Bird flu is believed to have circulated among U.S. dairy cows on a limited scale for approximately four months before being officially confirmed by federal authorities, according to a recent research paper funded by the U.S. Department of Agriculture (USDA), the Centers for Disease Control and Prevention (CDC), and the National Institute of Allergy and Infectious Diseases.
The first case of H5N1 virus infection in a dairy cow was reported in Texas on March 25, following reports of reduced milk production in several states. The USDA has attributed the introduction of the virus to wild birds, which can carry it, with the outbreak subsequently spreading as infected cows were transported to other states.
The research, led by University of Arizona biologist Michael Worobey, analyzed genetic sequences provided by the USDA and concluded that the virus was likely introduced to cattle from a wild bird source and then circulated locally for about four months before being detected.
Despite criticism for not disclosing more detailed data, the USDA has confirmed only one human case related to the current outbreak, with symptoms limited to conjunctivitis believed to be from contact with infected cow milk. The CDC has stated that the risk of infection to the general public is low.
The virus's ability to spread during this undetected period has raised concerns among experts, with some noting that by the time the outbreak was recognized, it had already surpassed containment efforts.
Observations of reduced milk production and changes in milk quality, along with decreased feed consumption in dairy cattle, began in January. These observations, along with the detection of influenza A virus (including bird flu) in milk and nasal swabs from cows at a Texas dairy, prompted further investigation and testing by USDA laboratories.
Overall, researchers view the sharing of this data as a positive step in understanding and managing the outbreak.