After the H5N1 avian flu virus jumped from birds to dairy cattle in Texas last December, it was spread across the country by infected cows, contaminated machinery, and people who inadvertently carried the virus on their clothes and footwear from farm to farm, said USDA scientists on Thursday. Officials said the risk to the public was low because the virus has not shown signs of adapting to humans. “We should be, as we are, alert, not alarmed,” said Nirav Shah, the CDC’s principal deputy director.
To date, three workers on dairy farms have contracted mild cases of bird flu from infected cows, and the virus has been confirmed in 96 herds in 12 states, from Wyoming to North Carolina. Michigan has the most infected herds, 25, and Idaho has 23. Bird flu has also killed nearly 97 million birds, mostly chickens and turkeys, in domestic flocks since U.S. outbreaks began in February 2022.
“We’re really wanting [producers] to up their biosecurity” practices, said Mark Lyons, director of the USDA’s Ruminant Health Center, during a teleconference with CDC and FDA officials. “Ultimately, that is the thing that gets us through.” USDA veterinary epidemiologist Kammy Johnson said, “Enhanced biosecurity is really crucial.”
Two USDA reports pointed to multiple pathways for spread of the disease among cattle, people, and equipment. Cows from infected herds were moved between farms after symptoms appeared. Farms shared equipment, including cattle trailers. Veterinarians, milk haulers, feed trucks, and contractors stopped frequently at dairy farms. A portion of dairy farm employees work at more than one farm, and they often have family members who work at other livestock farms. One of the reports was based on questionnaires given to farmers with infected herds. The other described a USDA investigation of the spread of H5N1 on Michigan dairy and poultry farms.
Genomic sequencing indicated that migratory waterfowl, often blamed for carrying the virus, were not a factor in its spread among dairy cattle.
“Phylogenic analysis and epidemiology support a single introduction into this novel host followed by onward transmission,” said one of the reports. The other said, “Transmission between farms is likely due to indirect epidemiological links related to normal business operations such as numerous people, vehicles, and other conveyances moving on and off the affected dairy premises, with many of these indirect links shared between premises.”
The appearance of the avian flu virus in cattle raised the possibility that it would mutate into “something much different,” said Shah, so it was important to quell the virus at its source. Samples from the most recent case of bird flu among farmworkers showed the virus remained avian, he said. “We did not see any sign … it was more able to transmit” to people.
Eleven of the 94 farms with infected herds have applied for USDA financial assistance, said Lyons. Up to $28,000 is available per farm over three months. Lyons said 20 states were considering enrollment in a voluntary USDA program to test milk for the H5N1 virus.
“Mortality and culling is 2 percent or less on average” among cows with bird flu, and fewer than 10 percent of cows in infected herds become ill, said the USDA, based on responses to the farmer questionnaires. Older cows are more likely to become ill, with fever, reduced appetite, and a decline in milk production as the most common symptoms. The animals recover after a couple of weeks, officials say, although there have been reports of symptoms lasting longer.
The USDA reports are available here.