Researchers are concerned by H5N1 bird flu’s ability to mutate, reassort

SALEM, Ohio — Wild birds are reservoirs for influenza A, which has caused outbreaks in past years among wild and domestic birds. They carry the virus in their GI tract and expel it through their feces. While some birds may become ill and die, the virus usually fades away on its own. This is not the case with the current outbreak, H5N1, which arrived in the U.S. from Europe and Asia in 2022.

Commonly called bird flu, the current strain is highly pathogenic avian influenza, or HPAI. It is extremely contagious, deadly and can wipe out entire flocks within days. Once found, birds are destroyed to prevent the spread to other flocks.

The outbreak of H5N1 also has expanded to mammals around the globe, including domestic and large cats, fox, polar bears, raccoons, marine animals such as dolphins and seals and livestock like dairy cattle. As of Jan. 20, there have been 67 human cases in the U.S. caused primarily by contact with infected birds or dairy cattle.

“I hate to use the word unprecedented, because it gets thrown around so much,” said Dr. Andy Bowman, from Ohio State University’s College of Veterinary Medicine. “But this is truly unprecedented.”

The introduction of infection in many new species concerns researchers in the U.S. because of the viruses’ ability to both mutate and reassort, which could eventually lead to more severe human infection.

The good news is that vigilant biosecurity practices can help prevent the spread of the virus to people and other species.

According to the U.S. Department of Agriculture, more than 130 million birds in commercial and backyard flocks in all 50 states have died from H5N1 or been destroyed to prevent the spread of the disease since February 2022. The most surprising development to veterinarians is the virus’ detection in dairy cows last year.

H5N1 was found in a dairy cow in March 2024, when veterinary researchers treating a cow with a difficult case of mastitis discovered the virus in her milk. According to the American Veterinary Medical Association, one of the clues that led researchers to test for H5N1 was illness and death among barn cats at the farm. Since then, there have been 929 confirmed cases of bird flu in dairy herds in 16 states, including Ohio and Michigan.

“Until 2024, I would have told you cows can’t get bird flu,” Bowman said. “We’ve never had (bird) flu in cows before.”

Understanding influenza

Influenza is a broad term for several viruses with similar disease-causing characteristics designated with letters A, B, C and D. Influenza D is most common in bovine; B and C are found in people, with C most common in children. Avian influenza is caused by influenza A virus.

“Influenza A gets most of the research attention,” said Bowman. In its virulent form, influenza A has been found to be the root cause of several serious flu outbreaks, including the famous 1918 pandemic, which killed an estimated 50 million people, and a less-deadly pandemic that circulated in 2009.

Bowman teaches veterinary and public health students about viral infectious diseases. He also oversees a research team investigating the epidemiology of influenza, especially in swine. Pigs are a mixing vessel for influenza. While some species can contract only one type of influenza, swine can be infected with several of them. Pigs are mammals. Viruses that infect pigs can more easily move to people. People can infect pigs with influenza, also.

According to Bowman, influenza pandemics have often moved from wild birds to domestic poultry, then to pigs and finally to people. In each species, the virus can mutate or can reassort with endemic viruses and become more fit.

Reassortment occurs when one of the surface proteins on the virus recombines and can be identified by a different subtype. H1N1, for example, instead of H5N1. This may occur if a mammal’s cell is infected by two different viruses simultaneously. The reassortment might create a more pathogenic virus that can cause more severe disease and spread more easily.

According to the CDC, the 2009 H1N1 flu pandemic appears to have reassorted from swine. The flu outbreak was unusual because it caused severe respiratory disease in young adults and children. People over age 50 were found to have antibodies against the strain, likely from exposure to similar strains in the 1960s.

“The most predictable thing about flu is that it’s unpredictable,” Bowman said. “If (H5N1) goes to pigs, there are so many viruses (in swine) to reassort. It could enter pigs and not be a good fit, but maybe it could find a (virus) friend to reassort.”

In that case, reassortment might lead to more human infections. But at this point, it’s unclear what symptoms the disease might cause, or how serious it might be.

“In the other species, it’s not showing up with (only) respiratory symptoms,” Bowman said. “We’re seeing neurologic system diseases. The animals die, and then we find it.”

While influenza is typically a respiratory disease in humans, according to the National Science Foundation, HPAI symptoms in birds include loss of balance, trembling, lethargy, swelling of the head, hemorrhaging and respiratory distress. In marine mammals, symptoms include respiratory issues, neurological issues, tremors and paralysis.

In October 2024, the National Veterinary Services Laboratory confirmed HPAI in at least one pig on a small Oregon farm. In order to eliminate the virus, those animals were destroyed, which is also the protocol with poultry.

With cows, recovery and re-testing are the common protocols. Bowman worries about managing the virus in other animals. If the virus continues to infect other species, decisions will have to be made about the best way to inhibit the virus’s spread.

“If pigs get it, how are we going to manage it?” Bowman said. “Like birds or like cows?”

Protecting flocks, herds and humans

Preventing infections is the top priority. Pennsylvania officials have taken aggressive measures to keep bird flu out of its dairy herds, particularly after dealing with an H5N1 outbreak from April 2022 to March 2023 in more than 60 poultry flocks that impacted more than 4 million birds. The state announced mandatory bulk testing of milk in November 2024 to aid in the early detection of HPAI, shortly before the USDA made it a nationwide requirement.

“Pennsylvania’s large number of farms with both dairy cattle and poultry present unique risks that demand extra vigilance,” said Pennsylvania’s Agriculture Secretary Russell Redding, in a statement announcing the bulk milk testing mandate.

The state-funded Center for Dairy Excellence has been giving away biosecurity kits and PPE at events like the Pennsylvania Farm Show and Keystone Farm Show.

Ohio farms have been on high alert, said Jim Chakeres, executive vice president of the Ohio Poultry Association. A dozen commercial flocks in Ohio have tested positive for H5N1 over the last month, impacting more than 3 million birds. The affected flocks have been in Mercer and Darke counties.

Ohio State Veterinarian Dr. Dennis Summers suggests following the USDA recommendations, especially regarding the use of personal protective equipment.

According to the USDA, hands should be washed thoroughly prior to putting on and removing PPE, which includes: properly fitted safety goggles or a face shield (if there is risk of liquid splash); disposable gloves; boots or boot covers; NIOSH-approved 1 particulate respirator (e.g., N951 filtering facepiece respirator); Disposable fluid-resistant coveralls (i.e., blue Tyvek); and disposable head cover or hair cover.

While the state of Ohio doesn’t distribute PPE, they encourage producers to access the funding resources available from USDA.

Summers also encourages consumer awareness.

“Don’t drink raw milk,” he said. Pasteurization was developed to remove pathogens from milk in order to prevent illness. “There are many reasons to never consume raw milk, but especially right now.”

In the U.S., indoor domestic cats have also been infected with H5N1. In Oregon, the infection originated in raw poultry meat that was included in pet food. Cooking meat to the recommended safe temperature of 165 F prevents infection from influenza and other pathogens, for pets and humans alike.

Biosecurity

The Ohio Poultry Association has active ongoing surveillance programs and has developed enhanced biosecurity protocols to protect birds from any disease, especially avian influenza and especially now. The OPA recommends keeping birds inside. If flocks must remain outdoors, their space should be fenced and covered to protect them from wild birds.

“Bring your birds inside, especially during migratory seasons, which are changing because of climate change,” Chakeres said. In addition to protecting flocks from wild birds, their enhanced protocol recommendations protect birds from people.

“We can carry virus on our shoes,” he said. “Even for a small backyard flock, we recommend one dedicated pair of shoes for going into the barn. Keep them at the barn and change there. Don’t walk across the yard and then go into the facility.”

Other tips include:

• Don’t share equipment with other farms.

• Don’t permit external people to come in — only essential personnel.

• Wash hands thoroughly before and after contact with any animals.

• Change and wash clothes.

“Some farms have car wash and truck wash stations as vehicles enter and exit the farm,” Chakeres said. “Some even have shower-in, shower-out capabilities.”

While these recommendations come from the OPA, Bowman notes they are appropriate guidelines for all species. He even suggests preventing delivery drivers from coming to a farm building.

“Examine every delivery, every visitor,” Bowman said. “Everything is a potential threat.”