US Could See Spike in Bird Flu Cases After Migration Season

The U.S. could see an increase in bird flu cases as the peak of spring migration comes to an end, experts warned.

Parts of the country are currently experiencing an H5N1 bird flu outbreak, with confirmed cases in dairy herds across nine states and in two humans—dairy workers in Michigan and Texas. But things could get worse, as the nation's birds finish off their spring migration.

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In spring, birds usually return to their breeding grounds, meaning they are making cross-country journeys, often stopping to take breaks along the way. The stopover locations can act as hotbeds for the spread of bird flu, especially if they are near farmland, where infected birds can leave contaminated saliva and feces, transmitting the disease to poultry animals.

Spring migration in the U.S. usually occurs between March and June, with the peak generally occurring between April and May—it varies in different parts of the country.

BirdLife's Senior Flyway Conservation Officer Willem Van den Bossche told Newsweek: "When birds are concentrating the risk is highest that infected birds will pass on the virus to other birds."

He and his team see the highest concentrations during stopovers for some waterbird species "at feeding sites, night roosts, etc," he added.

As it gets later into spring, "birds will pair up and the risk of transmission to a larger proportion of the population decreases," he explained.

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The U.S. is yet to discover the impact of the past few weeks, which have seen birds migrate and use resting spots throughout the country.

Bossche also spoke about the wetlands and their capacity to keep influenza A viruses (which can cause bird flu) alive. He pointed Newsweek to a study published in the Science of The Total Environment journal in 2022.

Scientists "assessed the persistence of 12 influenza A viruses in lake water over one year" and concluded that "wetlands represent a medium in which infectious influenza A viruses may reside."

They wrote: "Our results provide evidence that IAVs may remain infectious for extended periods, up to or even exceeding one year, when maintained in surface waters under ambient temperatures.

"Therefore, wetlands may represent an important medium in which infectious IAVs may reside outside of a biotic reservoir."

In other words, some surface waters are able to keep bird flu viruses alive for a long time, in some cases for over a year, and Bossche pointed to the risk this may present to poultry that then drink from that water.

He said: "The mutations predominantly happen in poultry (including backyard poultry), so implementing biosecurity is crucial (including for backyard poultry).

"Strict biosecurity measures should be put in place to prevent farm-to-farm spread and 'spillback' to wild birds."

Veterinary epidemiologist Victoria Hall previously told the New Scientist: "There are a lot of species – we think primarily shorebirds, seabirds and waterfowl – that often can carry different strains of avian influenza without showing signs of illness. And they mix strains as they all gather together in migratory groups, and then they take off again and spread it."

Bossche is also concerned about the summer months, when waterbirds molt.

"When waterbirds start to molt, they will concentrate again at safe places (large wetlands) and congregate before migration," he said.

Reuters looked at this topic when it tracked the migration path of the mallard duck, which has had the most cases of avian flu recorded - 1,449 between 2022 and 2024, according to the USDA's Animal and Plant Health Inspection Service.

April is when they are most likely to use farms for food and water during their journey, before they get to cooler states for breeding season in July.

The National Audubon Society told Newsweek: "Due to the spread of avian flu, we suggest monitoring birds at a safe distance and checking regularly whether local, state, or federal wildlife agencies are recommending wild bird feeders be taken down. If at least one local agency advises they be removed, we recommend following those guidelines."