg quality of laying hens exposed to a high ambient temperature[END_REF][START_REF] Etches | Behavioural, physiological, neuroendocrine and molecular responses to heat stress[END_REF], but St-Pierre et al. (2003) showed that this limit can be lower depending on air humidity. Heat stress effects become noticeable when temperatures exceed 30 • C [START_REF] Ertas | Effects of vitamin E and vitamin C dietary supplementation on egg production and egg quality of laying hens exposed to a chronic heat stress[END_REF][START_REF] Seven | The effects of dietary Turkish propolis and vitamin C on performance, digestibility, egg production and egg quality in laying hens under different environmental temperatures[END_REF] and are accentuated by the fact that chickens cannot dissipate heat efficiently because of the insulating property of feathers and the lack of sweat glands.

Apart from the intensity and duration of the heat stress itself, several factors affect a bird's sensitivity to high temperatures. The most frequently reported factors are the age of the bird, the cyclic variations of the temperature, and genotype. For example, egg shape is less affected by high temperature in old than in young hens (Tůmova and Gous, 2012) but laying intensity is more affected by a severe heat stress (37 • C) at the end than at the beginning of the laying period [START_REF] Bordas | Performance of homozygous naked neck layers, heterozygous naked neck layers and normal plumage layers of a brown-egg dwarf line subjected to constant or fluctuating temperature [Performances de ponte de poules de génotypes NaNa (Cou nu homozygotes), Nana+ (Cou nu hétérozygote) et na+na+ (plumage normal) d'une lignée naine (dw) de type "oeuf brun" soumises à; une chaleur constante ou avec des fluctuations[END_REF]. Cyclic variations of temperature reduce the effect of heat stress by providing recovery periods for birds during the cooler periods [START_REF] Mashaly | Effect of heat stress on production parameters and immune response of commercial laying hens[END_REF]. Finally, regarding genotype effects, the most productive genotypes are more affected than the less productive ones, and the brown egg genotypes, which are heavier and more feathery, are more affected than the white egg ones [START_REF] Franco-Jimenez | Differential effects of heat stress in three strains of laying hens[END_REF][START_REF] Melesse | Effect of long-term heat stress on key enzyme activities and T3 levels in commercial layer hens[END_REF].

Despite the large number of studies on chicken heat stress, predictors for heat tolerance are not yet easily accessed. It is difficult to synthesize the results from multiple studies because of the heterogeneity of the experimental conditions, especially regarding age, genotype, intensity of heat stress, and cyclic variation of temperature during stress. To overcome this difficulty, we performed a meta-analysis of data collected from a large number of published studies evaluating the main effects of heat stress on laying hens to produce a quantitative synthesis of the literature results.

MATERIALS AND METHODS

Literature Search and Paper Filtering Criteria

A detailed search of studies from 1970 to 2013 was carried out in the ISI Web of Science and PubMed through key word searches and cited references of other published papers. Keywords and search strings included terms related to robustness (robustness, tolerance, and resistance), heat stress (heat stress, high ambient temperature, high environmental temperature, etc.), laying hens (laying hen, layer, chicken or poultry), genotype by environment interaction (genotype by environment interaction or genotype-environment interaction), and egg production (egg production, egg quality, laying rate or performance).

The numerous papers obtained by this search were then filtered before inclusion in the database. Papers were not eligible for inclusion if: 1) they studied other poultry species like quail or meat-type genotypes such as broiler breeder hens, 2) they were conference proceedings without details on results or on conditions, 3) they were literature reviews with no original data, 4) they did not specify the genotype, the age of the hens, the daily amplitude of temperature variation or the group size, 5) the experiment was performed under one temperature, 6) the experimental temperatures were below 15 • C or above 35 • C, 7) the exposure time was too short to be considered as long-term (less than one week), and 8) the parameters recorded, even if interesting, were too scarce and rarely measured (e.g., heat shock proteins).

After the filtering step, the final database included data collected from 131 papers with 99 to 1,335 data points per trait (see Appendix). Because the raw data from each study was not available, the final database was made up of the mean values for each trait per heat treatment as summarized in tables and graphs for the retained papers. Similarly, in most cases standard er-rors were not reported for each trait, as such, in order to account for variability in the mean estimates due to differences in sample sizes among studies, we instead estimated the number of independent statistical units (NIS) and included them in the database. For example, in a study presenting the results obtained during 2 weeks of egg collection, with an egg production rate of 50%, and 32 hens in each heat temperature group, reared in cages of 2 hens with a common feeder for 2 cages, we counted 32 independent samples for body weight, 16 for egg production rate and egg mass, 8 for feed intake and feed conversion ratio, and 224 for egg weight (i.e., 14 d × 32 hens × 50% egg production rate). The detailed description of the database and the complete list of references included can be found as supplementary data.

Database Recording

Parameters recorded included minimum and maximum daily temperatures, and mean daily temperature (T). We also recorded genotype (GEN) in 3 categories: commercial white birds (CW, 48.8% of final data), commercial colored birds (CC, 24.5%), and experimental lines or indigenous flocks (EXP, 26.7%). To maintain a sufficient number of data per category, it was not possible to define more detailed categories of genotypes. We note that EXP represented a heterogeneous group, as it included both local breeds, generally less productive under standard conditions but well adapted to local conditions, as well as experimental lines selected on a single trait (as opposed to commercial lines selected on wider number of traits) and characterized by a small effective population size. Although more homogeneous, the CC and CW groups exhibited pronounced variability as they included several different commercial lines issued from selection programs over the time span included in the database (1970 to 2014). Group size (GS), representing single versus collective rearing (56.9% and 43.1% of the cases, respectively) was also included in the database. For all traits recorded between X and Y weeks of age of hens, we attributed a mean age value (A; in weeks) corresponding to the mean of X and Y. The mean hen age in the database was 39.6 wk, with 25.8% of data coming from hens younger than 30 wk, 36.1% from 30 to 40-wk-old hens, 17.1% from 40 to 50-wk-old hens, 11.4% from 50 to 60-wk-old hens, and 9.6% from hens older than 60 wk. Finally, daily amplitude of temperature variation (Δ) was calculated as the difference between maximum and minimum temperatures. In the database, 64% of the data arose from studies with no amplitude of variation or variation less than 3 • C. For the remaining studies, the daily variation of temperature averaged 9.5 • C.

Traits included in the database were hen-day egg production rate per flock (EPR; in %), egg weight (EW; in g), egg mass (EM; in g.d -1 ), daily feed intake (FI; in g.d -1 ), feed conversion ratio (FCR) calcu- lated as the ratio of feed consumption to egg mass (in g.g -1 ), Haugh units (HU), shell thickness (ST; in mm), shell strength (SS; in g), shell proportion (SP; in %), yolk proportion (YP; in %), albumen proportion (AP; in %), body weight (g), and mortality (%). Weekly mortality rate (MORT) was calculated by dividing the total mortality during the experiment by the length of the experiment (in weeks). We calculated a relative weekly body weight change (BW c ) during heat exposure as:

BW c = 100 × ⎛ ⎝ 1 - BW 2 BW 1 1 T 2 -T 1 ⎞ ⎠ ,
where BW 1 and BW 2 were body weight at the beginning and at the end of the heat stress period, respectively, and T 1 and T 2 the hens' ages (in weeks) at the beginning and at the end of the heat stress period, respectively.

Summary statistics for these traits are presented in Table 1.

Statistical Analysis

Egg production rate, mortality, body weight change, and shell proportions were expressed in percentages, which were below 20% or above 80% and were, thus, transformed by arcsine square root, while FCR was log-transformed before analysis. The corresponding abbreviations for transformed traits are, respectively, EPR t , MORT t , BWc t , SP t , and logFCR.

The analysis consisted of two steps. In the first step, data were pre-corrected for a random study effect through the MIXED procedure of SAS (SAS Institute Inc., 2009). In the second step, we ran the GLMSELECT (SAS Institute Inc., 2009) procedure on the residuals of the first step to automatically select the best model for each trait. Models were allowed to in-clude the fixed main effects of genotype (GEN), group size (GS) as fixed effects, and covariates for linear effect of temperature (T), nonlinear effects of temperature (T 2 , T 3 ), linear effects of age and amplitude of temperature variation, and all possible two-by-two interactions. In addition, we introduced observation weights into the model to account for study-specific differences in the variability of data measured on differing numbers of independent samples (NIS), particularly as the NIS varied considerably among the traits and studies considered. For example, mean albumen proportions were calculated on 19 to 2421 NIS depending on the study. However, weighting observations using the raw NIS (which would correspond to giving the latter study in the previous example 127 times more weight than the former) would lead to attributing a disproportionate influence to the few large-scale experiments and largely excluding data from small to medium-sized studies, even though small studies are often more accurate in their monitoring of heat stress than large studies, so that data quality of small-scale studies is quite reliable. To avoid this, the NIS were split into 5 quantiles for each trait, from 1 (20% of studies with the smallest NIS) to 5 (the 20% of studies with the largest NIS), and these categories were used as weighting factors in the analyses of variance. A stepwise procedure was used to successively introduce new effects in the model up to the entry significance level of P < 0.15, which is the common standard in stepwise regressions in SAS. The best-fitting parsimonious model was then selected on the criterion of lowest predicted residual sum of squares. The final selected models for each trait are presented in Table 2.

To assess whether a trait was a good indicator of sensitivity to heat stress, we calculated an index of sensitivity (IS) in a "standard" situation (defined as singlecaged birds of CW genotype, 30-weeks-old, reared under constant ambient temperature), as the relative difference between estimated performances at thermoneutrality (TN, 15 to 20 • C) and under heat stress

IS = 100 × HS S -T N S T N S ,
where HS S is the mean of estimated values between 15 and 20 • C (6 estimated values, i.e., 1 estimated value by degree) and TN S the mean of estimated values between 30 and 35 • C (6 estimated values, i.e., 1 estimated value by degree).

For traits for which an interaction between temperature (T, T2 , T3 or Δ) and one other effect was fitted in the model, we similarly calculated an index showing the differences between levels of fixed effects or pre-defined levels of covariates (30, 50, and 70 wk for age, 0, 5, and 10 • C for amplitude of temperature variation). For example, for an interaction between temperature and genotype, we calculated:

IS C W = IS IS C C = 100 × HS C C -T N C C T N C C IS E X P = 100 × HS E X P -T N E X P T N E X P ,
where IS X is the index of sensitivity for the level X of genotype (X = CW, CC, EXP), HS X and TN X the means of estimated values under HS and at TN for the level X of genotype. Finally, when an interaction between two effects A and B (except temperature) was fitted in the model, we calculated a similar index showing the importance of this interaction as:

IN T A X X ,B Y Y = 100 × P A X ,B Y -P A X ,B Y P A X ,B Y ,
where P A X ,B Y (P A X ,B Y ) were the means of estimated values between 15 and 35 • C for level X (X ) of effect A and level Y (Y ) of effect B. Only combinations in which one effect was fixed (X = X or Y = Y ) while the other was varying were tested. For example, for an interaction between age and genotype, we calculated the difference between CC and CW birds at 30 weeks, the difference between CC birds at 30 and 50 weeks, but not the difference between CC birds at 30 weeks and CW birds at 50 weeks.

To test the significance of indices defined above, estimated data (one point by degree) were analyzed with a GLM procedure with model 1 for IS, model 2 for an interaction between temperature and another effect, and with model 3 for interactions between two effects except temperature:

Model 1 : EV ij = μ + TZ i + e ij Model 2 : EV ijk = μ + TZ i + E1 j + TZ i × E1 j + e ijk Model 3 : EV ijk = μ + E1 j + E2 k + E1 j × E2 k + e ijk ,
where EV ij(k) is the estimated value at j (k) degrees (j = 15 to 20 and 30 to 35 • C in model 1, k = 15 to 20 and 30 to 35 • C in model 2, k = 15 to 35 • C in model 3), μ the general mean, TZ i the fixed effect of the i th temperature zone (TN, HS), E1 j and E2 k the fixed effects of level j of effect 1 and level k of effect 2 (j, k = CC, CW, EXP for genotype, 30, 50, 70 wk for age, 0, 5, 10 • C for amplitude of temperature variation, Individual or Collective for group size), TZ i ×E1 j the interaction between the i th temperature zone and the j th level of effect 1, E1 j ×E2 k the interaction between the j th level of effect 1 and the k th level of effect 2, and e ij(k) the residual for estimated value at j (k) degrees.

RESULTS

The list of significant main effects and interactions for the 13 modeled traits is presented in Table 2. Amplitude of variation of temperature was found to be insignificant as both a direct effect and in interactions with other effects and will thus not be included in the Tables of results. Interactions were present 17 times in the fitted models, involving all effects. The selected fitted models can be found in Table 3. Adjusted R 2 of the models are sometimes null or close to zero (e.g., for HU or YP), which implies that these traits are not affected by heat and that factors affecting these traits are not included in our model. However, it is also important to remember that data have been pre-corrected for the random effect of the study, which generally is the largest cause of variability in a meta-analysis. Indeed in our case, the pseudo-R 2 calculated for the initial model including only the random study effect averaged at 0.65.

Index of Sensitivity and Direct Effect of Temperature

The index of sensitivity describes the intensity of response to heat stress for each trait in the reference situation between TN and HS (Table 2). Mortality and body weight change were the most affected traits and varied sharply with temperature (+126.1% and -129.7%, respectively), which is partly because their proportions were close to zero under TN conditions. Other traits such as feed intake, shell strength, egg production, and egg mass were also strongly affected by heat as their values decreased by 9.0 to 22.6% between TN and HS in the standard situation. Egg weight, feed conversion ratio, shell thickness, and percentage showed lower but non-negligible variation with temperature (-3.9 to -7.2%). In contrast, Haugh units and albumen and yolk proportions did not vary with temperature.

The temperature effect included only quadratic direct or interaction effects for AP (Table 2) or both linear and quadratic direct or interaction effects for EPR t , FI, EW, ST, BW ct , MORT t , EM, SP t , SS, and ST (Table 2). For ST and SP t , the model also included a cubic term. A nonlinear relationship could be due to the presence of quadratic or cubic terms (FI, EM, ST, and SS) or to the nonlinear transformation of the traits before analysis (EPR t and SP t ). For shell characteristics and egg production rate, the values initially increased at lower temperatures, reached their maximum between 20 and 22 • C, and declined at higher temperatures (e.g., Figure 1A for shell proportion). Conversely, mortality decreased at low to medium temperature before increasing at high temperatures (Figure 1B). For most traits however, we observed a continuous increasing (AP) or decreasing (FI, EM, EW, FCR) trend with increasing temperature.

Genotype Effect

Genotype by temperature interaction This interaction was found for EPR t and EM (Table 2), and was mostly due to a difference in heat sensitivity between the commercial colored group and the other genotypes (Figure 2). At TN, EPR was equivalent for the 3 genotypes (Table 4), while under HS egg production was lower for CC than for CW (-8.4%, P = 0.001) and tended to be lower than in EXP (-5.7%, P = 0.08). At TN, egg mass of CC was 5.9% higher than that of EXP (P < 0.0001), but under HS no difference of EM between EXP and CC was observed (P = 0.52). This result can be interpreted by looking at the egg weight of CC that was constantly higher compared to EXP, both at TN and HS (3.4 and 3.7%, respectively, P < 0.0001). The fact that CC hens have higher EW but lower EPR at HS, leads to the lack of difference in EM observed between CC and EXP.

Genotype by age interaction A genotype by age interaction was found for egg weight (Table 2), due to a difference between EXP and CW genotypes (Table 5). Egg weight was not different between commercial hens and EXP at 30 weeks (+4.3%, P = 0.12), but egg weight increased more with age for EXP than for commercial birds (+15.8% and +6.2%, respectively), leading to 4.6% heavier eggs in EXP birds at 70 wk (P = 0.04, Table 5).

Genotype by group size interaction This interaction was found for egg weight and egg mass (Table 2) and was due to a lower sensitivity to the group effect in CW birds than in CC and EXP birds (-0.4% vs -2.8 to -7.9% for EW, -8.3% vs -14.1 to -24.1 for EM, Table 5). In both individual cages and in group housing, egg weight was higher in commercial birds than in EXP, but a difference was found between CW and CC only in collective systems. Similarly, for egg mass, no difference was found between genotypes in individual cages, but commercial birds showed better performances than EXP birds in collective systems (Table 5).

Genotype main effect Genotype differences were observed for FI and FCR (P = 0.02, Table 2), EXP birds eating 2.1 to 2.8 g.d -1 less and exhibiting a 0.06 to 0.08 higher FCR than commercial birds (Table 3). The effect of the genotypes on FI was nevertheless minor compared to the effect of the temperature itself (-34.1 g between estimated values at 20 and 35 • C in the standard situation).

Hen Age Effect

Age by temperature interaction Age by T or T 2 interactions were included in our models for EW, FI, and EM (Figure 3 for EM and EW) and SS, even if weakly significant for the latter (P = 0.10, Table 2). Egg weight, egg mass, and feed intake were more affected by heat in older than in younger hens (respectively, -13.1%, -18.0% and -26.7% at 70 wk vs. -7.2%, -9.0%, and -22.7% at 30 wk between TN and HS, Table 4). On the contrary, shell strength decreased more with heat in younger than in older hens (-14.3% at 30 wk, -6.4% at 70 wk, between TN and HS, Table 4). In both cases, this pattern led to the presence of a significant or nearly significant difference between 30-wk-old hens and 70-wk-old hens under TN (P = 0.002 for SS, <0.0001 for EW and EM, 0.10 for FI). Under HS, the differences between young and old hens vanished for FI and SS and were largely reduced for EW and EM (Table 4).

Age by group size interaction This interaction affected EPR (P < 0.0001) and EM (P < 0.0001, Table 2). At 30 weeks, egg production rate and egg mass were, respectively, 6.7% and 8.3% lower in group-caged birds than in individual cages. At 50 and 70 weeks, these differences between group sizes disappeared (Table 5).

Table 4.

Mean fitted values for each trait under thermoneutrality (TN, at temperatures between 15 and 20 • C) and under heat stress (HS, between 30 and 35 • C) for traits exhibiting an interaction between temperature and other effects. 3 Relative difference "B-A" is calculated as

P B -P A P A
, where PA and PB are the means over the whole temperature range (15 to 35 • C) of estimated performances of levels A and B of effect 2, within effect 1. 4 Traits within interaction with no common superscript are significantly different (P < 0.05).

Main age effect

An age effect was fitted for BWc t (P = 0.04), SP t (P = 0.11), ST (P = 0.04), and HU (P = 0.0002, Table 2). These effects were very low, leading to weekly decreases of 0.03 to 0.06% for HU, ST, and SP. For BWc, this effect changed the temperature above which hens started to gain weight. At 70 wk, body weight change was positive whatever the temperature. At 50 and 30 weeks, body weight change became positive above 32 • C and 28 • C, respectively (Figure 4).

Group Size

Group size by temperature interaction Group size by temperature interaction was fitted to EPR t (P < 0.0001), EM (P = 0.04), EW (P = 0.002), SS (P = 0.14), and FI (P < 0.0001, Table 2). For egg production and egg mass, the advantage of individual cages compared to collective ones was reduced at high temperatures (Figure 5 for EPR). Indeed, EPR and EW were 11.3% and 7.2% lower in collective than in individual cages at TN, but only 5.8% and 4.1% lower at HS (Table 4). On the contrary, the difference between group sizes slightly increased with heat for feed intake and egg mass. Mean values in collective cages were lower than those in individual cages by 22.6% for FI and by 9.0% for EM at TN, and by 24.7% and 9.7%, respectively, under HS.

DISCUSSION

In previous studies evaluating the effect of heat stress on production and feed efficiency traits, the effects of age, genotype, group size, and amplitude of temperature have been mostly studied separately and largely on CW hens reared in single battery cages. A few attempts have been made to model changes in traits with ambient temperature, but they never studied simultaneously the effects and traits analyzed in this study. For example, [START_REF] Marsden | Quantitative review of the effects of environmental temperature on food intake, egg output, and energy balance in laying pullets[END_REF] modeled the effects of temperature on various metabolic and production traits considering only the genotype effect, while Al-Saffar and Rose (2002) considered the genotype, group size, and temperature program but only on egg related traits. None of these reviews included the effect of hen age or interactions between traits.

One of the main results of this study is to have highlighted the existence of interactions between temperature and the different factors (age, group size, and genotype) for four main traits: 1) feed intake, 2) egg production, 3) egg weight, and 4) egg mass. This underscores that it is incorrect to apply the results obtained from one study done under specific conditions of genotype, hen age, and group size to other conditions, unless the mechanisms leading to the aforementioned interactions are well understood. This also enhances the importance of providing the equations corresponding to fitted models, which will allow people to apply the appropriate equation for a given situation.

Joint analysis of independent studies would be facilitated by a standardization of experimental procedures. For example, mortality recording is not standardized between studies. Age or causes of mortality are usu-ally not recorded, and it is difficult to summarize the available data on mortality although it is influenced by chronic heat stress. Similarly, some other effects such as air velocity or humidity could not be included in the models because of the paucity of records, even if they are known to influence the response to heat stress [START_REF] Yahav | Ventilation, sensible heat loss, broiler energy, and water balance under harsh environmental conditions[END_REF]. This information should be systematically included in a general effort of standardization of data recording in future experiments.

Concerning the temperature effect, our study confirmed that large negative effects of heat stress (more than 10% of decrease compared to the optimal value) are observed above 30 • C. However, a real impact of heat stress on some traits was observed below the threshold value of 30 • C. This was the case for feed intake, with a 10% decrease observed already at 24 • C, and for important production traits such as egg mass, egg production rate, egg weight, and shell strength with losses of 5% (i.e., values comparable to what is forecast as heat related losses in the USA, St-Pierre et al., 2003) at lower temperatures (24 to 29 • C). This observation is relatively novel since previous studies generally considered heat stress in layers to start only above 30 • C. To avoid any heat stress effect, it is recommended that ambient temperature remains below 25 • C. As feed intake is the trait that is the most largely affected at the lowest temperature, we can hypothesize that this trait has a higher adaptive value than the other traits, as a lower feed intake is associated with a reduced need to dissipate heat. Moreover, there is probably a cascading effect of the reduced feed intake on the other traits, as they are all correlated.

Furthermore, the impact of using cyclic temperatures appears to be much more limited than previously thought. This underlines the severe consequences that climate change may have on egg production, considering that the Intergovernmental Panel on Climate Change (IPCC) scenarios are suggesting that temperatures could rise by +2 to +4 • C by the end of the century.

Genotype Effects

The high egg production rate and egg mass in the CC hens under TN temperatures followed by a stronger drop under heat stress (HS) reflect the limited ability of the CC hens to cope with high temperatures. Highperforming hens have been shown to be more sensitive to environmental changes [START_REF] Chen | Performance comparison of dwarf laying hens segregating for the naked neck gene in temperate and subtropical environments[END_REF] but differences exist between genotypes. For example, the ability of the CW hens to tolerate heat stress better than the CC hens is well documented [START_REF] Marsden | Quantitative review of the effects of environmental temperature on food intake, egg output, and energy balance in laying pullets[END_REF][START_REF] Franco-Jimenez | Differential effects of heat stress in three strains of laying hens[END_REF][START_REF] Melesse | Effect of long-term heat stress on key enzyme activities and T3 levels in commercial layer hens[END_REF][START_REF] Melesse | Assessing the thermo-tolerance potentials of five commercial layer chicken genotypes under long-term heat stress environment as measured by their performance traits[END_REF], and several hypotheses have been proposed. At elevated environmental temperatures, animals dissipate body heat by sensible heat loss (radiation, convection, and conduction) and evaporative heat loss (DEFRA, 2005;[START_REF] Mutaf | Surface wetting and its effect on body and surface temperature of domes-tic laying hens at different thermal conditions[END_REF]. However, sensible heat loss is reduced in brown birds because of heavy feather coverage that limits heat dissipation [START_REF] Franco-Jimenez | Differential effects of heat stress in three strains of laying hens[END_REF]. Furthermore, brown birds are heavier and, therefore, require more energy for maintenance [START_REF] Franco-Jimenez | Differential effects of heat stress in three strains of laying hens[END_REF]. This was also the case in our database, where, at onset of lay (before 25 wk), CC were on average 174 g heavier than CW birds. [START_REF] Marsden | Quantitative review of the effects of environmental temperature on food intake, egg output, and energy balance in laying pullets[END_REF] showed that energy available for production is maximum at 23 • C for brown birds and at 24 • C for white birds. In our analysis birds from experimental lines (EXP) were mostly composed of White Leghorn hens and it is, therefore, not surprising that they showed a similar response to HS as the CW birds. Moreover, the EXP was a heterogeneous group. It was composed on the one hand of experimental lines selected on one unique selection criterion (residual feed intake, clutch length, etc). This might confer an advantage under HS conditions compared to commercial birds selected on multiple criteria selection indexes and that exhibit high performance for several production traits shown here to be sensitive to heat. On the other hand, the EXP group also included indigenous breeds coming mostly from hot countries and, thus, adapted to heat, and lines segregating for genes like the naked neck, sex-linked dwarf gene or frizzle genes, which have been shown to improve tolerance to heat [START_REF] Chen | Effect of high ambient temperature and naked neck genotype on performance of dwarf brown-egg layers selected for improved clutch length[END_REF][START_REF] Chen | Performance comparison of dwarf laying hens segregating for the naked neck gene in temperate and subtropical environments[END_REF][START_REF] Cahaner | Effects of the genetically reduced feather coverage in naked neck and featherless broilers on their performance under hot conditions[END_REF][START_REF] Zerjal | Performance comparison of laying hens segregating for the frizzle gene under thermoneutral and high ambient temperatures[END_REF].

The small difference between the commercial lines in changes in feed intake and FCR in response to increasing temperatures was expected. Reducing feed intake during heat stress is a physiological reaction to reduce diet-induced thermogenesis and the consequent metabolic energy to dissipate, thus, increasing the tolerance to heat stress. The differences in feed intake between the commercial and EXP hens can be explained considering that some EXP lines were selected for low residual feed intake, and others were carry-ing the sex-linked dwarf mutation, that causes a reduced appetite due to thyroid insufficiency [START_REF] Guillaume | The dwarfing gene dw: its effect on anatomy, physiology, nutrition, management. Its application to poultry industry[END_REF][START_REF] Marks | Growth, Feed Intake, and Feed Conversion of Dwarf and Nondwarf Broiler-Type Chickens[END_REF]. In both cases, a relatively low feed intake was expected. Small variation in BWc observed at HS in CW hen temperatures could be explained by the smaller size of the White leghorns compared to Brown hens and their higher tolerance to heat.

Age Effects

The combined effect of temperature and age can be explained by the larger drop in the level of production for age groups having the highest values in optimal conditions. Shell strength was more affected in young birds and EW was more affected in old ones. A similar scenario was reported by Tůmova and Gous (2012) who found a stronger impact of heat in highly productive laying hens compared to less productive broiler breeder hens.

When hens get older, egg weight and egg surface increase, implying an increased need for calcium to build shell matrix (Tůmova and Gous, 2012). However, the capacity to absorb calcium decreases with age [START_REF] Cordts | New alternatives for improving egg shell stability through breeding[END_REF]. This can explain why at TN, when heat stress does not affect production, shell strength is lower in old than in young hens. Under heat stress, birds hyperventilate to dissipate heat leading to an additional lack of bicarbonate ions required for shell structure (Tůmova and Gous, 2012). This should increase the disadvantage of old hens, however, old hens show a sharp decrease in egg weight under heat stress, thus limiting their needs in calcium, which may in turn explain why young and old hens have similar shell strength under HS.

Similarly, the interaction between temperature and egg weight can be linked to the trend observed with age and temperature, for egg number and feed intake. When birds get older, their needs increase for both maintenance and production as hens and eggs get heavier. Under HS, feed intake decreases sharply, and less resources are available both for maintenance and for production. As egg production does not decrease at the same rate as feed intake, there are proportionally less resources available for each egg, especially for older hens producing larger eggs, so that the age effect on egg weight is completely erased by the heat stress.

Group Size

The relationship between group size and ambient temperature has been less studied, as most of the studies on heat-stress effects in laying hens have been performed in single cages. In our database, almost 60% of the studies were done in individual cages, and only two considered both collective and individual rearing. In agreement with our results, [START_REF] Saki | The effect of cage density on laying hen performance, egg quality and excreta minerals[END_REF] showed that group-caged hens have lower egg production, egg weight and egg mass than hens kept in single cages under neutral temperatures of 18 to 22 • C. This could be due, on the one hand, to the lower feed intake of hens reared in group-cages compared to hens kept in single cages (Table 4), and, on the other hand, to the higher activity levels in group-caged hens [START_REF] Guo | The effect of group size and stocking density on the welfare and performance of the hens housed in furnished cages during summer[END_REF], both leading to a reduction of energy resources available for production. The absence of large differences for egg production between group and individual cage systems under HS (Table 4) could reflect that above a certain temperature, HS outweighed stress represented by the collective cage. For other traits such as FI and EM the stronger effect of heat stress in collective groups than in individual cages may be due to the greater difficulty of the former to dissipate heat. [START_REF] Etches | Behavioural, physiological, neuroendocrine and molecular responses to heat stress[END_REF] mentioned that under heat stress, caged birds tend to increase the distance between birds and to lift their wings from the body to increase the surface of evaporative heat loss. These behavioral adaptations are probably more difficult in collective cages than in individual ones. For further studies, it will be important to include the information on surface available per bird to make a realistic estimation of this effect.

CONCLUSIONS

This analysis provides a quantification of main effects that should be taken into account to predict the expected impact of chronic heat stress in given conditions. Age and genotype are crucial factors for resilience to heat stress in laying hens as both contribute strongly to variation in egg production and quality. The genotype plays a vital role in determining egg production rate, egg weight, egg mass, and feed intake. Commercial colored birds, which generally are heavier, have more difficulties in handling heat stress. The effect of age was more prominent on egg production rate, egg weight, and feed intake, which calls for consideration of the production length of hens in rearing conditions where house temperatures are not controlled. Finally, group size (single or group-cage) is important for egg production rate, egg mass, egg weight, and feed intake. This meta-analysis points out that shell strength, feed intake, egg mass, egg production rate, and relative body weight change are good indicators of robustness in laying hens as they are more sensitive to temperature.

Figure 1 .

 1 Figure 1. Fitted curves for evolution of (A) shell proportion (SP) and (B) weekly mortality rate (MORT) with temperature, in the standard situation (commercial white genotype, 30-wk-old birds, single-caged, daily amplitude of variation of temperature = 0 • C).

Figure 2 .

 2 Figure 2. Fitted curves showing the interaction between genotype and temperature for (A) egg production rate (EPR) and (B) egg mass (EM). Full line stands for commercial white, dotted line for commercial colored, and dashed line for experimental and indigenous lines.

  mass, EPR: egg production rate, FI: feed intake; EW: egg weight; SS: shell strength. 2 Traits within line with no common superscript are significantly different (P means of estimated values for levels A and B of effects at thermo-neutrality. means of estimated values for levels A and B of effects under heat stress. 5 Ind: Individual rearing; Coll: collective rearing.

Figure 3 .

 3 Figure 3. Fitted curves showing the interaction between age and temperature for (A) egg weight (EW) and (B) egg mass (EM). Full line stands for 30-wk-old birds, dotted line for 50-wk-old birds, and dashed line for 70-wk-old birds.

Figure 4 .Figure 5 .

 45 Figure 4. Fitted curve showing the effect of age on relative weekly body weight change (BWc). Full line stands for 30-wk-old birds, dotted line for 50-wk-old birds, and dashed line for 70-wk-old birds.

Table 1 .

 1 Summary statistics for each trait.

	Trait 1	Unit	N	Mean	Std Minimum Maximum
	FI	g.d -1	1097	99.55	19.90	23.20	162.00
	EPR	% 1323	76.18	15.03	18.00	100.50
	EW	g 1335	56.04	6.11	37.60	72.00
	EM	g.d -1	1281	43.07	10.39	10.40	70.38
	FCR	g.g -1	881	2.355	0.569	1.150	4.990
	HU	-	254	81.49	10.84	48.72	104.53
	ST	mm	307	0.3565 0.0387	0.2670	0.4900
	SS	g	99	3199	704	1958	4895
	YP	%	170	28.19	2.27	24.08	34.99
	AP	%	138	61.82	2.56	54.61	67.01
	SP	%	251	9.736	1.507	69.24	14.31
	MORT %.wk -1	159	0.5310 0.9198	0.00	6.78
	BWc	%.wk -1	529 -0.3483 2.5671	-14.9511	19.7880
	1 FI: feed intake; EPR: egg production rate; EW: egg weight; EM: egg
	mass; FCR: feed conversion ratio; HU: Haugh units; ST: shell thickness;
	SS: shell strength; SP: shell proportion; YP: yolk proportion; AP: albu-
	men proportion; MORT: mortality rate; BWc: relative weekly body weight
	change.

Table 2 .

 2 Trait mean estimated values at thermo-neutrality (TN, 15 to 20 • C) and under heat stress (HS, 30 to 35 • C); index of sensitivity (IS); and models fitted to each trait.

		EPR 1,2	FI	EW	EM	FCR 2	HU	ST	YP	AP	SP 2	BWc 2	MORT 2	SS
		(%)	(g.d -1 )	( g )	( g . d -1 )	(g.g -1 )		(mm)	(%)	(%)	(%)	(%.wk -1 ) (%.wk -1 )	(g)
	TN	86.91	112.82	58.09	48.48	2.313	-	0.3631	-	61.48	9.86	-0.6828	0.2610	3513
	HS	77.06	87.27	53.93	44.11	2.224	-	0.3439	-	62.23	9.43	0.2028	0.2900	3009
	IS	-11.3	-22.6	-7.2	-9.0	-3.9	-	-5.3	-	1.2	-4.3	-129.7	126.1	-14.3
	P 3	<0.0001 <0.0001 <0.0001 <0.0001 <0.0001	-<0.0001	-<0.0001 <0.0001	<0.0001	0.0010 <0.0001
						Significance level of main effects 4
	T	<0.0001				0.0002		<0.0001			0.0239	0.0610	0.0089	0.0788
	T 2	<0.0001 <0.0001		<0.0001			<0.0001		0.0003 <0.0001	0.0002	<0.0001 <0.0001
	T 3							0.0179			0.0129
	A	<0.0001	0.0120 <0.0001 <0.0001		0.0002	0.0039			0.1142	0.0411		0.0040
	GS	<0.0001 <0.0001 <0.0001	0.0008									0.0521
	GEN	<0.0001	0.0243 <0.0001	0.0018	0.0180						0.0795
						Significance level of interactions
	T×A			<0.0001
	T×GS	<0.0001 <0.0001	0.0022	0.0354									0.1360
	T×GEN	<0.0001
	T 2 ×A		0.0112	0.0046 <0.0001									0.0972
	T 2 ×GS
	T 2 ×GEN				0.0494
	A×GS	<0.0001			<0.0001
	A×GEN			<0.0001
	GS×GEN			0.0061	0.0233
	Adj R 2	0.34	0.43	0.37	0.24	0.02	0.05	0.30 0.00	0.09	0.17	0.04	0.16	0.42

(HS, 30 to 35 • C):

Table 3 .

 3 Equations of solutions.

		I					-6.777 10 -2															-385.7 + 0.3662 T
		G	T -0.2769 + 4.990 10 -3	+ 2.656 10 -3 A	T -10.39 + 1.092 10 -2		T -4.228 + 5.359 10 -2	-0.3715 (CC only)	+ 0.9582 (CW only)	T -14.45 + 3.510 10 -3	+ 0.1166 A	+ 4.326 (CC only)	+ 7.010 (CW only)
	Terms specific to several categories 3	CC CW	0.1426 3.402 10 -2	-5.769 10 -3 T -1.674 10 -4 T	2.813 2.078		4.627 6.098 -0.1253 A	-8.818 10 -2 A		4.410 1.957	-4.833 10 -3 T 2 T 2 -7.960 10 -4		-2.425 10 -2 -3.465 10 -2								-1.880 10 -3 3.950 10 -3
															T 3			T 3
	Common Terms 2		1.011 + 2.647 10 -2 T -6.973 10 -4 T 2	-2.966 10 -3 A	115.1 -2.801 10 -2 T 2	+ 0.2075 A -2.020 10 -4 T 2 ×A	48.87	+ 0.5125 A-1.771 10 -2 T 2 ×A T×A + 2.144 10 -4		44.35 -3.587 10 -3 T	+ 0.1336 A -1.650 10 -4 T 2 ×A		0.9192 -2.625 10 -3 T	83.44 -4.906 10 -2 A	T 2 + 1.055 10 -5 0.1157 + 2.905 10 -2 T -1.016 10 -3	-1.680 10 -4 A	61.17 + 1.001 10 -3 T 2	T 2 + 6.752 10 -6 0.2017 + 1.467 10 -2 T -5.650 10 -4	-5.033 10 -5 A	T 2 0.5426 -3.752 10 -3 T + 8.871 10 -5	+ 2.440 10 -4 A	T 2 0.1198 -7.003 10 -3 T + 1.740 10 -4	1921 + 193.5 T-4.844 T 2	-13.04 A + 1.008 10 -2 T 2 ×A
	Trait 1		EPR t (%)	FI (g)		EW (g)			EM (g)			logFCR (g.g -1 )	HU	ST (mm)		AP (%)	SP t (%)	BWc t (%.wk -1 )	MORT t (%.wk -1 )	SS (g)	1 FI: feed intake; EPR

t : egg production rate on transformed scale; EW: egg weight; EM: egg mass; logFCR: feed conversion ratio on log-transformed scale; HU: Haugh units; ST: shell thickness; SS: shell strength; SP t : shell proportion on transformed scale; YP: yolk proportion; AP: albumen proportion; MORT t : mortality rate on transformed scale; BWc t : relative weekly body weight change on transformed scale. 2 T: mean temperature; T 2 : squared mean temperature; T 3 : cubed mean temperature; Δ: daily amplitude of temperature variation; A: hen age.

Table 5 .

 5 Means of fitted values for traits exhibiting interactions between effects, except temperature.

	Effect 1 1	Means of estimated values by Relative difference between levels of effect
			level of effect 2 2		2 within effect 1(%) 3
			Egg Production Rate (%)
	Group	Ind	Coll		Coll-Ind
	30 wk	83.3 a,4	77.7 b		-6.7
	50 wk	78.7 b	77.2 b,c		-1.9
	70 wk	73.6 c	76.7 b,c		-4.1
				Egg Weight (g)
	Age	30 wk	50 wk	70 wk	50-30 wk 70-50 wk	70-30 wk
	CW	55.9 d,e	57.6 c,d	59.3 b,c	3.1	3.0	6.2
	CC	55.5 d,e	57.9 b,d	60.5 a,b	4.5	4.3	8.9
	EXP	53.5 e	57.8 b,d	62.0 a	7.9	7.3	15.8
	Group	Ind	Coll		Coll-Ind
	CW	55.9 a	55.6 a		-0.4
	CC	55.5 a	53.9 b		-2.8
	EXP	53.5 b	49.3 c		-7.9
				Egg mass (g.d -1 )
	Group	Ind	Coll		Coll-Ind
	30 wk	46.4 a,b	42.6 c		-8.3
	50 wk	46.9 a,b	45.4 b		-3.2
	70 wk	47.4 a,b	48.2 a		1.7
	Group	Ind	Coll		Coll-Ind
	CW	46.4 a	42.6 b		-8.3
	CC	46.2 a	39.7 c		-14.1
	EXP	45.0 a	34.1 d		-24.1

1 CW: commercial white birds, CC: commercial colored birds; EXP: experimental birds. 2 Ind: individual rearing, Coll: collective rearing.

at INRA Institut National de la Recherche Agronomique on March

27, 2015 http://ps.oxfordjournals.org/ Downloaded from

at INRA Institut National de la Recherche Agronomique on March 27, 2015 http://ps.oxfordjournals.org/ Downloaded from

FI: feed intake; EPR t : egg production rate on transformed scale; EW: egg weight; EM: egg mass; logFCR: feed conversion ratio on logtransformed scale; HU: Haugh units; ST: shell thickness; SS: shell strength; SP t : shell proportion on transformed scale; YP: yolk proportion; AP: albumen proportion; MORT t : mortality rate on transformed scale; BWc t : relative weekly body weight change on transformed scale.

2 EPR, FCR, SP, BWc and MORT were transformed before

analyses. 3 P: significance of the difference between

 TN and HS. 4 T: mean temperature; T 2 : squared mean temperature; T 3 : cubed mean temperature; Δ: daily amplitude of temperature variation; A: hen age; GS: group size; GEN: genotype.

CC: commercial colored genotypes; CW: commercial white genotypes; G: group-caged birds; I: individually-caged birds. at INRA Institut National de la Recherche Agronomique on March 27, 2015 http://ps.oxfordjournals.org/ Downloaded from

CW: commercial white birds; CC: commercial colored birds; EXP: experimental birds. at INRA Institut National de la Recherche Agronomique on March 27, 2015 http://ps.oxfordjournals.org/ Downloaded from

ACKNOWLEDGMENTS

The authors would like to thank the following institutions for their support and collaboration in this project: AgroParisTech (Paris, France), Wageningen University (Wageningen, The Netherlands), and Institut National de la Recherche Agronomique (Nouzilly, France).

Utlwanang Moreri benefited from a joint grant from the European Commission and "Institut de Sélection Animale", the layer breeding division of Hendrix Genetics within the framework of the Erasmus-Mundus joint doctorate "European Graduate School of Animal Breeding and Genetics." We would also like to thank Wendy Brand-Williams and Andrea Rau for the English revision of the manuscript (INRA/AgroParisTech, Jouy-en-Josas, France).

SUPPLEMENTARY DATA

Supplementary data is available at PSA Journal online.


DDS Antoine Couatarmanach

PhD William Sherlaw

DDS, Rennes Pierre-Marie Prigent

PhD Cyrille Harpet

DDS Valérie Bertaud

Dentists' perspectives on barriers to providing oral health care in French psychiatric hospitals with on-site dental clinics

published or not. The documents may come

Dentists' perspectives on barriers to providing oral health care in French psychiatric hospitals with on-site dental clinics

Introduction

In the past ten years, several reviews and meta-analyses have confirmed that people with mental health disorders suffer from poorer oral health than the general population [START_REF] Kisely | A Systematic Review and Meta-Analysis of the Association Between Poor Oral Health and Severe Mental Illness[END_REF][START_REF] Kisely | Advanced dental disease in people with severe mental illness: systematic review and meta-analysis[END_REF][START_REF] Matevosyan | Oral Health of Adults with Serious Mental Illnesses: A Review[END_REF] . Many causes have been identified, including the effects of psychiatric medications, a lack of dental hygiene, high smoking rates, unhealthy diets and poorer access to oral health care [START_REF] Moore | Promoting physical health for people with schizophrenia by reducing disparities in medical and dental care[END_REF] . In turn, oral diseases can have deleterious consequences on both the physical and the mental health of these patients [START_REF] Kisely | No Mental Health without Oral Health[END_REF] . This situation puts people with mental health disorders in the paradoxical position of being "objectively" in greater need for oral health care but less able to access it than the general population [START_REF] Jamieson | The mouth as a site of structural inequalities; an introduction[END_REF] . This situation has been highlighted for people receiving psychiatric care in the community (out-patients) [START_REF] Corridore | Oral Health Status and Oral Health-Related Quality of Life in Italian Deinstitutionalized Psychiatric Patients[END_REF][START_REF] Denis | Oral health in schizophrenia patients: A French Multicenter Cross-Sectional Study[END_REF] as well as, and even more so, for people hospitalised in psychiatric facilities (psychiatric in-patients) [START_REF] Ashour | Association between obesity/overweight and dental caries in psychiatric patients[END_REF][START_REF] Bertaud-Gounot | Oral health status and treatment needs among psychiatric inpatients in Rennes, France: a cross-sectional study[END_REF][START_REF] Chu | Factors associated with dental caries among institutionalized residents with schizophrenia in Taiwan: a cross-sectional study[END_REF][START_REF] Djordjevic | Prevalence of dental caries in hospitalized patients with schizophrenia[END_REF][START_REF] Gurbuz | Oral health and treatment needs of institutionalized chronic psychiatric patients in Istanbul, Turkey[END_REF][START_REF] Lewis | The oral health of psychiatric in-patients in South Wales[END_REF][START_REF] Nikfarjam | Oral Health Status in Three Long Term Care Units of Schizophrenic Patients in Chaharmahal-Bakhtiari Province, Iran[END_REF][START_REF] Teng | Oral health of psychiatric inpatients: a survey of central Taiwan hospitals[END_REF][START_REF] Velasco | Dental health among institutionalized psychiatric patients in Spain[END_REF][START_REF] Zusman | An assessment of the dental health of chronic institutionalized patients with psychiatric disease in Israel[END_REF] . Dedicated dental clinics inside psychiatric hospitals might be considered as a useful tool to enhance access for psychiatric inpatients, and some authors have called for such services [START_REF] Chu | Comparison of oral health between inpatients with schizophrenia and disabled people or the general population[END_REF] . Indeed, three epidemiological studies investigating the dental status of psychiatric inpatients reported that the hospital where the study was conducted did benefit from such in-site dental clinics [START_REF] Bertaud-Gounot | Oral health status and treatment needs among psychiatric inpatients in Rennes, France: a cross-sectional study[END_REF][START_REF] Djordjevic | Prevalence of dental caries in hospitalized patients with schizophrenia[END_REF][START_REF] Lewis | The oral health of psychiatric in-patients in South Wales[END_REF] . Despite evidence showing that the availability of dental clinic within psychiatric institutions facilitates hospitalized patients' oral health [START_REF] Ponizovsky | Effect of Implementing Dental Services in Israeli Psychiatric Hospitals on the Oral and Dental Health of Inpatients[END_REF][START_REF] Stevens | Perceived oral health needs in psychiatric inpatients: impact of a dedicated dental clinic[END_REF] , the evidence indicates that barriers persist in these situations.

Recent studies investigating barriers to oral care for people suffering from mental health disorders, based on literature review [START_REF] Slack-Smith | Barriers and enablers for oral health care for people affected by mental health disorders[END_REF] and qualitative inquiries, have focused on the perceptions of either dental professionals [START_REF] Scrine | Providing oral care for adults with mental health disorders: Dental professionals' perceptions and experiences in Perth, Western Australia[END_REF] and mental health professionals [START_REF] Scrine | Enhancing oral health for better mental health: Exploring the views of mental health professionals[END_REF] . However, no studies have focused on the barriers faced by psychiatric in-patients. Considering the unique environment psychiatric institutions constitute, there is a need for research focused on those specific situations. Despite an ongoing deinstitutionalization process and strong regional disparities, the delivery of psychiatric care remains largely hospital-based in France [START_REF] Coldefy | The Evolution of Psychiatric Care Systems in Germany, England, France and Italy: Similarities and Differences[END_REF][START_REF] Verdoux | Focus on psychiatry in France[END_REF][START_REF] Salisbury | An international comparison of the deinstitutionalisation of mental health care: Development and findings of the Mental Health Services Deinstitutionalisation Measure (MENDit)[END_REF] . Even if no systematic data are available, previous studies indicates that in-site dental clinics do exist in this country [START_REF] Denis | Oral health in schizophrenia patients: A French Multicenter Cross-Sectional Study[END_REF][START_REF] Bertaud-Gounot | Oral health status and treatment needs among psychiatric inpatients in Rennes, France: a cross-sectional study[END_REF] .

The aim of this study is to identify, from the perspective of dentists working in psychiatric hospitals in France, the barriers that still remain for oral health, even when an in-site dental clinic is available. In order to recruit participants, psychiatric hospitals were first contacted by telephone to establish whether dental consultations were available within the institution, starting with the closest from the researchers' work place, and then widening the search progressively to other hospitals elsewhere in France. When the hospital reported the existence of an in-site dental clinic, the dentist was identified and contacted by mail or telephone in order to explain him/her the goals and methods of this study. When he/she agreed to participate, an appointment was settled for a face-to-face interview. Before starting the interview, a consent form was delivered, informing the participant that his/her discourse would remain anonymous and that he/she would be able to withdraw from the study.

Semi-structured interviews were conducted, starting systematically with a question about their previous professional trajectory, and what led them to work in a psychiatric hospital. From this common starting point, participants were then asked about the difficulties they were facing delivering dental care in those facilities. Considering that the objective of the interviews was to highlight dentist's perceptions, and facilitate the emergence of unknown issues, participants were encouraged to provide accounts of their experiences and elaborate on the problematic issue [START_REF] Stewart | Qualitative research in dentistry[END_REF] .

Interviews were fully transcribed with Sonal® (V 2.1.41). Given the lack of previous knowledge on this particular matter, a conventional content analysis approach was chosen in order to highlight unrevealed themes [START_REF] Hsieh | Three Approaches to Qualitative Content Analysis[END_REF] . The coding process was performed manually and independently by two researchers in order to limit personal bias in the interpretation of dentist's responses. Codes were then discussed and organised in themes in an inductive way.

Interviews were coded consecutively, when they were available, allowing the investigator to compare them during the process of data collection. Saturation was considered to have been reached when no new themes could be extracted from the last interview. At this step, multiple readings of the interviews were performed in order to verify key themes and identify recurrences and differences among the interviews. Collective discussions finally allowed to confirm key themes and regroup them into broader dimensions.

A c c e p t e d M a n u s c r i p t

Results

A total of 15 psychiatric hospitals were contacted. Five didn't benefit from on-site dental clinics. Among the 10 dentists that were identified, 2 refused to participate arguing the lack of time. Eight interviews were finally conducted between January 2018 and March 2019. The average duration of the interviews was 54 minutes (minimum 24 min, maximum 89 min).

Characteristic features of the interviewed dentists are summarized in table 1.

The content analysis of the interviews permitted to highlight three dimensions of barriers to oral health care perceived by the dentists working in psychiatric hospitals: an individual dimension related to the patient situation and the dentists-patient relationship; a "local organization" dimension related to interprofessional relationships; a systemic dimension related to the organization of the health care system.

Individual dimension

Themes were classified as an individual matter when they were related, in the dentist's perception, with the personal and medical situation of the hospitalised patient. Every dentist who was interviewed identified mental disorder itself as a barrier to dental care. Despite the huge diversity of pathologies (from schizophrenia to severe depression) affecting the patients, dentists described a general lack of hygiene. Pharmacological treatments were also generally pointed out as a negative factor, especially for their impact on salivation. Psychoactive substance use was also seen as a major and frequent issue for its consequences on oral health.

A recurrent consequence of mental disorder identified by the dentists was its impact on the involvement in the dental care process they could expect from the patients. This lack of involvement can express itself in patient refusal:

We can see people that refuse dental treatment, they come but you can't touch them, and they run away… and you can't treat them against their will.

There are 30 to 40 % of the patients who refuse dental care, they say "No, I'm OK like that, don't touch my mouth", and the nurse call us to say that he's not going to come to the appointment, and we're not going to force him to come.

The relationship between mental disorder and the patient's social situation was also regularly pointed out, to explain the issue of involvement in dental care:

A c c e p t e d M a n u s c r i p t

We can see patients who experience precarious situations, who get here [in the hospital] because of this precariousness, or because their mental condition makes them face precarious situations, and so dental care, body care, are not really considered. This lack of involvement was also linked, from the dentists' perspective, with the relatively low importance of aesthetics for some patients:

One of his front teeth was took off, I don't know when, not so long ago. He doesn't seem to care, he's 35… He's missing a lateral [upper lateral incisor] but well… However, this low involvement was not seen as systematic. Many patients were described as really looking after dental care:

I have a proportion of patients who are really concerned about their dental care, really concerned.

Mental disorders were also seen as impacting on the dental care relationship. Psychiatric inpatients were described as being in need for specific approaches. Dentist behaviour and time were recurrently identified as the main issues in order to create a favourable care relationship:

With general population, it's the patient who have to adapt: "a little bit on the left, turn the head". In psychiatry, it's the opposite, it's for us, the dentist, to adapt.

Sometimes the talking can last for half an hour, for one hour… Well it depends, in

order to really motivate them, because it might not be the proper time for starting treatments, but it's still a part of the story.

Questions were asked about the need for violence management, but this wasn't described as a real issue by the interviewed dentists:

Well they have difficulties and sometimes they can have violent behaviours, but when they are in this kind of states, they usually don't come here. If they do come here, I know what I can and cannot do and so I never got molested.

Finally, mental disorder was sometimes described, on the contrary, as a factor that can facilitate the dental care relationship. This had mainly to do with the fact that patients with mental disorders wouldn't hide their reactions and feelings:

Every patient is scared, but here [in the psychiatric hospital], I have never faced faintness, never ever. Because the patient expresses it when he is not feeling well. He says it.

A c c e p t e d M a n u s c r i p t

He's going to agitated, to refuse… Well, I'll know it. Whereas in private practice, some patients will hold their feelings in, interiorise, until they faint and fall down.

"Local organization" dimension

The "local organisation" dimension regrouped themes pertaining to the nature of the relations between the dentist and his/her environment in the institution. These relations, and how they were managed and carried out, appeared to be an important matter of interest for the interviewed dentists. The related issues were usually mentioned before any questions were asked about them. First of all, the quality of relationships among the different members of the dentistry unit were seen as an important enabler for the delivery of dental care. Beyond the need for assistance expressed by dentists for several tasks (as appointments management, sterilization, ordering materials), the importance of good relationships with dental assistants was specifically highlighted for their ability to perform multiple functions and their experience dealing with mental health issues:

They are able to take care of radiology, gynaecology… I used to have a radiology assistant for dental assistant.

As former psychiatric nurse, my assistant has a deep knowledge of psychiatric disorders and she even knows personally some of the patients that have been hospitalised for a long time.

Relationships with other professionals from the psychiatric ward were also considered as a major matter of interest. The nature and efficacy of communication with nurses, general practitioners and psychiatrists varied according to the hospital involved, and so it was perceived as a barrier where this communication was difficult, and as an enabler in hospitals where it was easy. The main contact for dentists appeared to be the general practitioner:

Relationships are principally mediated by the general practitioner, he's the one who address the patients.

General practitioners have the kindness to coordinate for me, and to tell me when there's a problem.

A c c e p t e d M a n u s c r i p t

Interactions with the nurses in psychiatrics units were also described as an important feature for the organization of appointments, as well as for preparing the patients before dentist's appointment, or the improvement of daily dental care. This relation was frequently described as non-efficient:

The main barrier is the motivation of nursing teams. It varies a lot depending on the unit.

Relations with the psychiatrist were usually described as poor, if not inexistent. All such limitations in the relations dentists can have with other staff members were seen as a symptom of the specialized orientation of the psychiatric hospital, relegating the somatic dimension:

Here we are in a psychiatric hospital where… I mean, in fact, somatic issues do not have a proper place here.

Limitations in inter-professional relationships were perceived as having multiple consequences, particularly for the organisation of appointments. For example, there was a consensus on the need for regular dental check-up visits for psychiatric inpatients, but those were described as difficult to set up: I fight for this all the time but I have to admit that I'll never get it. I used to hope we could organise a regular follow-up of the patients, but that's some something we'll never get.

The "local organisation" dimension also included technical issues related to the quality of dental office equipment, as well as access to specific treatment facilities such as nitrous oxide sedation and general anaesthesia. All of the interviewed dentists were satisfied with their material organisation for conventional treatment, and several were able to perform treatment under nitrous oxide sedation. On the other hand, access to general anaesthesia was indicated as a recurrent issue, highly dependent on the medical offer in the region, since none of the psychiatric hospitals were equipped to perform general anaesthesia within the facility:

We don't perform treatments under general anaesthesia here. There's a huge need for it, but it's unmet. Sometimes, the general practitioner, rarely, can react and send the patient to [university hospital] which is 100 km away.

A c c e p t e d M a n u s c r i p t

Systemic dimension

The systemic dimension comprised themes implicating the whole health system. The question of the training and professional development of the different professionals involved was systematically addressed, and appeared as a debatable topic. Training of dentists on the specific issues of psychiatry was seen as an important enabler by some dentists, but others considered it to be irrelevant, the most important thing being tacit knowledge that could be acquired only by experience:

It's not a matter of training, it's a matter of stance… I don't know… The question is how you interact with the patients.

It is noteworthy that only one of the eight dentists who were interviewed reported that he had benefitted from a special course on mental health issues.

There was more consensus about the need for training and professional development of nurses in oral health issues. A recurrent enabler to be described was the training of nurses (in psychiatric wards) who could act as correspondents for oral care in the units. Some dentists mentioned its existence in other institutions and it was perceived as something that needed to be developed in psychiatric hospitals:

The organization is more coherent, as there's an identified person in every unit in charge of the oral health issue. This person, after time spent observing dental appointments, seeing what's going on, can communicate with the other staff members in the unit.

Another important barrier identified by the participants was the tendency, in psychiatric hospitals, to shorten the length of hospitalization. It was perceived to be the outcome of the general process of de-institutionalization, leaving mental care of patients to either the community or the housing facilities separated from the hospital. All of the interviewed dentists expressed difficulties arising from the increased turn-over of patients in the hospital; it was seen as a threat that could compromise dental care:

The main problem is that the patients leave the hospital more and more quickly nowadays. They used to stay longer, now they leave as soon as possible, and it's not easy to follow up… A c c e p t e d M a n u s c r i p t Finally, financial issues were also highlighted, especially for their impact on recruitment. Indeed, the remuneration for working in psychiatric hospitals, based on fixed salary scales, was unanimously described as lower than in private practice. In a context of declining demographics of dentists, the interviewed dentists who were close to retirement expressed the fear that it might be difficult to find someone to replace them. This was then perceived as a potential barrier. However, when asked about their motivation for working in psychiatric hospitals, participants did not mention the economic incentive. On the contrary, they described their choice as the consequence of a social mandate: I found myself a bit morally constrained to stay here, because I think that's a population who really needs to be taking care of, and who faces real difficulties to find dental care solutions. I also work in a private practice and I'm really proud of that. But I'm always keeping in mind the sense of state. Always in the service of the state.

Discussion

This study highlights the barriers dentists working in psychiatric hospitals in France are facing in delivering dental care. First, barriers were identified as linked to the individual situation of the patients. Such barriers have been identified for a long time and strategies have been proposed to overcome these 30 . In fact, it appears from the interviews that dedicated dental clinics inside psychiatric hospitals help to create the conditions to reduce such individual barriers.

Organisation-related barriers were described as being much more resistant by the participants.

The lack of communication between dental professionals and psychiatric teams has also been previously identified in other contexts, described as the consequence of a "siloed approach" that would characterise the professionals' attitude [START_REF] Slack-Smith | Barriers and enablers for oral health care for people affected by mental health disorders[END_REF] . Findings from this study confirm the lack of interprofessional practice even when all of the actors are in the same hospital.

The main systemic barrier identified by all of the participants was related to the consequences of deinstitutionalization. This finding is consistent with the hypothesis formulated by Kisely that "the move to care in the community may have had the unintended effect of reducing access to dental care" [START_REF] Kisely | No Mental Health without Oral Health[END_REF] . However, this study's design does not permit confirmation of such a Interviewed dentists agreed about the fact that the existence of a dental service inside the hospital represents a great opportunity. They argued that they are able to provide care in such facilities that would have been impractical to achieve in a private practice. These findings are consistent with the consensus on the need for dedicated dental clinics reached through a recent Delphi technique survey involving a panel of French dentists committed to special care dentistry 32 .

Education and professional training have been previously identified as major enablers for enhancing access to care for people in need for specific oral care 33 . That the participants did not necessarily agree on the importance of education should not in our view put its importance into question. Findings on the perception of training and education might cast more light on the actual situation of the dentists working in psychiatry in France than on the opportunities that education may create if it was implemented.

Data collection appeared to be problematic at first given the absence of systematic data on the existence of dental clinics within psychiatric hospitals in France and the overall diversity of psychiatric care delivery [START_REF] Coldefy | The Evolution of Psychiatric Care Systems in Germany, England, France and Italy: Similarities and Differences[END_REF] . However, once the dentists were identified and contacted, they showed great interest for the study and were enthusiastic to participate. This could indicate that actors in the field acknowledge the need for research in this area. Although the findings from this study can't be generalised for all psychiatric hospital-based dental care delivery, it highlights for the first time the barriers that are faced in this specific situation.

Conclusion

The findings of this study on dentists' perspectives help to identify a diversity of remaining barriers to oral health care in psychiatric hospitals with on-site dental clinics, especially on the organizational (the lack of interprofessional relationship) and the systemic aspects (deinstitutionalization-related risks). Consequently, they offer insights into the conditions in which such facilities could be usefully introduced. The question of whether or not this kind of dental service inside the psychiatric hospital should be implemented remains a practical and real question for stakeholders and managers; findings from this study claim for its

Acknowledgement

The researchers express their gratitude to the participants for giving their time to be interviewed.

A c c e

  p t e d M a n u s c r i p t Methods This study was designed as a qualitative inquiry based on semi-structured interviews with dentists working within psychiatric hospitals in France. The protocol was validated from the ethical point of view by University Hospital ethics committee in Rennes (notice n°19.01).

A c c e

  p t e d M a n u s c r i p t hypothesis. This issue remains nonetheless of great importance in a context of intense discussions about how long term psychiatric care should be organised in the future 31 .

A c c e

  p t e d M a n u s c r i p t development. Further research should be conducted to consider the perspectives of other actors within the psychiatric institution, including patients and medical staff members.

A c c e

  p t e d M a n u s c r i p t A c c e p t e d M a n u s c r i p t

A c c e p t e d M a n u s c r i p t