Just 10 viral particles of the H5N1 bird flu that caused hundreds of influenza outbreaks in U.S. dairy cattle can cause infection in cows, a new study shows.
The research also hints at why the outbreaks have confounded scientists, farmers and livestock handlers hoping to contain and prevent the disease – an effort likely complicated by the fact that the virus has an affinity for cow mammary glands rather than airways.
Tests for transmission among cows through milk machinery or through feeding of calves, and between birds and cattle through shared indoor air, didn’t show results of disease spread.
Researchers are continuing to pursue more answers, but for now the transmission mystery endures, meaning scientists can’t yet provide evidence-based recommendations for practices that would stop the spread.
“How it spreads from cow to cow becomes a very important question. We need to understand if there’s a way to change milking practices or farming practices, whatever it is, to limit cow-to-cow transmission because we think spillover is going to happen again. It’s just a matter of time,” said senior author Andrew Bowman, professor of veterinary preventive medicine at The Ohio State University.
“And right now we don’t have a great way to prevent either that spillover or cow-to-cow transmission once it happens.”
The study was published recently in Nature Communications.
Highly pathogenic H5N1 avian influenza A viruses are associated with wild birds and poultry, but since 2021 a group of these viruses of clade 2.3.4.4b have circulated globally in mammals and were first reported in U.S. dairy cattle in March 2024.
To date, 1,053 outbreaks of the originally detected virus genotype (B3.13) have been confirmed in dairy herds in 17 states, which have been brought under control through a national milk testing strategy that halted movement of herds producing milk in which the virus was detected. Current federal data suggests small numbers of infections in cows are confirmed in Idaho, Utah and Texas.
“This work has all been done in response to the unprecedented spillover of avian influenza into dairy cattle,” Bowman said. “Initially, we had no idea that cows could even be infected with influenza, let alone that the mammary gland was involved. That in and of itself was a major paradigm shift: It’s not respiratory.”
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