The U.S. Department of Agriculture said Tuesday that the strain of bird flu currently circulating among dairy cattle appears to be only transmissible via mammary glands, or udders, rather than respiratory droplets, a finding that gives officials greater optimism about managing the risks facing dairy workers.
This finding could have significant implications for the spread of bird flu from dairy cattle to humans, with dairy workers being most at risk. So far, three people in the United States have confirmed cases of bird flu: One dairy worker in Texas and two in Michigan.
“This is principally mediated by milk. This virus has an exquisite fondness for mammary tissue and is expressed in milk,” Eric Deeble, acting senior adviser for USDA’s bird flu response team, told reporters Tuesday.
Public health officials have been vigilant since the spring with the discovery of the highly pathogenic bird flu strain H5N1 in dairy cattle, which has spread to 13 states as of June.
Deeble told reporters that although H5N1 circulates via respiratory droplets among poultry, none of the studies conducted so far demonstrate respiratory spread among cattle. The virus also does not appear to affect other tissues in an infected cow.
“The disease, while it is H5N1, does appear to operate very differently in dairy cattle than it does in poultry,” said Deeble. “We have no evidence of respiratory transmission in any case involving dairy cattle.”
Research published Tuesday in the Centers for Disease Control and Prevention journal Emerging Infectious Disease found that H5N1 lives for several hours on surfaces essential to the milking process, including steel and rubber.
“Our data provide compelling evidence that dairy farm workers are at risk for infection with H5N1 virus from surfaces contaminated during the milking process,” wrote the authors of the study, led by Valerie Le Sage from the University of Pittsburgh Center for Vaccine Research.
Le Sage and her team strongly recommended the use of personal protective equipment for dairy workers during the milking process and sanitizing milking machines in between each cow.
CDC representative Demetre Daskalakis told reporters that there is still little risk of widespread human infections of bird flu, but the risk of spread among dairy workers is higher.
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Daskalakis, head of the National Center for Immunization and Respiratory Diseases, said this elevated risk among dairy workers is why the agency has “really focused a lot of efforts on education as well as a strategy to prevent exposure, including encouraging appropriate use of personal protective equipment.”
So far, public health agencies have not identified mutations in bird flu that would enable it to spread between humans directly.
