Science New data challenge assumed transmission route and some call for restricting cattle.
The bird flu virus spreading through dairy cattle in the United States may be expanding its reach via milking equipment, the people doing the milking, or both, U.S. Department of Agriculture (USDA) representatives reported today at an international, virtual meeting held to update the situation.
The avian virus may not be spreading directly from cows breathing on cows, as some researchers have speculated, according to USDA scientists who took part in the meeting, organized jointly by the World Organisation for Animal Health and the U.N.’s Food and Agricultural Organization. “We haven’t seen any true indication that the cows are actively shedding virus and exposing it directly to other animals,” said USDA’s Mark Lyons, who directs ruminant health for the agency and presented some of its data. The finding might also point to ways to protect humans. So far one worker at a dairy farm with infected cattle was found to have the virus, but no other human cases have been confirmed.
USDA researchers tested milk, nasal swabs, and blood from cows at affected dairies and only found clear signals of the virus in the milk. “Right now, we don’t have evidence that the virus is actively replicating within the body of the cow other than the udder,” Suelee Robbe Austerman of USDA’s National Veterinary Services Laboratory told the gathering.
The virus might be transmitted from cow to cow in milk droplets on dairy workers’ clothing or gloves, or in the suction cups attached to the udders for milking, Lyons said. (In a 30 March interview with Science, Thijs Kuiken, a leading avian influenza researcher at Erasmus Medical Center in the Netherlands, had suggested the milking machines might be responsible because the components may not always be disinfected between cows.)
The influenza virus causing the outbreak, an H5N1 subtype that is known as clade 2.3.4.4b, has devastated wild birds and poultry around the world for more than 2 years, and researchers at first thought migratory birds were responsible for spreading it to all of the affected dairy farms. But USDA scientists now think the movement of cattle, which are frequently transported from the southern parts of the country to the Midwest and north in the spring, may also have played an important role. And they floated the possibility, without naming specific herds or locations, that all affected cows may trace back to a single farm.