Heat and social stress effects on pig immune measures.

Forty-eight domestic pigs were used to evaluate the effects of heat and social stress on immune indices. Pigs were brought together in groups of three per pen and video-taped for the first 72 h. Video tapes were viewed to determine time spent in aggressive and submissive behaviors. Social status of each pig was determined from outcomes of agonistic interactions. Pens of pigs were housed in either a thermoneural (control, 24 degrees C) or heat-stress (33 degrees C) air temperature. Immune measures were determined from blood samples obtained on d 0, 7, 14, 21, and 28 after grouping. Social status had an effect (P < .05) on lymphocyte proliferation in response to pokeweed mitogen: socially intermediate pigs had a higher proliferative response than socially dominant or subordinate pigs. Many immune measures showed a significant interaction between heat and social stress over days of the study. Generally, socially dominant or submissive pigs had alterations in immune function (elevated numbers of neutrophils, decreased antibody production) compared with socially intermediate pigs. In conclusion, heat and social stress interact in their effect on the pig's immune system. Although one might have predicted immunosuppression among submissive pigs, there also seemed to be immunological costs to dominant pigs as well. These data also have implications in design of stressor research in that social behavior should be measured or controlled.