Swine influenza virus infection in different age groups of pigs in farrow-to-finish farms in Thailand

We conducted longitudinal monitoring in 6 farrow-to-finish farms in the central region of Thailand from 2008 to 2009. Nasal swabs and serum samples were collected periodically from clinically healthy pigs consisting of sows, fattening pigs, weaned piglets and pigs transferred from other farms. A total of 731 nasal swabs were subjected to virus isolation and 641 serum samples were subjected to detection of SIV antibodies against H1 and H3 subtypes using the hemagglutination inhibition test and ELISA. Twelve SIVs were isolated in this study and eleven were from piglets aged 4 and 8 weeks. Phylogenetical analysis revealed that SIVs isolated from different farms shared a common ancestor. Antibodies against SIVs were detected in fattening pigs on farms with no SIV isolation in the respective periods studied. These observations suggested that piglets aged 8 weeks or younger could be a main target for SIV isolation. Farm-to-farm transmission was suggested for farms where pigs from other farms are introduced periodically. In addition, antibodies against SIVs detected in fattening pigs could be a marker for SIV infection in a farm.The present study provided important information on SIV surveillance that will enable better understanding of SIV ecology in farrow-to-finish farms.Swine influenza virus (SIV) is one of the pathogens that cause respiratory diseases accompanied with coughing and sneezing in pigs [1]. This virus is considered an important pathogen not only from the viewpoint of animal health but also from that of public health [1-3]. Pigs can play the role of a 'mixing vessel' producing a novel influenza virus by genetic reassortment [4] as they have dual susceptibility to both human and avian influenza viruses [5]. Both receptors, namely, the sialic acid linked to galactose by an α2,6 linkage (SAα2,6Gal) for human viruses and an SAα2,3Gal for avian viruses, are expressed on epithelial cells of the tracheal and pulmonary structures of pigs [6,7]. The segmented nature