H5N1 avian flu spreads among US dairy cows and workers, warning of virus mutations

US health officials say the human health risk from a strain of bird flu that spreads through dairy cows remains low after a third human case was recorded in Michigan.

The two previous cases in dairy workers in Michigan and Texas reported mild symptoms of conjunctivitis that officials believe were caused by direct contact with milk or other bodily fluids from infected dairy cows.

However, the Centers for Disease Control and Prevention (CDC) said a third person, a dairy worker in Michigan, had developed respiratory symptoms.

“Like the previous two cases, this individual is a dairy farm worker who had contact with infected cows, making this a new example of possible cow-to-human transmission,” the CDC said in a statement.

“This is the first reported human case of H5 in the United States, reporting symptoms more typical of acute respiratory illness associated with influenza virus infection, including A(H5N1) viruses.”

This marks only the fourth case of human infection with this strain of the H5N1 virus, or HPAI (highly pathogenic avian influenza), in U.S. history.

Combined with low transmissibility and mild symptoms, the CDC still considers the overall risk to human health to be low.

“This case does not change CDC's current human health risk assessment of avian influenza A(H5N1) to the U.S. general public because all three sporadic cases involved direct contact with infected cattle,” the CDC said.

“Risk depends on exposure, and in this case the relevant exposure is to infected animals,” the report said.

So far, avian flu has been detected on 68 dairy farms in nine states in the United States.

Matt Koci, professor of poultry science at North Carolina State University, told ABC it was important to contain the spread of the virus before it mutates and becomes more contagious.

“At least in cattle, this strain is not fatal; they seem to recover after a few days to a few weeks,” Prof Koc said.

“So far there are no indications that the virus is mutating or undergoing changes that would pose a greater risk to humans.”

Prof Koç said the poultry industry, which has been dealing with avian influenza outbreaks since 2022, has adapted to operating under stricter biosecurity controls, but the dairy industry is still catching up.

“This is something dairy workers, like poultry workers, have never had to deal with before,” he said.

Infected milk

High levels of the virus have been detected in raw milk samples taken from infected cows, but recent testing by the Food and Drug Administration (FDA) has not found live virus in retail products.

Health officials and experts say pasteurized dairy products pose no risk to humans.

“Pasteurization is done at temperatures and for times far beyond the limits at which the virus can survive,” Prof Koc said.

Genomic analysis of the virus in cattle revealed that the highly contagious strain was circulating long before the first case was detected in Texas on March 23.

Scientists from the U.S. Department of Agriculture and the National Veterinary Services Laboratory tracked the spillover from birds to dairy cows through early December 2023.

The analysis, published on the bioscience server bioRxiv, also shows that all cases of infection in U.S. dairy cows stem from a single infection event.

Symptoms of sick cows include reduced milk production, loss of appetite, fever, lethargy and thick milk that resembles colostrum.

Global reach

According to Nigel French, professor of infectious diseases, epidemiology and public health at Massey University, bird flu has been spreading around the world for several years.

“H5N1 has been circulating for some time, but this new variant, the 2.3.4.4b strain, which is spreading around the world and causing major problems for wild birds, appears to have originated in Eurasia and spread to every other continent except Oceania,” he said.

The virus has also been found in South American sea lions, Arctic walruses and European mink.

The migratory birds that spread the virus around the world do not pass through Australia, but the virus continues to spread to birds that do pass through Australia.

A different strain of bird flu, known as H7N3, was detected on two poultry farms in Victoria this week.Hundreds of thousands of birds have been culled to curb the spread of the virus.

In 2020, a similar strain caused an avian flu outbreak at a free-range chicken farm in Lethbridge, Victoria, which led to the culling of more than 400,000 birds.