U.S. health officials are closely monitoring the recent leap of avian flu from birds to cattle — and earlier this month, to a Texas dairy farmer.
The Centers for Disease Control and Prevention stresses the current public health risk is low — the virus isn’t known to spread among people or through the food supply, is rarely caught from exposure to infected animals, and where human infections have occurred, is often mild and manageable.
But the leap of the H5N1 influenza strain, widespread among wild fowl and a regular threat to domestic poultry, into cattle was unexpected, and a worrisome development, as the more the virus spreads, the more chances it has to mutate. The COVID-19 pandemic was a stark lesson in the risk of “spillover,” when pathogens spread from animals to humans.
People have frequent contact with cattle, creating opportunities for more exposure. Experts suspect that it may be transmitted among animals in a herd, or by contaminated milking equipment.
California, the nation’s leading milk producer, is working out the details of a testing program for all incoming animals to comply with a new federal requirement, according to the California Department of Food and Agriculture.
Worried dairy farmers are taking biosecurity steps to help protect herds, although testing of the state’s animals remains voluntary.
Some farmers are putting tarps on their water supplies to fend off migratory birds and are requiring sanitary precautions such as disinfecting tools and boots for people who travel from ranch to ranch, like hoof trimmers, according to Anja Raudabaugh, CEO of Western United Dairies in Turlock.
But there is no state or federal requirement to routinely sample cows or milk. Clinical signs of flu occur in only a fraction of cases, so some experts fear that the virus may be hiding in untested animals.
Avian flu used to be just a bird problem. But now it’s been detected in 33 dairy cattle herds in eight states. It doesn’t kill cows, but causes a dramatic reduction in milk production.
While California remains off the list of infected states, “it’s a very dynamic situation that could change from day to day,” said Dr. Terry W. Lehenbauer, director of UC Davis School of Veterinary Medicine’s Veterinary Medicine Teaching and Research Center in Tulare.
“Wild birds introduced bird flu to Midwest cattle; it could happen in California too,” according to Dr. Michael Payne, director of the California Dairy Quality Assurance Program.
There are two major clues that the outbreak is bigger and started earlier than experts thought.
One hint was found in the milk supply. This week, the U.S. Food and Drug Administration discovered fragments of the inactivated H5N1 bird flu virus in milk purchased from stores around the country. Because milk is pasteurized, it is safe — but this is evidence that infected cows aren’t being counted.
The other was detected in the genome. A new genetic analysis reveals that the virus jumped from birds into dairy cows mid-December — likely as a single incident. This is earlier than previously thought and suggests it has spread more widely, according to University of Arizona evolutionary biologist Michael Worobey.
There is currently no evidence of human-to-human transmission. If the virus does spread to humans, it can be treated with the medicine Tamiflu, the U.S. Centers for Disease Control and Prevention said at a Wednesday briefing. The CDC said that a testing plan is in place, as well as two potential vaccines.
In birds, this is the worst flu outbreak in U.S. history, according to U.S. Department of Agriculture data. The virus also has been detected in other mammals, such as elephant seals and sea lions in South America, where it is very deadly.
Pigs, which are routinely tested for flu, are still negative. This is reassuring because pigs have both avian and human cell receptors that act as “mixing vessels” for flu viruses.
The new federal order, which requires testing all lactating cows before moving across states, will aid the disease investigation. Labs and veterinarians that find the virus, or antibodies against it, must report it to USDA.
California ships many dairy cattle in and out of state, said Lehenbauer. Most cows coming into California are heifers that do not yet produce milk. Cattle leaving the state tend to be steers headed to feedlots for beef production.
To help prevent infection of the state’s 1.72 million animals, California requires that cattle originating from a state that has infected herds be inspected and receive a clean bill of health. Cattle can’t enter if they’re from a suspicious herd.
“Our ‘ratcheted up’ approach at our border crossings, and quarantine for cattle coming in from affected areas, has really assisted in preventing further spread of that virus,” said Raudabaugh.
And too-restrictive regulations could disrupt the critical supply chain that provides the nation’s milk, Lehenbauer said.
“Given our current situation, I think that the procedures and policies that we have in place are adequate and appropriate. … We don’t want to put undue restrictions on on our animal production systems,” said Lehenbauer. “I’m very confident that our milk and dairy product supply is very safe and wholesome.”
But this approach is drawing criticism from some experts who think the lack of testing of healthy animals is short-sighted.
“This isn’t good, folks,” said Dr. Eric Topol, director of the Scripps Research Translational Institution in San Diego, on Twitter. “I’m not worried about H5N1 transmission to humans (yet) but … the lack of testing of asymptomatic cattle,” among other issues, “detracts from confidence.”