Disease Resilience Indicators Based on Phagocytosis Assays on Blood from Young Healthy Pigs

Abstract Infectious diseases impact the swine industry through increased morbidity and mortality, leading to large economic losses and reduced animal welfare. Disease resilience is the ability of an animal to maintain production performance under pathogen exposure and has gained traction as an approach to help reduce the impact of infectious disease. For this purpose, a wean-to-finish polymicrobial natural disease challenge model was set up, in which, using a continuous flow system, batches of 60 or 75 young healthy Landrace × Yorkshire barrows from one of seven breeding companies of PigGen Canada were entered into a quarantine nursery before their entry in a nursery/finisher that was seeded with multiple pathogens. The objective of this study was to evaluate phagocytosis assays that can be conducted on blood from young healthy pigs as potential indicator traits to select for disease resilience in high-health nucleus breeding herds. Immune cell proportions and their phagocytic activity were evaluated in blood collected at ~26 days of age in the quarantine nursery on 985 pigs. Genetic parameters, including heritabilities and genetic correlations with disease resilience phenotypes collected on these pigs and an additional 2,220 pigs from the same companies in the polymicrobial challenge were estimated using GBLUP in ASReml 4.2, with the fixed effect of batch, the covariate of entry age, and random effects of pen by batch, litter, and additive genetics, using a genomic relationship matrix derived from genotypes of 650K single nucleotide polymorphisms. Results showed moderate heritability estimates for immune cell proportions (average = 0.36; range = 0.22-0.42) and the % of phagocytizing cells for each cell type (0.32; 0.19-0.48), but low estimates for mean fluorescence intensity (MFI) of the phagocytizing cells of each type (0.2; 0.05-0.3). Estimates of genetic correlations with disease resilience traits ranged from -0.93 to 0.72, but had substantial standard errors. Granulocyte % had a high positive genetic correlation with number of health treatments across the challenge nursery and finisher (0.72 ± 0.5), while a greater % phagocytizing eosinophils was genetically correlated with decreased growth rate in the challenge nursery (-0.26 ± 0.2) but greater growth rate (0.33 ± 0.2) and reduced mortality in the finisher (-0.78 ± 0.6) and across the challenge nursery and finisher (-0.50 ± 0.4). Greater MFI of phagocytizing eosinophils was also genetically correlated with reduced mortality across the challenge nursery and finisher (-0.93 ± 0.5). These results provide insights into potential indicator traits that can be used to select for disease resilience without the need to subject animals to disease, with the overall aim to improve animal welfare and increase production performance in herds that are subject to hard-to-manage infectious pathogens. This project was supported by Genome Canada, Genome Alberta, PigGen Canada, and USDA-NIFA grant #2017-67007-26144.