Animal-Based Factors Prior to Infection Predict Histological Disease Outcome in Porcine Reproductive and Respiratory Syndrome Virus- and Actinobacillus pleuropneumoniae-Infected Pigs

A large variety in clinical manifestation in individual pigs occurs after infection with pathogens involved in porcine respiratory disease complex (PRDC). Some pigs are less prone to develop respiratory disease symptoms. The variation in clinical impact after infection and the recovery capacity of an individual animal are measures of its resilience. In this paper we examined which ones of a range of animal based factors (rectal temperature, bodyweight, skin lesion scores, behavior, natural antibody serum levels, serum levels of white blood cells and type of T and granulocyte subsets) when measured prior to infection are related to disease severity. These animal based factors and the interaction with housing regime of the piglets (conventional or enriched) were modelled using linear regression to predict disease severity using a dataset acquired from a previous study using a well-established experimental co-infection model of porcine respiratory disease virus (PRRSV) and Actinobacillus pleuropneumoniae. Both PRRSV and A. pleuropneumoniae are often involved in PRDC. Histological lung lesion score of each animal was used as a measure for PRDC severity after infection. Prior to infection higher serum levels of lymphocytes (CD3+), naive T helper (CD3+CD4+CD8-), CD8+ (as well as relatively higher levels of CD8+), memory T helper (CD3+CD4+CD8+ ) cells and higher relative levels of granulocytes (CD172a) related to reduced disease severity in both housing systems. Raised serum concentrations of natural IgM antibodies binding to keyhole limpet hemocyanin (KLH) were also related to reduced disease severity after infection. Increased levels of skin lesions at the central body part (after weaning and before infection) were related to increased disease severity in conventional housing systems only. High resisters showed a lower histological lung lesion score, which appeared unrelated to sex. Body temperature, behavior and growth prior to infections were influenced by housing regime but could not explain the variation in lung lesion scores after infection. Raised basal lymphocyte counts and lower skin lesion scores are related to reduced disease severity independent or dependent of housing system respectively. In conclusion, our study identifies intrinsic animal based measures using linear regression analysis that predicts resilience to infections in pigs.