Three highly pathogenic avian flu viruses that have infected poultry and wild birds in the U.S. Midwest appear unlikely to present a significant risk to humans. But the presence of the viruses in North America has scientists scrambling to understand their potential long-term threat.
Iowa's agriculture department said on Thursday that the avian flu has turned up in initial tests at five more farms in Iowa, including a commercial egg operation housing up to 5.5 million birds.
If the virus is confirmed at the farms in additional tests, the total number of American cases could surpass 20 million birds and result in the biggest death toll in a bird flu outbreak in U.S. history.