Dynamics of influenza A virus transmission in swine herds and analysis of risk factors for recurrent infections
An investigation of the circulation of influenza A virus (IAV) and dynamics of the disease in swine populations is described in this thesis. The advent of more complex swine production systems can lead to changes in the pattern of disease making it difficult to predict IAV outbreaks and develop control programs. Factors such as host immunity, genetic and antigenic characteristics of the viruses, and fluctuations of temperature and humidity (seasonality), can be associated with the maintenance of the infection in swine populations. A longitudinal study evaluated, in two different cohorts, the dynamics of IAV circulation in multi-source nursery herds and identified risk factors associated with infection in nursery pigs. In addition, the circulation of IAVs between weaning and market age was characterized on the basis of development of antibody response and molecular epidemiology of detected viruses. Finally, a cross-sectional study was used to evaluate the association between environmental measurements such as temperature and humidity, and the presence of antibodies for two specific IAV. Virological tests showed a high prevalence of swine influenza and that influenza virus can circulate during the nursery phase in a cyclical pattern, with pigs becoming infected more than once with the same virus within a short time. The recurrent infection with the same virus was associated with the presence of maternal antibodies for a heterologous virus. In addition, the existence of antibodies titers for IAV other than the isolated virus confirmed that more than one viral subtype can circulate in the same population. Temperature and relative humidity can also be associated with development of influenza infection in swine populations based on cumulative exposure to the viruses. In conclusion, the research findings described in this thesis demonstrate that IAV transmission in complex swine systems can involve multiple virus subtypes and be influenced by environment and management factors, making the control of IAV complicated.