An HSUS Report: Human Health Implications of U.S. Live Bird Markets in the Spread of Avian Influenza

After millions of years of existing as a harmless infection of aquatic birds, some strains of avian flu have developed the ability to mutate into highly pathogenic forms that may not only be deadly for birds, but potentially more dangerous for humans as well. Experts from the World Health Organization, the World Organization for Animal Health, and the Food and Agriculture Organization of the United Nations have joined leading scientists in implicating industrialized poultry production for providing fertile ground for the transformation of benign bird flu into highly pathogenic strains. Live bird markets can be the conduit by which waterfowl viruses enter into industrial poultry populations and have been identified as playing a critical role in the emergence of H5N1, a deadly strain of avian influenza threatening to trigger a human flu pandemic. In response, Asian countries are shutting them down. In the United States, live bird markets have been tied to domestic outbreaks of highly pathogenic avian influenza and have been described by U.S. Department of Agriculture poultry researchers as the “missing link in the epidemiology of avian influenza,”1 yet hundreds of these retail storefront slaughter markets still operate across the country, processing tens of millions of birds annually. Background on Avian Influenza The primordial source of all influenza viruses—avian and mammalian—is aquatic birds.2 In nature, the influenza virus has existed for millions of years as a harmless, intestinal, waterborne infection of waterfowl, particularly ducks,3 who are infected as ducklings. Studies of ducks on Canadian lakes show that up to 30 percent of juvenile birds are actively shedding the virus.4 The birds excrete such massive titers of virus5 that researchers have been able to culture it straight out of a spoonful of lake water.6 Under the right conditions, the virus is estimated to be able to persist for years in cold water.7 With such high concentrations of virus and its highly efficient transmission8 and environmental stability, scientists estimate that virtually all of the millions of ducks in the world become infected sometime during their lives.9 Avian influenza multiplies in the ducks’ intestinal lining and is then excreted into the water of ponds, lakes, and other reservoirs. Ducks touching down to drink ingest the virus, and the cycle continues as it has for perhaps 100 million years before the first person became infected with the flu.10 The ducks are not infected for long;11 most only shed virus for a few days. However, the fecal-oral route of infection for aquatic birds is so efficient that this narrow period of contagion is thought to be enough to keep the virus spreading throughout the millennia.12 Millions of years of evolution have so tailored the parasite to its host, a so-called “optimally adapted system,” that the virus seems completely innocuous to the ducks.13 The virus exists in an “evolutionary stasis” in waterfowl,14 remaining unchanged despite its rapid mutation rate.15 Influenza viruses do not tend to retain any new mutations in ducks because they found their perfect niche. Sharks, for example, have remained basically unchanged for 100 million years, even while other species evolved around them, presumably because sharks had already evolved to be such perfected predators.16 Similarly,