A porcine model for polymicrobial respiratory infections with swine influenza virus and Staphylococcus aureus

Influenza A virus (IAV) is a significant problem worldwide, and respiratory disease is further complicated by secondary bacterial infection. The emergence of highly pathogenic strains of IAV in conjunction with the increase of antibiotic-resistant bacteria threatens human health. A large-animal model effective for study of polymicrobial infection comparable to humans must therefore be developed. IAV has been studied extensively in small animals, including mice, rats and ferrets. However, these species frequently require IAV adaptation, reducing the capacity of these models to adequately represent human infection. Furthermore, species commonly used lack likeness to humans in both the presentation of symptoms and in lethality of infection. However, pigs are naturally susceptible to unadapted IAV and are considered to be the 'mixing vessel' for the recent pandemic IAV virus. Pigs are also susceptible to infection with Staphylococcus aureus, the most commonly isolated bacteria from IAV-infected human adults. Therefore, the use of pigs in the study of polymicrobial respiratory infections would be ideal for characterizing a host immune response comparable to humans, as well as for the development of diagnostics and therapeutics. Using this novel model, we determined that pigs are susceptible to Staphylococcus aureus, swine IAV, and polymicrobial infection. Furthermore, we showed that IAV infection predisposes pigs to Staphylococcus aureus pneumonia, and this susceptibility is dependent on day post-IAV infection.