Genetic characterization of Thai swine influenza viruses after the introduction of pandemic H1N1 2009.

Pandemic H1N1 2009 (pH1N1), influenza virus containing triple reassortant internal genes (TRIG) from avian, human, and swine influenza viruses emerged in 2009 as a highly infectious virus that was able to be transmitted from humans to pigs. During June 2010-May 2012, influenza virus surveillance was conducted in Thai pig population. Twenty-three samples (1.75%) were successfully isolated from total of 1,335 samples. Interestingly, pH1N1 (7 isolates, 30.34%), reassortant pH1N1 (rH1N1) (1 isolate, 4.35%), Thai endemic H1N1 (enH1N1) (3 isolates, 13.04%), reassortant H3N2 with pH1N1 internal genes (rH3N2) (9 isolates, 39.13%), and reassortant H1N2 with pH1N1 internal genes (rH1N2) (3 isolates, 13.04%) were found. It should be noted that rH1N1, rH1N2, and rH3N2 viruses contained the internal genes of pH1N1 virus having a TRIG cassette descendant from the North American swine lineage. Although all isolates in this study were obtained from mild clinically sick pigs, the viruses were still highly infective and possibly may play an important role in human-animal interfacing transmission. In addition, the TRIG cassette may have an influence on antigenic shift resulting in emergence of novel viruses, as seen in this study. Continuing surveillance of influenza A natural hosts, particularly in pigs is necessary.