Raw milk infected with the highly pathogenic bird flu may pose a risk of infecting humans, according to a new study funded by the National Institutes of Health.

The findings come as 52 cattle herds across nine states have been infected with the bird flu strain H5N1 and two dairy workers have tested positive for the illness.

Researchers at the University of Wisconsin-Madison and the Texas A&M Veterinary Medical Diagnostics laboratory tested raw milk to determine whether live virus was present and, if so, whether it could be transmitted to mice.

The scientists fed raw, or unpasteurized, milk from infected cows to five mice, all of whom demonstrated signs of illness on the first day. Although no mice died as a result of the infection, researchers discovered high levels of the virus in the nasal passages and lungs of the mice as well as moderate-to-low levels in other organs.

The researchers also tested the heating point of raw milk that would kill off the virus, proving that the theory behind the boiling process of pasteurization should kill the virus in the milk. In a separate experiment, refrigerating the raw milk did not result in any significant decline in virus particles.

The study authors noted, however, that these experiments were not necessarily indicative of the treatment and processing in the commercial milk supply.

“We must emphasize that the conditions used in our laboratory study are not identical to the large-scale industrial treatment of raw milk,” Yoshihiro Kawaoka, professor at the UW-Madison Department of Pathobiological Sciences, said in a press release, adding that the experiments “may not perfectly translate to real-world conditions.”

The U.S. Department of Agriculture and the Food and Drug Administration maintain that the commercial milk supply is safe. On Monday, the FDA published findings that none of the tested pasteurized milk samples from commercial shelves tested positive for live virus.

Public health agencies, however, have consistently warned that raw milk is dangerous not only for possible bird flu infections but also because of bacterial diseases, such as E. coli and salmonella.

“Do not drink raw milk,” Kawaoka said.

Raw milk purchases have risen consistently since the onset of the COVID-19 pandemic, but they have soared in recent weeks since the announcement of the bird flu outbreak among cattle in late March. European studies on raw milk have found that the product has immune system-boosting properties.

Raw milk producers contend that it is possible to prevent bacterial infections from raw milk if the farm is kept clean, the health of the cows is monitored, and the raw milk product is thoroughly tested.

Scientists and public health experts, however, are concerned about any possible mechanism for the bird flu to spill over into human infections.

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Earlier this month, the Biden administration announced a joint effort from USDA and the Department of Health and Human Services to spend $101 million to combat the spread of bird flu among cattle and mitigate the potential for human infections.

Part of this spending included an $8 million package from the FDA to monitor the commercial milk supply and better train staff on biosecurity protocols for contaminated milk.