A cross-sectional serological survey of the Dutch commercial poultry population for the presence of low pathogenic avian influenza virus infections.

After the discovery of poultry infected with highly pathogenic avian influenza (HPAI) virus of subtype H7N7 in the central area of The Netherlands on 28 February 2003, the hypothesis was put forward that an outbreak of the low pathogenic (LP) variant of H7N7 had preceded, unnoticed, the occurrence of the HPAI virus. Consequently, a cross-sectional serological survey of the Dutch poultry population was executed in the second week of March 2003. The basic requirements set were detection of a 5% prevalence of flocks exposed to LPAI virus with 95% confidence within the production type stratification level within each province in The Netherlands. Because of supposed higher risk of avian influenza infections in ducks, turkeys and free-range poultry, all the commercial flocks of these production types present in The Netherlands were sampled. The serological screening of 28018 sera from 1193 randomly selected poultry farms, located outside surveillance zones showed that LPAI H7 virus infections had occurred on three neighbouring farms all located in the southwest of The Netherlands. No antibodies against the neuraminidase N7 subtype were detected in the sera of these farms, indicating that the subtype was different from the HPAI H7N7 subtype that caused the avian influenza epidemic in 2003. In addition, evidence of infections with non-H5 or non-H7 subtypes of influenza A virus were obtained in two other farms located in the northeast and the southeast of The Netherlands. It was concluded that the HPAI subtype H7N7 outbreak was most likely not preceded by a significant circulation of a LPAI subtype H7N7 virus. Based on the Dutch experience, recommendations are made to detect avian influenza infections faster in the future.