Bird flu viruses may more frequently than previously believed infect mammary glands.

The identification of bird flu in dairy cow milk brought attention to the H5N1 virus’s hitherto unnoticed target—mammary glands. According to a recent study, it’s not just a cow problem.

Virologist Amie Eisfeld of the University of Wisconsin–Madison and colleagues report July 8 in Nature that an H5N1 virus isolated from an infected cow spread to the mammary glands of mice and some ferrets, popular stand-ins to study flu infections in humans, exposed to the virus directly in their noses. The mammary glands of mice and ferrets were infected with a bird flu virus that was recovered from an infected human in 2004. However, more research indicates that the virus isn’t particularly good at traveling via the atmosphere.

It is previously known that these primarily respiratory viruses can infect a wide range of other body tissues, including the brain (SN: 5/31/24). Prior research has suggested that the virus might infiltrate breast tissue. Cow mammary glands could become infected with a distinct strain of avian flu, according to a long-forgotten 1953 study. A different study discovered that ferret tissue might become infected with the 2009 pandemic virus.

The H5N1 virus that is presently circulating in cows in the United States also travels to the mammary glands, according to a recent study, indicating that the virus may be more frequently targeting the tissue specific to mammals than previously believed.

More than 135 dairy herds in 12 states have been impacted by the current H5N1 outbreak among American cows. While some infected calves show no signs at all, others may experience fatigue or fever, as well as decreased appetite and milk output.

Cow milk has been shown to contain the virus (SN: 4/25/24). Researchers reveal in the July Emerging Infectious Diseases that the bird flu virus can enter cows’ mammary cells by taking advantage of a ducklike protein covering their surface. These infections could be the reason behind the virus’s proliferation among cattle. Separate researchers found that the virus might spread from cow to cow through infected milking equipment, as published in the August Emerging Infectious Diseases.

There have also been cases of bird flu in the respiratory systems of cows. However, virologist Richard Webby of St. Jude Children’s Research Hospital in Memphis, Tennessee, notes that even while there is a lot of virus in that area of the body, there doesn’t appear to be much respiratory transmission as of now. Apparently, “unless you go directly to the udder, cows aren’t a really good host for this virus.”

In the latest investigation, Eisfeld and associates exposed mice and ferrets to a New Mexico cow’s H5N1 variation to see if the virus produced symptoms akin to those in cows and to get more insight into the virus’s mode of transmission.

The virus traveled throughout the body of mice and ferrets, affecting organs such as the kidney, heart, intestines, brain, and lungs. Additionally, mice and certain ferrets’ mammary glands were infected by the virus.

The study discovered that whereas infected female mice could pass the virus to pups consuming milk, direct contact did not result in any transmission. The fact that only one of the four ferrets in a nearby cage that were exposed to diseased animals also shown symptoms of infection suggests that the virus that is circulating among cows is still not very effective at transmitting through the air.

What does this mean for people, then? Overall risk is still minimal, according to public health professionals. However, compared to the general public, farm workers who have close contact with animals are more likely to contract bird flu from cows. Four Americans have so far experienced mild infections as a result of handling sick animals. It’s recommended that everyone who eats dairy stay away from raw milk. However, milk found in grocery stores is still safe to drink: on June 28, the U.S. According to the Food and Drug Administration, all H5N1 viruses in milk are successfully killed by a commonly used pasteurization method.

However, one area of study that experts are closely monitoring is if the virus is changing in a way that would make it more likely to spread. Cows may be mixing vessels for the exchange of genes between human and bird viruses since their cells have entry points for both bird and human flus (SN: 5/14/24). This may result in novel influenza strains that could more effectively infect humans.