Public health experts are closely monitoring Colorado, following the unexpected spread of H5N1 bird flu into America’s dairy cattle herds and a potential cluster of human cases there. On July 14, Colorado officials announced that five workers involved in culling 1.8 million chickens at an H5N1-infected egg farm in Weld County had tested positive for the virus. The strain infecting these workers appears closely related to the virus affecting cows in Colorado and at least 12 other states.
On Friday, the Centers for Disease Control and Prevention confirmed a sixth case among the Colorado poultry workers. Nearly 70 individuals involved in the culling operation were tested for H5N1 after showing symptoms, according to a spokesperson for the Colorado Department of Public Health and the Environment.
All six Colorado cases were mild, with symptoms ranging from traditional flu signs like fever and cough to conjunctivitis, which has also been observed in some infected dairy workers during the outbreak. This is the first instance of multiple human cases reported on a single farm in the U.S., raising concerns about whether the virus has mutated or if specific environmental factors facilitated its spread.
A recent study led by flu virologist Yoshihiro Kawaoka of the University of Wisconsin-Madison found evidence suggesting that the virus currently circulating in cows has acquired some ability to bind to receptors in the human upper respiratory tract. However, other labs have produced conflicting data. The primary concern with a large cluster of human cases is the increased potential for human-to-human transmission, especially to immunocompromised individuals.
Mike Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, cautioned that it is too early to assess the risk in Colorado without more data. “If we get 7, or even 70 more cases of conjunctivitis, what does that mean? Could this be a precursor to a respiratory infection, to influenza being transmitted from person to person? No one knows.”
He referenced a situation in Michigan earlier this summer where 54 farmworkers exposed to infected cows exhibited flu-like symptoms and were tested by state public health officials. Only two of those individuals tested positive for H5N1.
A serological study of Michigan farmworkers released by the CDC on Friday provided additional reassurance. None of the blood samples collected from people exposed to infected dairy cows who showed no symptoms contained H5N1 antibodies, indicating they had not been infected.
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