A strain of avian (bird) flu appears to be killing seals off the New England coast, heightening fears among scientists that mammal-to-mammal transmission could be happening.
If so, it would be a step towards something health experts have long dreaded: A strain of H5N1 bird flu that might spread easily among people, with potentially devastating effects.
"We report an HPAI A (H5N1) virus outbreak among New England harbor and gray seals that was concurrent with a wave of avian infections in the region," said a team led by Dr. Wendy Puryear. She is a virologist at the Cummings School of Veterinary Medicine at Tufts University in North Grafton, Mass.
Overall, there were 164 known cases of deaths to harbor seals and 11 such cases of gray seals in Maine during June and July of 2022. Puryear and her team reported their findings March 15 in Emerging Infectious Diseases, a journal of the U.S. Centers for Disease Control and Prevention.
The "unusual mortality event" among the seals tracked closely with the second wave of an H5N1 outbreak among Atlantic coast seabirds last year, the researchers noted.
That outbreak had also spread to millions of commercial poultry, resulting in a disease-limiting cull of over 70 million domestic poultry in the United States.
The presence of highly pathogenic avian influenza (HPAI) A (H5N1) bird flu was detected in samples taken early in 2022 by the team from over a thousand New England seabirds. The disease continued to show up among birds throughout the summer.
Puryears' group also used swab tests to monitor any presence of H5N1 in seals. Tests taken of 132 stranded seals in the first half of 2022 showed no sign of the virus in the mammals, but by June and July the strain was detected in 19 harbor seals sampled.
"We're always concerned when it's in mammals, just because they're closely related to humans," Dr. Ryan Miller, an infectious disease doctor at the Cleveland Clinic in Ohio, explained recently to HealthDay.
Any fast-spreading mutation of bird flu among humans could lead to a pandemic with death tolls that could dwarf those seen with SARS-CoV-2, the virus behind COVID-19.
Luckily, human cases have so far remained very rare.
"During the past 20 years, fewer than 900 confirmed human cases of H5N1 have been reported to the WHO [World Health Organization]," according to a report published last month in the Journal of the American Medical Association.
Still, "the historic case-fatality rate for human H5N1 infection has been high — more than 50%," the report's authors stressed. Some experts believe that this high death rate could be an overestimation, however, since many mild or asymptomatic infections may go unreported.
Did the New England seals in last year's outbreak pass H5N1 to each other? That's still unclear, according to the Tufts team.
All they can say at this point is that seals got infected "through environmental transmission of shed virus" with seabirds as the originating source. The affected seals rarely prey on seabirds or scavenge bird remains, however, so seal-to-seal transmission of H5N1 remains possible.
If individual seals did get the virus from close proximity to seabirds, the ease at which the virus transmitted between species is troubling, Puryears' group noted. In addition, novel mutations in the HPAI A (H5N1) strain were noted between infected seals, including certain changes "associated with mammal adaptation," the study authors added.
In commercial poultry or other animal food or fur production facilities, large-volume culling of animals can help rein in an outbreak. But as Puryear's team noted, that's just not feasible when it comes to populations of mobile, wild animals such as birds or seals.
So if seal-to-seal transmission of H5N1 is at least a possibility, is a variant of the virus that does the same in humans an inevitability?
The world had a scare earlier this month when an 11-year-old girl in Cambodia contracted H5N1 and later died. Her father also tested positive for the virus, but recovered.
In those cases, it was determined that each person had gotten the virus from direct contact with infected poultry, ruling out human-to-human transmission.
The infections noted in the New England seals aren't the only sign that mammal-to-mammal transmission is happening: A mink farm in Spain suffered an H5N1 epidemic last October, one of the first large outbreaks of the virus that was apparently driven by mammal-to-mammal transmission. Nearly 52,000 mink on the farm were culled as a result.
"These mink are jammed very, very closely together, by and large, and it spread among the mink," Dr. William Schaffner, medical director of the National Foundation for Infectious Diseases, explained recently to HealthDay. "Well, that's a little bothersome because this virus is not supposed to spread very much in mammals, although the mink have some viral receptors that are more accepting of this virus than some other animals," he noted.
"Fortunately, all the human beings who worked on that mink ranch were tested and were found to be negative, so it didn't get to the people," Schaffner added.
The risk to humans comes from the possibility that one of these avian flu strains will infect some animal — a pig, for instance — that is also susceptible to human influenza, Schaffner explained.
"When the two of them are together in the pig, it's like being in a test tube. Those two viruses can exchange genetic components, and it's possible for a bird flu virus to pick up that genetic capacity from the human virus of being able to spread readily from person to person," Schaffner said.
"Now, if that happens, then you have an incipient pandemic because the world's population would be susceptible to that virus and it could spread globally," Schaffner continued.
Other experts urge vigilance, but believe there's no immediate threat to people.
"It's a potential risk. The likelihood of that happening is, I think, still quite low compared to other risks that we have. But it's a real risk and it's something obviously we have to be prepared for, something we have to watch very carefully," said Dr. Aaron Glatt, chief of infectious diseases and chair of medicine at Mount Sinai South Nassau in Oceanside, N.Y.
It helps that the United States is emerging from its cold and flu season, with circulating rates of flu on the decline since the holidays ended, Miller said.
"The risk of it actually merging with one of those more transmissible human viruses is rather low right now," Miller explained.
And, bolstered by the lessons of the COVID pandemic, scientists around the world are ramping up vigilance of H5N1 and other viruses in animals. That could ultimately help in warning people early about a threat, and developing interventions that could save countless lives.
"The World Health Organization organizes a global surveillance for all kinds of influenza strains that are out there all the time, both in humans, in animals and in birds," Schaffner noted. "And we have the capacity to sequence these viruses to understand their genetic components and to do that very, very rapidly. Our surveillance system, the radar that's out there getting early information, is so much better than it was 10 years ago."
Find more about the H5N1 avian flu at the U.S. Centers for Disease Control and Prevention.
SOURCES: Emerging Infectious Diseases, March 15, 2023; Ryan Miller, DO, infectious disease, Cleveland Clinic, Ohio; William Schaffner, MD, medical director, National Foundation for Infectious Diseases; Aaron Glatt, MD, chief, infectious diseases, and chair, medicine, Mount Sinai South Nassau, Oceanside, N.Y.; Journal of the American Medical Association, Feb. 8, 2023, online