By Sama Marwan,
Every spring, horseshoe crabs emerge from Delaware Bay during the first full moon of May to mate and lay eggs. Soon after, migrating shorebirds flock to the beaches, feeding on the nutrient-rich eggs, often doubling their weight in just a week. This annual event, involving up to 25 bird species, is a global ecological marvel and a critical site for scientists monitoring potential pandemic threats.
This year, the urgency of their work has increased due to the spread of the H5N1 flu virus in U.S. dairy cattle and poultry. The team from St. Jude Children’s Research Hospital, funded by the National Institutes of Health, has been collecting bird droppings at these beaches for nearly 40 years. Founded by Dr. Robert Webster, the project focuses on understanding how flu viruses, originating in birds’ guts, spread and evolve.
Dr. Richard Webby, now leading the project that Dr. Webster started, recalls how surprising it was to discover that H5N1 was replicating in the intestinal tract of birds, not their respiratory system. Instead of staying confined to the lungs, the virus spread through bird droppings in the water. This discovery has been crucial to understanding how influenza viruses spread, especially along the Atlantic flyway, where birds migrate between South America and the Arctic.
Webby emphasizes the importance of knowing “normal” flu patterns to predict pandemics, likening it to forecasting tornadoes. The team has been tracking flu viruses for decades, discovering that while some viruses cause mild symptoms in birds, H5N1, a highly pathogenic avian influenza, is far more dangerous, wiping out entire poultry flocks.
Despite efforts like aggressive culling, H5N1 has continued to spread since it first appeared in the U.S. in late 2021. In addition to birds, it has infected mammals, including cats and sea lions, raising concerns about the virus evolving to infect humans. Though human-to-human transmission hasn’t occurred yet, studies show that small changes in the virus’s DNA could enable it to infect human lungs.
This spring, the St. Jude team collected over 1,000 bird droppings along Delaware Bay, testing them for H5N1. The samples are sequenced to track viral mutations. While the team didn’t find H5N1 in the Delaware Bay birds, they are investigating further with a mobile lab, which speeds up the process by screening samples for flu viruses in the field before sending them to the lab.
Although the virus wasn’t found this year, Webby remains vigilant, as the dynamics of flu transmission continue to evolve, and the team remains prepared for the next outbreak.
Tracking Avian Viruses: A Global Effort to Understand and Control H5N1
Dr. Pamela McKenzie and her team are on the front lines of avian virus surveillance, collecting between 800 and 1,000 bird droppings annually along the Delaware Bay beaches. Their mission to track viruses extends far beyond the East Coast.
After completing their work in Cape May, team member Kercher transported their mobile laboratory to northern Alberta, Canada, where they tested ducks breeding in the Peace River region—a yearly tradition for 45 years but the first to include the mobile lab. From Alberta, the team traveled to Tennessee to study ducks wintering there.
Meanwhile, H5N1 was spreading rapidly across North America, appearing first in cattle herds in the Midwest and California, with mild human infections reported among farmworkers. While cattle outbreaks slowed toward summer’s end, severe human cases began to emerge.
A teenager in Vancouver, Canada, was hospitalized with respiratory distress, and another individual in Louisiana became critically ill after exposure to a backyard flock. Both cases involved the D1.1 genotype of H5N1, distinct from the B3.13 genotype affecting cattle. Additional D1.1 cases were reported in Washington state, linked to individuals assisting with bird culling, though these infections were less severe.
In response, the St. Jude team relocated their mobile lab to a new site in northwest Tennessee, a significant wintering ground for mallards and other ducks. Swabbing 534 ducks in November and December, they detected the D1.1 genotype in about a dozen samples.
“This is the same strain wreaking havoc among humans and wild birds,” Kercher noted.
D1.1 is relatively new, emerging through a genetic reassortment where two viruses infect the same host and exchange genes. This process creates significant genetic shifts compared to gradual mutations. The team’s findings linked the virus to the Mississippi flyway, a migration route from central Canada to the Gulf of Mexico.
Recent surveillance data contributed to a preprint study led by Dr. Louise Moncla of the University of Pennsylvania, focusing on the evolution of viruses. The study found that the H5N1 outbreak in North America, beginning in 2021, stemmed from eight separate introductions of the virus by migrating waterfowl and shorebirds along the Atlantic and Pacific flyways.
The researchers believe aggressive culling efforts, which controlled previous outbreaks like the one in 2014, have been less effective this time due to continuous reintroduction of the virus by wild birds into farmed and backyard flocks.
The ongoing research underscores the need for enhanced surveillance and innovative strategies to combat this evolving threat to wildlife, agriculture, and public health.
They conclude that wild birds are an emerging reservoir for the virus in North America, and that surveillance of migrating birds is critical to stopping future outbreaks.
Webby and his team say they plan to continue their lookout. Come May, when the first full moon rises over the Delaware Bay, they’ll be back to do it all over again.
Kercher said what they found this year in the Delaware Bay was about what they’ve seen for the last 40 years: Shore birds are moving viruses around long distances.
“They stop in Delaware Bay to refuel, and then the viruses get moved around while they’re stopped over and then they carry it off again,” Kercher said.
There’s no way to know what lies ahead or whether the H5N1 virus will finally shape shift enough to become a danger to people. If it does, she said, they’ll be watching.
