Investigating a dead dolphin’s mystery: How’d it get the bird flu?

Veterinarians previously thought dolphins had low susceptibility to flu infections — and dolphins don’t feed on birds. So biologists were highly concerned when a dolphin with the bird flu died in Florida.

The infected dolphin was trapped between a dock and a seawall in the Big Bend region in March 2022. By the time wildlife officials arrived, it had died. They later found that it was infected with bird flu. The discovery — the first known case of a dolphin contracting bird flu in U.S. waters — sent a jolt of concern through wildlife officials.

Bird flu is highly contagious and deadly to many animals. If the virus, which can wipe out swaths of both bird and mammal populations, were to spread among dolphins, whales and manatees, it could be “catastrophic for these populations,” a recent paper published by the University of Florida and other institutions said.

Now, the new paper is bringing some clarity, offering clues as to how the dolphin may have contracted the virus. To better understand the virus and suss out how the dolphin may have contracted it, the UF team sent tissue samples to an enhanced laboratory at St. Jude Children’s Research Hospital, Memphis, Tennessee.

“We wanted to find out, OK, why would this dolphin come down with this?” said Michael Walsh, a veterinarian working with the aquatic animal health program at the University of Florida, and one of the authors on the study.

Here’s a look at what they found.

Examining the dolphin

Biologists were stumped with what they discovered inside the skull of a young male bottlenose dolphin that had died. When University of Florida veterinarians later performed a necropsy, they found inflammation of the dolphin’s brain and meninges (membranes that enclose the brain and spinal cord).

The inflammation stemmed from a highly pathogenic variant of bird flu. It was the first known case of bird flu in dolphins in U.S. waters, and only the second known diagnosis in the world.

The Centers for Disease Control and Prevention says the virus and its variants can cripple the poultry industry and fuel widespread mortality in wild birds, especially raptors such as eagles, owls, and vultures that might scavenge or prey upon infected animals.

The disease also can jump to mammals, either through the respiratory system, or when carnivorous such as foxes or mink eat infected birds.

Dolphins barely interact with birds. What wildlife officials didn’t know when they retrieved the dolphin in Big Bend, was that bird flu was rapidly spreading globally, and had hit the Gulf Coast.

“We had no idea there were birds dying on the coast when we picked up the dolphin,” Walsh said. He and his team later worked with bird researchers to connect the dots.

At the time of the dolphin’s death in March 2022, the virus was new to North America. The first bird flu detected occurred on a farm in Newfoundland, Canada, in December 2021, according to the New York Times.

Since then, it has spread aggressively. By January 2022, the virus was detected in wild birds in the Carolinas.

The Big Bend dolphin died in March, and that summer, the virus had jumped to mammals elsewhere, killing hundreds of gray and harbor seals along the coast of Maine. Was the Florida infection connected to a larger, possibly deadly outbreak?

After the UF team sent tissue samples to a lab, genetic analysis there showed that the dolphin’s bird flu strain was the same strain that was circulated along the Atlantic and Mississippi flyways — the routes that migratory birds take — during the spring of 2022. The Atlantic flyway passes over the area where the dolphin died.

The study then compared the dolphin’s strain to the variant responsible for the outbreak in seals in the Northeast and found them to be different. Also, genome sequencing revealed that the dolphin’s virus lacked markers normally associated with mammals passing on the virus.

In other words, the infection had not traveled down the east coast among mammals — a good sign.

What about local birds?

There were coastal bird die-offs from bird flu along the Big Bend area, but genetic testing revealed they were driven by a different strain of the virus.

A bird migrating along the Atlantic flyway might have been the source of the dolphins infection. But how? Dolphins don’t eat birds.

Walsh theorizes that the dolphin may have played with an infected bird.

“He was a young male. And it’s quite possible when they’re bored they’re playing, they’re investigating things. He may have come upon a bird that was sick and carrying the virus,” Walsh said.

“He probably mouthed it. They wouldn’t tend to eat it. But they might push it around and play with it. He may have been exposed to feces. Those things could have caused transfer.” Walsh also said that if the bird was still breathing, the virus could have entered the dolphin’s blow hole.

Global spread and human risk

Since 2022, the virus has continued to spread in North, Central and South America, in both birds and mammals.

Last year, the virus killed 24,000 sea lions along the coast of Chile and Peru before spreading south the the Antarctic region where it is now infecting elephant seals.

The virus can make people sick.

There have only been two cases of humans contracting the virus in the U.S.

In 2022, a person with direct exposure to poultry being culled for the disease tested positive. They had fatigue for a few days and recovered.

The most recent case occurred in Texas in April, when a person working with cattle that were presumed to be infected tested positive. Their only symptom was conjunctivitis.

Symptoms can range from mild (eye infection, upper respiratory symptoms) to severe illness (pneumonia).

Bird flu-related deaths have occurred in other countries.

According to the CDC, “people with close or prolonged, unprotected exposures to infected birds or other animals (including livestock), or to environments contaminated by infected birds or other animals, are at greater risk of infection.”

The CDC says the health risk to humans is relatively low because human-to-human transmission isn’t sustainable for the virus.

Is there still a risk to Florida marine mammals?

No other bird flu has been detected in Florida waters in either dolphins or manatees since the young male dolphin’s death, so he appears to have been a one-off, a victim of his own curiosity.

But the report emphasized that there seems to be no end in sight to the global spread of bird flu, and there is still much that is unknown about how the virus may evolve and affect marine mammals in the future.

Walsh said that wildlife officials have a better understanding now of how the virus made its way to a Florida mammal, and “we’ll have all this in the bank if something else happens.”

“We were all very concerned by where this could have gone,” he said. “And very fortunate it didn’t give us a long-term die-off in our area.”

Bill Kearney covers the environment, the outdoors and tropical weather. He can be reached at bkearney@sunsentinel.com. Follow him on Instagram @billkearney or on X @billkearney6.