Risk Factors for Exposure to Influenza A Viruses, Including Subtype H 5 Viruses, in T hai Free‐Grazing Ducks

Summary Free‐grazing ducks ( FGD ) have been associated with highly pathogenic avian influenza ( HPAI ) H 5 N 1 outbreaks and may be a viral reservoir. In J uly– A ugust 2010, we assessed influenza exposure of T hai FGD and risk factors thereof. Serum from 6254 ducks was analysed with enzyme‐linked immunosorbent assay ( ELISA ) to detect antibodies to influenza A nucleoprotein ( NP ), and haemagglutinin H 5 protein. Eighty‐five per cent (5305 ducks) were seropositive for influenza A. Of the NP ‐seropositive sera tested with H5 assays ( n  =   1423), 553 (39%) were H 5 ELISA positive and 57 (4%) suspect. Twelve per cent (74 of 610) of H 5 ELISA ‐positive/suspect ducks had H 5 titres ≥ 1 : 20 by haemagglutination inhibition. Risk factors for influenza A seropositivity include older age, poultry contact, flock visitors and older purchase age. Study flocks had H 5 virus exposure as recently as M arch 2010, but no HPAI H 5 N 1 outbreaks have been identified in T hailand since 2008, highlighting a need for rigorous FGD surveillance.