US health officials are struggling to monitor people who have been exposed to a bird flu virus circulating in dairy cows. The patchy surveillance means infections in people may go undetected, making it difficult to mitigate the virus’s spread. This raises the risk of the pathogen developing mutations that could make it more dangerous to people.
The virus, a strain of avian influenza known as H5N1, has killed millions of birds and thousands of mammals worldwide. It has been spreading in dairy cows across the US for almost five months, affecting more than .
On 15 July, the (CDPHE) announced that five workers on a poultry farm in the state had tested positive for the virus. Earlier this year, four people who had close contact with sick cows tested positive for H5N1, and in 2022 a US poultry worker contracted the virus. Everyone infected has only experienced mild symptoms, according to the (CDC) and the CDPHE. However, H5N1 can cause severe illness in humans. Out of the in people worldwide, about half were fatal.
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Fortunately, the virus hasn’t been found to spread between people, though that could change if it develops mutations that increase its transmissibility in mammals. This is why it is crucial to monitor for signs of illness in people exposed to H5N1, says at Johns Hopkins University in Maryland.
A spokesperson for the CDC says that as of 12 July, at least 1390 people have been monitored for the virus related to the cattle outbreak, and at least 61 of them have been tested for it. The organisation recommends that state and local health departments survey people who have close contact with sick cows for symptoms like eye redness, cough and fatigue .
But surveillance is a voluntary process and requires collaboration between the CDC, state and local health departments, farmers and workers. In some states, distrust of government has prevented officials from monitoring people on farms with sick cows, says at the National Milk Producers Federation (NMPF), a non-profit organisation in Virginia advocating for dairy farmers. Some farms may bar government agencies from checking on the health of their human employees or fail to alert those agencies when a worker falls ill.
This distrust may stem, in part, from the farms’ locations, he says. Most are in of the country, which have historically been underserved by government health agencies.
The NMPF, which has relationships with both the CDC and dairy farmers, is trying to bridge some of these issues, says Jonker. In the meantime, H5N1 surveillance continues to vary across states, with some workers on affected farms not being monitored at all, says Davis. As a result, cases in humans are most likely going undetected, she says.
One way to estimate how many people have contracted H5N1 from infected cows is to test workers for antibodies against the virus, says Davis. The CDC says it is working with certain groups, like the Michigan Department of Health and Human Services, to conduct these studies.
But much more surveillance is needed to truly understand how this outbreak in cattle affects public health, says Davis. This includes ramping up testing in cows, monitoring more workers and surveying people who drink raw milk, which contains high levels of the virus, she says.
