Avian influenza A(H5N1), commonly known as bird flu, has been infecting cows across the United States, and scientists have uncovered a concerning new development. A study published in the journal *Emerging Infectious Diseases* has revealed that the virus from infected dairy cows can remain infectious in unpasteurized milk and on the surface of milking equipment for at least an hour. This finding raises concerns about the potential for increased infection risk among dairy workers during the milking process.
While the primary focus of the study wasn’t the transmission of the virus through milk, the research does confirm that live viruses can persist in raw milk. Avian influenza A(H5N1) primarily infects birds, but it has occasionally spread to other animals, including mammals. The virus was first detected in cows in the U.S. in March 2023, and it has since been found in multiple states across the country.
Since the initial detection, three individuals in the United States have contracted the H5N1 virus following exposure to infected dairy cows. Two of these cases involved eye infections, while the third individual experienced mild respiratory symptoms. However, scientists are still working to understand precisely how the disease is being transmitted from cow to cow and from cow to human.
Adding to the complexity of the situation, researchers are still trying to determine how H5N1 initially entered the cow population. “How [H5N1] got into the cows in the first place is a question that is a little bit mind-boggling to scientists,” study lead author Dr. Le Sage, a microbiologist at the University of Pittsburgh, told Live Science.
Previous research had already established that H5N1-infected cows could shed the virus in their milk. In 2023, the Food and Drug Administration (FDA) reported that a sample of retail milk products tested positive for H5N1 genetic material. However, none of the viruses detected in these products were “live” or infectious because the products had undergone pasteurization. Pasteurization is a heat treatment process that effectively kills most viruses and microbes.
The recent study sought to investigate whether the H5N1 virus could remain infectious in unpasteurized milk. To conduct their experiment, Dr. Le Sage and her colleagues collected raw milk samples from cows, added H5N1, and then monitored the samples for an hour. These contaminated milk samples were also pipetted onto stainless steel and rubber materials commonly found in milking equipment.
After an hour, the scientists discovered that the virus showed very little sign of decay on either material. More alarmingly, the viruses appeared infectious when exposed to cells in laboratory dishes. “Because the virus remains stable on these surfaces, if they’re not cleaned up pretty soon after [contamination], the milk is just there with the virus in it,” Dr. Le Sage said. “It does pose a threat to the dairy workers that are in the milking parlors,” she added.
For comparison purposes, the scientists also placed H5N1 viruses in a saline solution. Surprisingly, the pathogens did not survive for an hour in this liquid. “There is something about the milk that is making it [H5N1] more stable, and allowing it to survive for longer periods of time,” Dr. Le Sage said. “But we don’t know what component is causing this.”
Due to laboratory safety concerns, the scientists have not yet tested the virus’s infectiousness over a longer period. Despite this limitation, the study underscores the importance of dairy farmers taking appropriate precautions, including wearing personal protective equipment when handling infected cows. Dr. Le Sage emphasized, “Masks and face shields do their job. If workers were provided these and wore them, they should be protected.”
The Centers for Disease Control and Prevention (CDC) has issued recommendations for personal protective equipment in situations involving potential exposure to H5N1, but the use of such safety equipment remains a matter of ongoing discussion and implementation.
While personal protective equipment can help reduce the risk of contracting the virus, “it’s not 100%,” said Dr. Hanson, the policy director at the nonprofit Center for Food Safety. However, he provided reassurance, stating, “the good news is that if we do a proper job of pasteurizing the milk from these cows, it’s safe.”
In general, the CDC considers the risk of H5N1 infection to the general public as low. However, individuals who work closely with infected dairy animals face a higher risk. There is currently no evidence suggesting that the virus spreads between people.
