Management of Clinical or Subclinical Immune Activation

Besides infectious diseases, environmental constraints and farm management practices can also be a cause of chronic stimulation of the immune system of farm animals. This stimulation can in turn worsen animal welfare and impair production performances. When not related to a pathogenic infection, suspected causes of immune activation include increased nonpathogenic microbial environmental pressure, animal flora dysbiosis, and transient immune system or endocrine alterations. In pigs, immune activation can be observed not only in animals housed in facilities with poor hygiene conditions, but also in animals housed in standard environmental farm conditions (when compared to animals housed in enriched environments), and in response to specific challenging events such as weaning. Depending on the study, various indicators like white blood cell counts, blood concentrations of acute phase inflammatory proteins, immunoglobulins, oxidative stress products and anti-oxidant blood potential demonstrate this immune stimulation. Experimental studies show that amelioration of the housing environment (adequate environmental temperature or hygiene, welfare friendly housing) and mitigation of stress factors are the most direct ways to reduce this unspecific immune stimulation. Few data also suggest the possible positive influence of environmental housing factors such as outdoor housing, ambient temperature and stocking density on animal resistance to viral and bacterial infections (for instance E. coli or porcine circovirus infections).

However, other strategies could also be considered. Immune activation is usually accompanied by an inflammatory response which, in growing animals, impairs growth. Inflammation down-regulates feed intake, and is simultaneously a nutrient liberating (skeletal muscle catabolism) and nutrient consuming (acute phase protein synthesis, fever) process. Upon chronic inflammation, some amino-acids display more severe and longer alterations, possibly related to their role either as an energetic substrate or as specific precursors / substrates for immune processes. Based on this knowledge, various strategies have been tested, with the aims of adequately supplying the metabolism of the immunologically challenged pig, limiting inflammation and minimizing growth deterioration. For example, among nutritional approaches, moderate feed restriction during inflammation produced mixed results, while tryptophan supplementation is more promising. Genetic selection for animals exhibiting lower inflammatory traits or lower metabolic alterations during inflammation is another approach that deserves to be explored. As an example, the differences in response to a moderate inflammatory challenge of two lines of pigs divergently selected for residual feed intake are presented.