Cats suffer H5N1 brain infections, blindness, death after drinking raw milk

On March 16, cows on a Texas dairy farm began showing symptoms of a mysterious illness now known as H5N1 bird flu. Their symptoms were indescribable, but their milk supply dropped significantly and became thick and creamy yellow. The next day, farm cats that had consumed some of the raw milk from sick cows also became ill. While the cows largely recovered, the cats were not so lucky. They developed depressed mental states, stiff body movements, loss of coordination, circular movements, heavy discharge from the eyes and nose, and blindness. By March 20, more than half of the farm’s approximately 24 cats had died of the flu.

In a study published today in the journal Emerging Infectious Diseases, researchers from Iowa, Texas and Kansas found that cats had the H5N1 virus not only in their lungs, but also in their brains, hearts and their eyes. The results are similar to those observed in cats experimentally infected with H5N1, also known as the highly pathogenic avian influenza virus (HPAI). But, on the Texas dairy farm, they present a worrying warning about the potential for transmission of this dangerous and evolving virus.

Contaminated milk was the most likely source of fatal infections in cats, the study authors concluded. Although it cannot be completely ruled out that the cats became ill from eating infected wild birds, the milk they drank from the sick cows was full of virus particles and genetic data shows almost exact matches between the cows, their milk and the cats. “Therefore, our results suggest cross-species mammal-to-mammal transmission of the HPAI H5N1 virus and raise new concerns regarding the potential for spread of the virus within mammalian populations,” wrote the authors, veterinary researchers at the Iowa, Texas and Kansas.

Early outbreak data from the Texas farm suggests the virus is increasingly able to spread to mammals, and data from elsewhere shows the virus is spreading widely in its new host. On March 25, the U.S. Department of Agriculture confirmed the presence of H5N1 in a dairy herd in Texas, marking the first time H5N1 was transmitted to cows. Since then, the USDA has documented infections in at least 34 herds across nine states: Texas, Kansas, Michigan, New Mexico, Idaho, Ohio, South Dakota, North Carolina and Colorado.

The Food and Drug Administration, meanwhile, has detected genetic traces of H5N1 in about 20 percent of commercial milk samples. Although commercial milk is still considered safe (pasteurization is expected to destroy the virus and initial testing by the FDA and other federal scientists confirms this expectation), the findings suggest an even wider spread of the virus among dairy cows from the country.

Cows are just the latest addition to H5N1’s surprisingly broad host range. Amid a global outbreak in recent years that has devastated wild bird populations and poultry farms, researchers have documented unexpected and often fatal outbreaks in mammals. Since 2022, the USDA has detected H5N1 in more than 200 mammals, from zoo felines to harbor seals, mountain lions, raccoons, skunks, squirrels, polar bears, black bears, foxes and bottlenose dolphins.

“The recurrent nature of global HPAI H5N1 virus outbreaks and the detection of spillover events in a wide host range are concerning and suggest increasing adaptation of the virus in mammals,” the authors wrote. “Monitoring HPAI viruses in domestic livestock animals, including cattle, is necessary to elucidate the evolution and ecology of influenza virus and prevent cross-species transmission.”

In the meantime, now is definitely not the time to start drinking raw cow’s milk. Although drinking raw milk is still dangerous because it carries the threat of various nasty bacterial infections, H5N1 also appears to be infectious in raw milk. And unlike other influenza viruses, H5N1 has the potential to infect organs other than the lungs and respiratory tract, as seen in cats. The authors of the new study note that a 2019 consumer survey found that 4.4 percent of adults in the United States consumed raw milk more than once in the previous year, which suggests that greater public awareness of the dangers of raw milk is needed.