The clinical features, pathobiology, and epidemiology of influenza infections in pigs

Endemic influenza in pigs is caused by influenza A virus subtypes H1N1, H1N2, and H3N2. Clinical disease results in high morbidity and low mortality, with increased rectal temperature being the most consistent clinical sign. As influenza viruses infect the respiratory epithelial cells and replicate, the prominent microscopic lesions of necropurulent bronchitis and bronchiolitis and the macroscopic lesions of bronchopneumonia are produced. Natural transmission of influenza occurs year round, and important risk factors for transmission include large herd sizes, large numbers of pigs per pen, and farrow-to-finish farms with continuous flow. Both human-to-pig and pig-to-human zoonoses play a significant role in influenza ecology and epidemiology, highlighting the importance of understanding the pathogenesis of these viruses in their hosts and the risk factors for movements within and between these two species.