Multiyear Serological Surveillance of Notifiable Influenza A Viruses in Belgian Poultry: A Retrospective Analysis.

Surveillance of notifiable avian influenza (NAI) virus is mandatory in European member states, and each year a serological survey is performed to detect H5 and H7 circulation in poultry holdings. In Belgium, this serological monitoring is a combination of a stratified and a risk-based approach and is applied to commercial holdings with more than 200 birds. Moreover, a competitive nucleoprotein (NP) ELISA has been used as first screening method since 2010. A retrospective analysis of the serological monitoring performed from 2007 through 2013 showed sporadic circulation of notifiable low-pathogenicity avian influenza (LPAI) viruses in Belgian holdings with a fluctuating apparent flock seroprevalence according to years and species. Overall, the highest apparent flock seroprevalence was detected for the H5 subtype in domestic Anatidae, with 20%-50% for breeding geese and 4%-9% for fattening ducks. Positive serology against non-H5/H7 viruses was also observed in the same species with the use of the IDScreen influenza A antibody competition ELISA kit (ID-vet NP ELISA), and confirmed by isolation of H2, H3, H6, and H9 LPAI viruses. Among Galliformes, the apparent flock seroprevalence was lower, ranging between 0.3% and 1.3%. Circulation of notifiable LPAI viruses was only observed in laying hens with a similar seroprevalence for H5 and H7. Based on ID-vet NP ELISA results, no circulation of LPAI viruses, regardless the subtype, was observed in breeding chickens and fattening turkeys. Retrospectively, the use of an ELISA as first-line test not only reduced the number of hemagglutination inhibition tests to be performed, but also gave a broader evaluation of the prevalence of LPAI viruses in general, and might help to identify the most at-risk farms.