H5N1 influenza A viruses from 2002 are highly pathogenic in waterfowl

Waterfowl are the natural reservoir for all influenza A viruses, and these viruses are usually nonpathogenic in wild aquatic birds. In late 2002, an outbreak of H5N1 influenza virus caused death among waterfowl in two Hong Kong nature parks. Genetic analysis showed that these viruses were novel H5N1 genotypes. In February 2003, a closely related H5N1 was isolated from human patients. This study aims to answer two questions: (1) Are the novel H5N1 antigenically different from the H5N1 viruses of 1997 and 2001? (2) Are they more pathogenic to ducks than previous genotypes? Antigenic analysis of the new isolates showed a different reactivity pattern from previous H5N1 isolates, suggesting antigenic drift has occurred. The new 2002 isolates caused acute disease in mallard ducks, including severe neurological symptoms and death. In contrast, previous H5N1 isolates and a 2003 human isolate did not cause any disease. Since 1961, this is the first time that influenza virus is reported to kill wild aquatic birds. (C) 2004 Elsevier B.V. All rights reserved.