Influenza A virus epidemiology in breed-to-wean farms and infection dynamics in nursery pigs

Abstract

Influenza is an economically important disease in pigs and a public health threat. Breed-to-wean (BTW) farms play a central role in influenza epidemiology and control because piglets born in BTW farms can maintain, diversify and, at weaning, disseminate influenza A virus (IAV) to other farms. Despite the importance of piglets in influenza epidemiology, there is limited information on IAV infection parameters in piglets, risk factors that impact IAV prevalence in piglets at weaning, and how strategies that are implemented in BTW farms affect IAV infections in weaned pigs. In this thesis, I aimed to: 1) estimate herd-level prevalence and seasonality of influenza in BTW farms, 2) evaluate farm factors associated with IAV infection in piglets at weaning, 3) assess transmission patterns and parameters of influenza in nursery pigs based on IAV prevalence at weaning, and 4) evaluate the impact of maternally-derived antibodies (MDA) at weaning on IAV infection parameters in nursery pigs. Results from my work showed that IAV herd-level prevalence in piglets at weaning is seasonal with higher infection rates in winter and spring. I also found that among all the factors evaluated, IAV sow vaccination and influenza negative status of replacement breeding females (gilts) at entry to the herd were significantly associated with fewer IAV infected piglets at weaning. My results also indicate that groups of piglets with different prevalence at weaning had different transmission patterns and parameters after weaning and these patterns were characterized by 1, 2 or no peaks of infection after weaning. Lastly, I reported that if pigs had high levels of strain-specific MDA at weaning, IAV infection occurred later and was of shorter duration after weaning. Knowledge of seasonality and what factors are associated with influenza in BTW farms may help producers and veterinarians to better use and allocate influenza control strategies such as vaccination. Lower prevalence at weaning due to high strain-specific MDA levels may help decrease IAV spread and infections in nursery pigs. Reducing the burden of IAV in pigs should decrease IAV-associated economic losses and the generation of novel strains, including strains with pandemic potential.