Wild aquatic birds, including ducks, are the natural reservoir of influenza type A viruses and play an important role in the viruses’ ecology and propagation. Influenza A viruses can occasionally be transmitted to other avian and mammalian hosts, including humans, and can cause outbreaks of severe disease. Influenza viruses in wild aquatic birds have long been in a state of evolutionary equilibrium (stasis), and infected hosts usually show no signs of disease. Most avian influenza viruses replicate preferentially in the gastrointestinal tract of wild ducks, are excreted at high levels in feces, and are transmitted through the fecal-oral route .
However, since late 2002, H5N1 outbreaks in Asia have resulted in mortality among waterfowl in recreational parks, domestic flocks, and wild migratory birds, which contradicts the previous belief that influenza A viruses are usually nonpathogenic to the Wild waterfowl. The evolutionary stasis between influenza virus and its natural host may have been disrupted, prompting us to ask whether waterfowl are resistant to H5N1 influenza virus disease and whether they can still act as a reservoir for these viruses. To better understand the biology of H5N1 viruses in ducks and attempt to answer this question, researchers inoculated juvenile mallards with 23 different H5N1 influenza viruses isolated in Asia between 2003 and 2004. All virus isolates replicated efficiently in inoculated ducks, and 22 were transmitted to susceptible contacts. Viruses replicated to higher levels in the trachea than in the cloaca of both inoculated and contact birds, suggesting that the digestive tract is not the main site of H5N1 influenza virus replication in ducks and that the fecal-oral route may no longer be the main transmission path. The virus isolates’ pathogenicities varied from completely nonpathogenic to highly lethal and were positively correlated with tracheal virus titers. Nevertheless, the eight virus isolates that were nonpathogenic in ducks replicated and transmitted efficiently to naïve contacts, suggesting that highly pathogenic H5N1 viruses causing minimal signs of disease in ducks can propagate silently and efficiently among domestic and wild ducks in Asia and that they represent a serious threat to human and veterinary public health.
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Melissa Lawrence, Social Marketing Administrator, VERTICES, LLC