A bottlenose dolphin found dead in a Florida canal last spring tested positive for a highly virulent strain of bird flu, scientists said Wednesday. The announcement came a week after Swedish officials reported finding the same type of bird flu in a stranded porpoise.
This version of the virus, which has spread widely among North American and European birds, has affected an unusually wide range of species. But these findings represent the first two cases documented in cetaceans, a group of marine mammals that includes dolphins, porpoises and whales.
It’s too early to say how often the virus infects cetaceans, but its discovery in two different species on two different continents suggests there have “almost certainly” been other cases, said Richard Webby, influenza virologist at St. Jude Children’s Research Hospital in Menfi, Tennessee.
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“Our surveillance activities on a global scale are never sensitive enough to detect the only two such events,” said Webby, who was not involved in the initial detection of the virus but is now working with the Florida team on the follow-up studies.
The virus has become so prevalent in birds that it wouldn’t be surprising to see the pathogen appear in other unexpected species, he added. “Unfortunately, I think this is just a kind of sign of what will happen if this virus doesn’t go away,” she added.
Experts point out that the risk to humans remains low. In the United States, the version of the virus that is circulating has caused only 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 offers the virus new possibilities to mutate and adapt to host mammals.
This strain of avian flu, known as Eurasian H5N1, spread rapidly through domestic poultry, affecting tens of millions of farmed birds, according to the Department of Agriculture. Compared to previous versions of the virus, this lineage has had a particularly heavy impact on populations of wild birds, edged eagles, owls, pelicans, and more.
This, in turn, has put mammals encountering wild birds at risk. As the outbreaks spread this spring, the virus appeared 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 was trapped between the stilts of a pier and a dam, said Dr. Michael Walsh, a veterinarian at the University of Florida’s College of Veterinary Medicine who leads the university’s marine animal rescue program.
When the rescuers arrived, the dolphin was dead, he said. The team, which regularly perform autopsies, collected a variety of samples from the dolphin and stored them until they could be analyzed in more detail.
At the time, scientists had no reason to suspect bird flu had found its way into dolphins and were in no particular rush, said 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 returned this summer, they revealed signs of inflammation in the dolphin’s brain and surrounding tissues, Walsh said. Scientists previously documented brain inflammation in virus-infected fox kits, which can cause neurological symptoms in birds and mammals.
Subsequent laboratory tests detected Eurasian H5N1 in the dolphin’s brain and lungs. “The brain tissue really showed a high level of the virus,” Walsh said.
It is not known whether the virus contributed to the dolphin’s death, as well as exactly how the animal contracted it. But it’s not hard to imagine a young dolphin investigating a sick bird near the coast, Walsh said, adding, “These animals are always curious about their environment and controlling things. So if he comes across a sick, dying or dead bird, he might be very curious about it. He could put it in his mouth.
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 many organs of the animal, including the brain, according to the agency.
So far, there is no evidence that the cetaceans are spreading the virus to each other, Webby said. And Webby’s team, which isolated and sequenced the virus detected in the Florida dolphin, found no signs that it developed mutations associated with adaptation to mammals. “It still looks like a virus you’d pick up from a bird,” he said.
But now that dolphins and porpoises are known to be susceptible, researchers can start looking for the virus more proactively, including in all previously collected tissue samples.
“Now, everyone will be on their guard for this,” Walsh said. “And this will help us understand how serious this problem is for coastal cetaceans.”
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