Explainer-What to know about bird flu in dairy cows and the risk to humans

By Julie Steenhuysen

CHICAGO (Reuters) – Texas officials reported on Monday that a farm worker tested positive for H5N1, or bird flu, that has spread to dairy cows in Texas, Kansas, New Mexico, Michigan and Idaho – the first time the virus has infected cattle.

Health officials and scientists say the risk to humans remains low, but many questions remain.

IS BIRD FLU A CONCERN?

The Texas case is only the second time bird flu has been confirmed in a human in the United States, with the first occurring in 2022 in Colorado in a person exposed to infected poultry. In both cases, the infections were mild.

The Texas farm worker’s only symptom was conjunctivitis, or pink eye. He is being treated with Tamiflu, which is used to treat human influenza.

The infections are from the same subtype of bird flu that has been infecting wild birds and poultry flocks globally for more than two years, also killing several mammal species that likely contracted the virus from consuming sick or dead birds.

WHAT ARE THE RISKS OF BIRD FLU?

Some outbreaks of avian influenza have caused serious or fatal infections among people who have close contact with wild birds or poultry. Currently, H5N1 is not capable of spreading easily among humans, but scientists have been on alert for changes that could facilitate human spread and spark a pandemic.

Dr. Thomas Inglesby, director of the Johns Hopkins Center for Health Security, said the Texas case “doesn’t change the overall risk of a major pandemic,” but stressed that any new case should be investigated to ensure it is not spreading from person to person.

An official with the U.S. Centers for Disease Control and Prevention (CDC) said for most people who are not exposed to infected animals, the risk is very low.

The CDC has analyzed the genetic sequences of the virus in the infected cattle and the dairy worker and determined that they lack changes that would make it better adapted to be transmitted among mammals.

DOES BIRD FLU AFFECT THE FOOD SUPPLY?

The risk of contaminated milk reaching consumers is of “no concern” according to the U.S. Department of Agriculture because pasteurization kills viruses and milk from sick cows is not being sold.

The CDC renewed its warning that people should not consume raw milk or cheese, which can contain a number of pathogens.

At this point, no beef cattle are known to have been infected with the virus.

IS THERE A BIRD FLU VACCINE FOR HUMANS?

The U.S. has a stockpile of bird flu vaccines matched with the strain currently circulating, as well as antivirals that could be used to treat human infections, Inglesby said. If there were a major epidemic or a pandemic, the U.S. would have to scale up “in a huge way,” he said.

The CDC has sample or “seed” strains of virus that manufacturers could use to make more vaccine that closely matches the circulating virus, an official said.

Flu vaccine suppliers Sanofi, GlaxoSmithKline and CSL Seqirus said in statements they are monitoring avian flu and stand ready to develop avian flu vaccines as needed.

HOW WERE THE COWS INFECTED WITH H5N1?

Richard Webby, a virologist at St. Jude’s Children’s Research Hospital in Memphis, Tennessee, said it is still not clear how the cows became infected, how many have been infected or how the virus has spread to other herds, but said there’s “reasonable evidence that there has been some cow-to-cow spread.”

(Reporting by Julie Steenhuysen; Editing by Caroline Humer and Bill Berkrot)

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