Four new cases of avian influenza were detected in farmworkers in Washington state this week, the latest in a drumbeat of human infections cropping up across the U.S. as the virus continues to spread among farm animals. 

Washington is the sixth state to report human cases of bird flu, which has spread rampantly in wild birds, poultry and cattle. Assuming the Centers for Disease Control and Prevention confirms the state’s positive tests, they will bring the U.S. tally to at least 31.

Flu experts said the risk of a broader outbreak among humans will continue to simmer if the virus remains uncontrolled in animals.

“The longer this virus hangs out in the environment, the more animals it spills over into, the more it changes in ways we don’t understand or predict, the more concerned we are going to be that this becomes the next global pandemic,” said Dr. Amber Itle, the Washington state veterinarian.

For now, however, health officials maintain that the virus has not made key genetic changes that would allow it to spread between people. 

“There is no evidence of any sustained human transmission. And that is a key milestone for all of us. It’s a key trigger for all of us. That’s when we really start to say, ‘Oh, something that’s markedly different about this virus,’” Dr. Umair Shah, Washington’s health secretary, said at a news conference Sunday. 

A CDC spokesperson said Tuesday morning that a federal team was being deployed to Washington and that risk to the public remained low. 

However, the virus is a growing concern for farmworkers on the front lines. The four latest cases were among workers who had culled birds at a commercial egg farm where an outbreak of avian influenza was reported in chickens.

Shah said the workers were part of a contract crew tasked with depopulating — or euthanizing — all chickens at the farm in Franklin County, in southeastern Washington, which had more than 800,000 birds. Itle said the workers wore full protective equipment: suits made of Tyvek, goggles and respirators. 

“It’s hot, it’s dusty. It’s a lot of birds in close proximity, and we’re getting a lot of viral exposure,” she said.

The four people who tested positive after that work reported mild respiratory symptoms and conjunctivitis, or pink eye — symptoms now understood to be common when people are infected. None of the workers were hospitalized. 

Dr. Richard Webby, director of the World Health Organization Collaborating Center for Studies on the Ecology of Influenza in Animals and Birds, said the Washington cases represented “more of the same” in a pattern observed across the U.S. as workers tasked with culling sick animals have fallen ill. 

However, one human case — in Missouri — has puzzled researchers and health officials, because that person did not have contact with animals. It is not clear how the person was exposed, and results of further tests that could offer clues are still pending, according to the CDC. Six health workers exposed to the patient reported respiratory symptoms.

Webby said the virus’ spread in cows for the past 10 months has raised experts’ level of concern because cows are mammals and interact with humans frequently. That increases the likelihood the virus could mutate to pass from person to person. 

“Just because it hasn’t happened in 10 months … it doesn’t mean it can’t,” said Webby, an infectious disease researcher at St. Jude Children’s Research Hospital in Memphis, Tennessee. 

Research suggests that incredible amounts of virus are shed in infected cows’ mammary glands, so the virus could spread through raw milk. (Milk sold in grocery stores is pasteurized, which inactivates the virus.) 

At the same time, the U.S. is preparing for flu season, when circulation of common influenza viruses increases in fall and winter. If two influenza viruses share the same host, they can share genetic information and mutate more quickly.

“You don’t want people affected with the seasonal flu virus to get a virus from animals and for those viruses to potentially recombine,” said Dr. Peter Rabinowitz, a professor of environmental and occupational health sciences at the University of Washington and the director of its Center for One Health Research.

Rabinowitz added that it is important to improve protection protocols for farmworkers and encourage them to get their flu shots. 

“This is a wakeup call. We need to do a better job of protecting workers,” he said.

Itle said cases in poultry were not unexpected in Washington state, because migrating birds passed through during summer and early fall. Recently, owls and other birds of prey were found dead, suggesting some wild animals had been stricken with the virus. 

Wild birds can spread the virus to farms through direct contact with poultry or livestock or via droppings, saliva or feed.  

More than 103 million birds have been sickened, killed or culled in the current bird flu outbreak, according to CDC statistics. The virus has been detected in poultry in 48 states. 

The CDC says 330 dairy herds have been affected. 

Washington has not had an outbreak among dairy producers, Itle said, but dairy farms there are now testing weekly.