Science Unexpected H5N1 outbreaks in cattle raise difficult questions about how to protect herds and people.
Texas officials today issued a “health alert” about the first confirmed case of a human infection with a bird influenza virus that has found its ways into dairy cows. The worker developed conjunctivitis, a mild eye infection that frequently occurs when avian influenza viruses jump into humans.
The case is the latest surprise in the global march of the flu strain, a subtype of H5N1 known as clade 2.3.4.4b that has devastated wild birds and poultry around the world for more than 2 years. The U.S. Department of Agriculture (USDA) says it has confirmed the virus has infected cattle at farms in Texas, Kansas, New Mexico, and Michigan, while Idaho has a “presumptive” outbreak at one dairy farm. Wild birds, which have been found dead on some farms, most likely contaminated cow feed or water.
Some evidence suggests the virus was transmitted between cows, but that remains unproven. And for now, USDA says its “initial testing has not found changes to the virus that would make it more transmissible to humans.” Still, the widespread occurrence of H5N1 in mammals has renewed worries that it may evolve to become more transmissible between people. And scientists are urgently trying to answer a host of questions, including how far the virus has spread among U.S. cows and how to prevent more herds and people from becoming infected.
Although cows routinely contract influenza viruses, this is the first time that a “highly pathogenic” bird flu strain has been found in them. USDA says about 10% of affected herds have become ill. Sick cows have a mild illness, and produce less milk, which is thicker than usual, resembling colostrum, the first milk produced after a calf is born.
USDA today stressed that the “current risk to the public remains low.” Contamination of commercial milk is of “no concern,” the agency said in a statement, because pasteurization reliably kills viruses, and milk from sick cows is not being sold. The U.S. Centers for Disease Control and Prevention (CDC) says people should not drink raw milk or eat products such as cheese that are made from it. The USDA statement noted that cats on farms have also become infected.
Antibody tests of herds should soon reveal how widespread the infection is and how long it has been infecting cattle. Lab experiments may clarify how a virus that typically causes respiratory disease wound up in cow udders, making it detectable in their milk, and whether other organs are infected. No evidence exists that the virus has infected beef cattle, but researchers say that could simply be because of a lack of surveillance, or because these animals show subtler symptoms than changes in milk production and its appearance.