INRA, Unité mixte de recherches sur le veau et le porc, F-35590 Saint-Gilles

Abstract 
This article shows how situations of acute (transport, mixing animals) or chronic (poor housing conditions) stress affect the immune function of farm animals. The stress response implies the release of corticotrope axis hormones that inhibit leukocyte activity, but also of numerous other immunoactive or immunosuppressive hormones and neuropeptides (growth hormone, prolactin, enkephalins…). The increase in the blood neutophil to lymphocyte ratio and the inhibition of the ability of lymphocyte to proliferate are markers of a stress response. Other tests bring more functional information because they measure the effect of stress on precise immune functions. Functions related to the innate immune system, which is the first line of defence of the organism, are sensitive to stress. Natural killer cell cytotoxicity is inhibited and data from rodent studies indicate that the inflammatory response can be dramatically disturbed. Some stressors enhance the production of inflammatory cytokines and increase the susceptibility to septic choc, whereas other stressors inhibit leukocyte migration to the infection site, thereby limiting the inflammatory response and delaying wound healing. Lymphocytes form the second line of defence, which is the acquired immunity. Stress can inhibit the development of lymphocyte response to antigens, for example the response to a vaccine. It decreases specific cellular responses but does not affect or even sometimes stimulates antibody production. Alteration of innate and acquired functions decreases resistance of animals to viral or bacterial infections.