Isolation of the Pandemic (H1N1) 2009 virus and its reassortant with an H3N2 swine influenza virus from healthy weaning pigs in Thailand in 2011

► Six H1N1 and two H3N2 influenza A viruses were isolated from pigs in Thailand in 2011. ► The H1N1 isolates were the A(H1N1)pdm09 viruses. ► The H3N2 viruses were novel reassortants with A(H1N1)pdm09 virus. ► The HA and NA genes of the H3N2 isolates originated from a Thai swine influenza virus. ► The internal genes of the H3N2 isolates originated from a A (H1N1) pdm09 virus. A total of 300 nasal swabs were collected from 5 pig farms in two provinces in the Eastern part of Thailand in February 2011 and were subjected to viral isolation of influenza A viruses. Two H3N2 and 6 H1N1 influenza A viruses were isolated from swabs collected from clinically healthy weaning pigs on farms in Chonburi and Chachoengsao provinces, respectively. The H3N2 isolates consisted of the hemagglutinin (HA) and neuraminidase (NA) genes closely related to Thai SIVs and derived from a cluster of human seasonal H3N2 strains circulating around 1996–1997. The remaining gene segments of the isolates originated from the Pandemic (H1N1) 2009 (A (H1N1) pdm09) virus. Antigenicity of the H3N2 isolates was distinguishable from a human seasonal vaccine strain in the 1996–1998 seasons that represented antigenicity of the seasonal strains around 1996–1998. Nasal swabs from a Chachoengsao farm yielded A (H1N1) pdm09 viruses in chicken embryonated eggs and MDCK cells. A (H1N1) pdm09 viruses isolated in this study grew poorly in MDCK cells. Deduced amino acid sequences of the HA1 region of the HA protein of egg isolated viruses were identical to the sequences directly amplified from original swab samples. Our result demonstrated that the A (H1N1) pdm09 virus has been established in the Thai pig population and this has resulted in genetic reassortment with Thai SIV that previously circulated among pigs.