In late March, the US Department of Agriculture (USDA) announced it had detected cases of bird flu in dairy cattle. Initially discovered in dairy farms in Texas, Kansas, and New Mexico, there are now 36 confirmed outbreaks in dairy herds in nine states.
Although the H5N1 virus circulates widely in wild birds, it is now circulating among dairy cattle in the US. The USDA has confirmed transmission between cows in the same herd, from cows to birds, and between different dairy cattle herds.
But the reported outbreaks are likely to be a major underestimation of the true spread of the virus, says James Wood, head of veterinary medicine at the University of Cambridge. “It’s likely there is going to be a fair amount of underreporting and underdiagnosis,” he says.
Tests by the Food and Drug Administration (FDA) of retail milk samples might give some indication of how widespread the virus is. The agency found viral fragments in one in five samples of commercial milk, although this virus had been deactivated by pasteurization so was not infectious.
So far there is only one confirmed human infection in the outbreak: someone in Texas who had close contact with dairy cattle. Their only reported symptom was conjunctivitis, and the individual was told to isolate themselves and take an antiviral drug for flu. But anecdotal reports of illness on dairy farms hints that infections among humans may be more widespread than official data suggests. Although human infections have tended to be rare, the virus is dangerous—just over half of the human cases recorded by the World Health Organization over the past two decades have been fatal.
Dairy workers are most at risk of possible infection in the current outbreak, but understanding the extent of any infections is extremely tricky, says James Lawler, professor of infectious diseases at University of Nebraska Medical Center. More than half of workers in the US dairy industry are immigrants, and many of them are undocumented.
These undocumented workers are unlikely to want to put themselves at risk by coming for testing, Lawler says. “There’s an inherent disincentive that many of the workers, because of their status as undocumented immigrants, are not raising their hands.” The result, Lawler says, is that it’s difficult for scientists to track any possible spread of the virus through humans.
Another issue is incentivizing owners of dairy farms to report when their animals seem sick. The USDA Animal and Plant Health Inspection Service specifically provides payments for poultry farmers who have to kill their livestock due to bird flu infections. Dairy farmers don’t get compensated for reporting infections, which incentivizes producers to keep quiet, upping the risk that outbreaks get out of hand and spread to other cattle or farm workers.
This presents a major problem for tracking the spread of the disease. “From the perspective of a producer, how is it going to benefit them to share or even test and understand if there’s a virus circulating in their herd?” Lawler says.
On April 24, the USDA ordered that dairy cattle must receive a negative test for the H5N1 virus at an approved laboratory before they can be moved out of state. Owners of cattle herds in which dairy cattle test positive must also provide data about their cows’ movements and other information. Laboratories and state veterinarians also have to report positive H5N1 tests to the USDA.
At the moment, scientists are focusing on raw milk as a risk factor for transmission to humans. While H5N1 infections in poultry usually result in respiratory illnesses, in cattle the virus seems to mainly target mammary glands, which might explain the high amount of virus found in raw milk from infected cows. At least two cats that drank milk from infected cows on a Texas farm later died and their bodies tested positive for the infection.
A surprisingly large number of Americans also drink raw milk. According to one study, 4.4 percent of US adults reported consuming raw milk at least once in the past year, while 1.6 percent reported consuming raw milk once a month or more often. On TikTok, a whole community of raw milk drinkers espouse their enthusiasm for the unpasteurized white stuff.
Neither Lawler nor Wood recommend drinking raw milk. “I for one would not be drinking unpasteurized milk, not that I would anyway, knowing there are many other diseases it can transmit,” says Lawler. Raw milk can carry bacteria such as salmonella, E. coli, listeria, and campylobacter, and was associated with 1,909 illnesses between 1993 and 2012, according to the Centers for Disease Control and Prevention. That number is likely to be a significant underestimate of the true number, as most cases of food poisoning go unreported.
The FDA recommends that people do not consume raw milk, and the sale of raw milk is already illegal in 24 states. In some states raw milk can be sold only on farms, while others permit sales in retailers. Tests conducted by the USDA indicate that pasteurization—a process of heating milk to kill contaminants—deactivates any infectious virus within the milk.
On May 1, the USDA reported that it had not found H5N1 virus in samples of raw ground beef taken from retail shelves in states where dairy cattle had tested positive for the virus. “These results reaffirm that the meat supply is safe,” the USDA said in a statement.
