During their lifespan, chickens are confronted with a wide range of acute and chronic stressors in their housing environment that may threaten their welfare and health by modulating the immune system. Especially chronic stressful conditions can exceed the individual’s allostatic load, with negative consequences for immunity. A fully functional immune system is mandatory for health and welfare and, consequently, also for high productivity and safe animal products. This review provides a comprehensive overview of the impact of housing form, light regime as well as aerial ammonia and hydrogen sulfide concentrations on the immune system in chickens. Certain housing conditions are clearly associated with immunological alterations which potentially impair the success of vaccinations or affect disease susceptibility. Such poor conditions counteract sustainable poultry production. This review also outlines current knowledge gaps and provides recommendations for future research.

Insufficient nutrient supply can impair the immune system, which is important for animal health and welfare. Since chicken can partly hydrolyze phytate, which is the primary phosphorus storage in plant seeds, a reduction of mineral phosphorus in the diets could be an option for more sustainable egg production. Laying hens require high concentrations of calcium that might inhibit the function of endogenous enzymes for phytate hydrolyzation. The objective of this study was to characterize the impact of standard and reduced dietary phosphorus and calcium concentrations on the number and functionality of immune cells in the peripheral and gut-associated immune system in a white and brown laying hen strain. Reduced mineral phosphorus enhanced several immune parameters such as B cells in blood and IgA concentrations in bile in both strains, and peripheral monocytes and T cells in cecal tonsils in brown hens. Reduced calcium levels resulted in lower numbers of T cells in blood and cecal tonsils in both strains, suggesting negative effects on adaptive immunity. Differences between the two strains were found in almost all immune parameters. Results suggest a potentially beneficial effect of reduced dietary mineral phosphorus on the immune system that is dependent on the genetic background.

Already during early life, chickens need to cope with chronic stressors that can impair their health and welfare, with stocking density being one of the most influential factors. Nevertheless, there is a gap in research on the influence of stocking density on laying hens during rearing and in the subsequent laying period. This study therefore investigated how stocking density during rearing affects the immune system and welfare of pullets, and whether effects are persistent later in life. Pullets were reared at either low (13 birds/m²) or high (23 birds/m²) stocking densities but in identical group sizes from wk 7 to 17. Afterward, hens were kept at the same stocking density (2.4 birds/m²) until wk 28. Blood and tissue samples (spleen and cecal tonsils) were collected at the end of the rearing period and in the laying period. The parameters evaluated encompassed number and distribution of leukocytes and lymphocyte subsets in blood and lymphatic tissue, lymphocyte functionality, plasma corticosterone concentrations as well as behavior and physical appearance of hens. At the end of rearing, pullets kept under high stocking density had lower numbers of T lymphocytes, especially γδ T cells in blood, spleen, and cecal tonsils and displayed a higher heterophil to lymphocyte ratio. These effects are mostly persistent during the laying period, although stocking density was identical at this time. Furthermore, birds from the high stocking density group showed less active behavior, more pecking behavior and worse physical appearance throughout both examination periods. In conclusion, stocking density during rearing affects pullets' immune system and behavior not only in the rearing, but also subsequently in the laying period, indicating a strong correlation between health and welfare during rearing and the laying period.

Determining the abundance of leukocyte subtypes, including lymphocyte subpopulations, not only in blood but also in lymphatic tissues, is inevitable to assess the immune status of an organism for research purposes. However, nucleated thrombocytes and erythrocytes exacerbate many hematological techniques in avian species. In order to enable a rapid discrimination of leukocyte subsets from lymphatic tissues of chicken, we adapted existing flow cytometric methods for counting leukocytes in chicken blood. We established staining and gating strategies allowing the flow cytometric characterization and enumeration of total leukocytes, thrombocytes, monocytes/macrophages, CD8α⁺ lymphocytes, CD4⁺ T cells, γδ T cells, and B cells in chicken spleen and CD8α⁺ lymphocytes, CD4⁺ T cells, γδ T cells, and B cells among intraepithelial lymphocytes in chicken cecal tonsils. For this, we prepared single‐cell suspensions of spleen and isolated intraepithelial lymphocytes from cecal tonsils without density centrifugation, and performed antibody staining of cells without subsequent washing steps to prevent cell loss and falsification of obtained cell counts. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC on behalf of International Society for Advancement of Cytometry.

Various aspects of activity, such as spontaneous activity, explorative activity, activity in open-field tests, and hyperactivity syndrome have been explored as causal factors of feather pecking in laying hens, with no clear results. In all previous studies, mean values of activity over different time intervals were used as criteria. Incidental observation of alternated oviposition time in lines selected for high (HFP) and low feather pecking (LFP), supported by a recent study which showed differentially expressed genes related to the circadian clock in the same lines, led to the hypothesis that feather pecking may be related to a disturbed diurnal activity rhythm. Hence activity recordings of a previous generation of these lines have been reanalyzed. Data sets of a total of 682 pullets of 3 subsequent hatches of HFP, LFP, and an unselected control line (CONTR) were used. Locomotor activity was recorded in pullets housed in groups of mixed lines in a deep litter pen on 7 consecutive 13-h light phases, using a radio-frequency identification antenna system. The number of approaches to the antenna system was recorded as a measure of locomotor activity and analyzed using a generalized linear mixed model including hatch, line, time of day and the interactions of hatch × time of day and line × time of day as fixed effects. Significant effects were found for time and the interaction line × time of day but not for line. All lines showed a bimodal pattern of diurnal activity. The peak activity of the HFP in the morning was lower than that of the LFP and CONTR. In the afternoon peak all lines differed with the highest mean in the LFP followed by CONTR and HFP. The present results provide support for the hypothesis that a disturbed circadian clock plays a role in the development of feather pecking.

Feather pecking (FP) is a serious behavioral disorder in laying hens, leading to feather damage, skin lesions, and often resulting in cannibalism. The mechanisms underlying FP are not clear yet, but recently the role of the immune system as a cause has been discussed. In humans, the interrelation between personality traits and the immune system is well-documented, with impulsivity and hyperactivity linked to distinct alterations in blood immune cell numbers and to elevated levels of pro-inflammatory cytokines. Similarly, FP in hens is associated with impulsivity and hyperactivity, suggesting a possible connection between FP and immune cell alterations. In this study numbers of leukocyte subsets in blood, spleen and cecal tonsils, along with mitogen-induced lymphocyte proliferative response and antibody concentrations across hens selectively bred for high (HFP) and low (LFP) feather pecking behavior were analyzed. Results showed that divergent selection altered FP behavior, with HFP hens showing about 10 times more pecking behavior than hens of the LFP line. HFP hens had lower numbers of T helper cells, CD4+ CD25high as well as B cells compared to LFP hens. Furthermore, HFP hens demonstrated a stronger proliferation of T cells when stimulated with ConA, while showed a weaker response in T cell-dependent B cell proliferation when stimulated with PWM, compared to LFP hens. Antibody plasma concentrations were similar between both lines. These findings highlight substantial immunological differences between HFP and LFP hens, especially in T cell immunity, and support the hypothesis that FP may be an immune- related behavioral response.