As the bird flu outbreak in dairy cows has ballooned, officials have provided repeated reassurances: The virus typically causes mild illness in cows, they have said, and because it spreads primarily through milk, it can be curbed by taking extra precautions when moving cows and equipment.
A new study, published in Nature on Tuesday, presents a more complex picture.
Some farms have reported a significant spike in cow deaths, according to the paper, which investigated outbreaks on nine farms in four states. The virus, known as H5N1, was also present in more than 20 percent of nasal swabs collected from cows. And it spread widely to other species, infecting cats, raccoons and wild birds, which may have transported the virus to new locations.
“There’s probably multiple pathways of spread and dissemination of this virus,” said Diego Diel, a virologist at Cornell University and an author of the study. “I think it will be really difficult to control it at this point.”
The outbreak, which officials first announced in March, has spread to at least 170 dairy farms in 13 states, according to the Department of Agriculture. It has also jumped into poultry farms and infected at least 10 farmworkers exposed to infected cows or poultry.
The exact origins of the outbreak remain unknown, but scientists believe that the version of H5N1 that is now circulating in dairy herds probably jumped into cows just once, most likely in late 2023 in the Texas Panhandle.
In the new study, the scientists focused on nine farms — five in Texas, two in New Mexico and one each in Kansas and Ohio — that reported outbreaks between Feb. 11 and March 19. When they analyzed samples of the virus taken from affected farms, they found that those samples were closely related to one collected from an infected wild skunk in New Mexico in February.
It’s not clear whether that skunk was connected to any of the affected dairies, but the finding suggests that the virus was already present in local wildlife in the early days of the outbreak.
Although many infected cows did recover on their own, the researchers found, two farms reported a spike in cow deaths. On the Ohio farm, 99 cows died over the course of a three-week outbreak, a mortality rate roughly twice as high as normal.
“I think the potential for this virus to cause very serious disease has been downplayed a bit,” said Richard Webby, an influenza expert at St. Jude Children’s Research Hospital, who was not involved in the new study. “That has probably hurt the response.”
Still, Dr. Diel noted, the cause of these deaths remains unknown. “Whether the mortality observed in those cases was due directly to influenza or whether the influenza infection led to a secondary bacterial infection, I think that’s a question that remains to be answered,” he said.
The findings provided more evidence that milk plays a major role in the spread of the virus, which was present more frequently — and at higher levels — in milk than in nasal swabs, blood or other kinds of samples.
But some cows did have virus in their noses. “I do think that our data shows that the respiratory route cannot be completely ruled out,” Dr. Diel said.
Intriguingly, asymptomatic cows were more likely to have the virus in their noses than obviously sick ones, the researchers found. One possible explanation, Dr. Diel said, is that the infection may start in the upper respiratory tract, where modest levels of virus accumulate, before moving to the mammary glands, which appear to be the primary site of viral replication.
Some infected cows also shed the virus in their urine, the researchers found. Asymptomatic cows, in fact, were more likely to have the virus in their noses or urine than in their milk, the scientists found. That suggests that testing milk samples may not be enough to give officials a full picture of asymptomatic spread, said Carol Cardona, a bird flu expert at the University of Minnesota who was not part of the research team. “Surveillance should be focusing on nasal swabs and urine,” she said.
Genomic sequencing suggested that on some farms infected cows passed the virus on to other species, including cats and raccoons.
The researchers also identified two infected wild blackbirds, which were found five to eight miles away from an affected Texas farm. The birds were carrying a version of the virus that was a close match to what was infecting cows on that farm and on one in Kansas.
That raises the possibility that wild birds might be involved in transporting the virus between farms or introducing it to other wild animals.
“We know these milking parlors are heavily contaminated with virus,” Dr. Webby said. “You can imagine a bird coming in and getting infected and flying away.”
That means that even strict controls on the movement of cows and farm equipment may not be enough to stamp out the outbreak, Dr. Cardona said: “This is not ending with cows.”
Source: nytimes.com
