Preliminary laboratory results of an epidemiological study on avian and swine influenza in Vietnam

Abstract

Background: To study the pig-poultry interspecies transmission, the French Agricultural Research Center for International Development (CIRAD) carried out a cross-sectional survey in pig and poultry farms and a longitudinal study in a local pig slaughterhouse in North Vietnam from April 2009 to February 2010. The preliminary laboratory results of this study are presented here. A screening of sera by a commercial competitive enzyme-linked immunosorbent assay (cELISA) for influenza A was done in Vietnam, and further serological analyses on cELISA positive sera and virological analyses were performed in the Pasteur Research Centre in Hong Kong. Results: A total of 2340 pigs were sampled in farms and 678 in slaughterhouse, with 22.3% positive by cELISA in farms, and 37.2% in slaughterhouse; 1554 poultry (ducks and chickens) were sampled in farms, with 30.4% positive by cELISA. In Hong Kong, 710 pigs and 37 poultry swabs were analyzed by isolation in cell culture and quantitative reverse transcription polymerase chain reaction (qRT-PCR), and 677 pigs and 472 poultry cELISA positive sera by hemagglutination inhibition (HI) tests and H5 pseudoparticle-based seroneutralization assay (H5pp). No virus was isolated nor detected by qRT-PCR in swabs from pigs and poultry. Among pigs, 13.3% of sera in farms and 17.6% in slaughterhouse were positive for at least one strain tested, 19.1% for poultry sera. According to the considered subsets, 5.5% to 11.3% of sera were positive by cELISA but remained negative by HI test and H5pp assay for the tested strains. Pig sera were tested positive against strains of H1N1 (12.1% in farms, 17.1% in slaughterhouse), H9N2 (1.7% in farms, 1.2% in slaughterhouse), and H5N1 (0.7% in farms, 0.1% in slaughterhouse), HI tests did not reveal evidence of infection by a H3 subtype. For poultry, some sera were positive against H5N1 (11.8%) and H9N2 (9.7%) strains, but no positive serum against H6 strains was detected. Sera in pigs and poultry were found to cross-react between several subtypes and strains within a subtype. Conclusion: Several strains were found in both pigs and poultry sera, suggesting the occurrence of inter-species transmission. The interpretation of analyses can be challenging due to the lack of information on the viral strains circulating in field and to the potential cross-reaction between different strains. Further analyses are carried out in order to identify the possible circulation of other strains and to evaluate the performance of the cELISA.