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The findings represent the first time a highly pathogenic form of the virus, which has devastated bird populations this year, has been detected in cetaceans.
A bottlenose dolphin found dead in a Florida canal this past spring tested positive for a highly virulent strain of bird flu, scientists said on Wednesday. The announcement came a week after Swedish officials reported that they had found the same type of avian influenza in a stranded porpoise.
This version of the virus, which has spread widely among North American and European birds, has affected an unusually broad array of species. But these findings represent the first two documented cases in cetaceans, a group of marine mammals that includes dolphins, porpoises and whales.
It is too soon to say how commonly the virus infects cetaceans, but its discovery in two different species on two different continents suggests that there have “almost certainly” been other cases, said Richard Webby, an influenza virologist at St. Jude Children’s Research Hospital in Memphis.
“Our surveillance activities on a global scale are never sensitive enough to pick up the only two events of this kind,” said Dr. Webby, who was not involved in the initial detection of the virus but is now working with the Florida team on follow-up studies.
The virus has become so widespread in birds that it would not be surprising to see the pathogen pop up in other unexpected species, he added. “Unfortunately, I think this is maybe just sort of a sign of what’s to come should this virus not disappear,” he added.
Experts emphasize that the risk to humans remains low. In the United States, the version of the virus that is circulating has caused just one documented human infection, in a person known to have had contact with poultry, according to the Centers for Disease Control and Prevention.
But the spread of the virus to new species poses potential risks to wildlife and provides the virus with new chances to mutate and adapt to mammalian hosts.
This strain of bird flu, known as Eurasian H5N1, has spread rapidly through domestic poultry, affecting tens of millions of farmed birds, according to the Agriculture Department. Compared to previous versions of the virus, this lineage has taken an especially heavy toll on wild bird populations, felling eagles, owls, pelicans and more.
That, in turn, has put mammals that encounter wild birds at risk. As the outbreaks expanded this spring, the virus turned up in foxes, bobcats, skunks and other species. The virus has also been blamed for a spike in seal strandings in Maine, where bird flu has been detected in both gray and harbor seals.
The Florida dolphin, a young male, was found in March in a canal in Dixie County, where area residents noticed that the animal had become trapped between the pilings of a pier and a sea wall, said Dr. Michael Walsh, a veterinarian at the University of Florida College of Veterinary Medicine who leads the university’s marine animal rescue program.
By the time rescuers arrived, the dolphin had died, he said. The team, which routinely conducts necropsies, collected a variety of samples from the dolphin and stored them until they could be analyzed in more detail.
At the time, the scientists had no reason to suspect that bird flu had made its way into dolphins, and they were not in a particular rush, said Dr. Walsh, who collaborated on the investigation with Dr. Robert Ossiboff, a veterinary pathologist, and Andrew Allison, a veterinary virologist, both at the University of Florida College of Veterinary Medicine.
When the results came back this summer, they revealed signs of inflammation in the dolphin’s brain and the surrounding tissues, Dr. Walsh said. Scientists have previously documented brain inflammation in fox kits infected with the virus, which can cause neurological symptoms in birds and mammals.
Subsequent laboratory testing turned up Eurasian H5N1 in the dolphin’s brain and lungs. “The brain tissue really showed a high level of virus,” Dr. Walsh said.
Whether the virus contributed to the dolphin’s death remains unknown, as does precisely how the animal contracted it. But it is not hard to imagine a young dolphin investigating an ailing bird near the shoreline, Dr. Walsh said, adding: “These animals are always curious about their environment and checking things out. So if he came upon a sick, either dying or dead, bird, he might be very curious about it. He might mouth it.”
The virus was also responsible for the death of a porpoise found stranded in Sweden in June, the Swedish National Veterinary Institute said last week. The pathogen was found in several of the animal’s organs, including the brain, according to the agency.
So far, there is no evidence that cetaceans are spreading the virus to one another, Dr. Webby said. And Dr. Webby’s team, which has isolated and sequenced the virus detected in the Florida dolphin, has not found any signs that it has picked up mutations associated with adaptation to mammals. “It still very much looks like a virus that you would pick up out of a bird,” he said.
But now that dolphins and porpoises are known to be susceptible, researchers can begin to look for the virus more proactively, including in any tissue samples they previously collected.
“Now, everybody’s going to be on guard for this,” Dr. Walsh said. “And that’ll help tell us how serious this really is for cetaceans on the coastlines.”