Background and purpose: We conducted serological and virological surveillance of pig farms in Taiwan from areas epidemic for low pathogenic avian influenza virus (AIV), H5N2 subtype, in order to determine the prevalence of AIV and swine influenza virus (SIV) in 2004.
Methods: Pig sera from 9833 animals from 1974 farms in 9 counties were examined using agar gel precipitation (AGP) to screen for the presence of antibody against influenza A virus. AGP-positive sera were subjected to hemagglutination-inhibition test against H1, H3, H5 and H7 AIV subtypes and H1 and H3 SIV subtypes. Nasal swabs from 881 pigs were also examined for the presence of SIV by virus isolation in specific pathogen-free embryonated chicken eggs. Virus isolates were identified by reverse transcriptase-polymerase chain reaction followed by DNA sequencing of hemagglutinin and neuraminidase genes.
Results: The AGP test on sera revealed the presence of antibodies against influenza A virus in 62.6% of farms and in 37.7% of pig sera. SIV antibodies to subtype H1 and H3 were found in 10.8% and 65.8% of sera, respectively. There were two peaks of the serological prevalence of SIV in pigs: one between January and February, and the other in October. By contrast, hemagglutinin tests against H5 and H7 AIV subtypes were negative in all sera, while there was a very low positive rate against H1 and H3 AIV subtypes. One H1N2 and one H3N1 viral isolate were obtained from nasal swabs of pigs. Phylogenetic analysis of hemagglutinin and neuraminidase genes revealed both isolates were reassortants of both classical and recent SIVs.
Conclusions: Different subtypes of SIV co-circulate among swine from different farms within the same county and may cause clinical outbreaks of the disease in Taiwan.