Science|How Did a Virus From the Atlantic Infect Mammals in the Pacific?
Sea otters and seals in the Pacific Ocean, off the coast of Alaska, are infected with a virus that once was seen only in animals in the Atlantic.
Tracey Goldstein, a biologist at the University of California, Davis, got curious when sea otters in the Pacific tested positive for phocine distemper virus — a cousin of canine distemper virus — in 2004, two years after a major outbreak among European harbor seals.
Genetic analysis showed that the infections in the two groups were connected. Dr. Goldstein wondered how a virus usually passed through direct contact with a sick animal had managed to get from one northern ocean to another.
Until 2002, the seas around the Arctic Circle remained largely frozen even during the late summer. That year, though, the Arctic Ocean between the North Atlantic and Pacific was passable at the end of the summer, she and her colleagues found.
Although sea otters don’t venture far from home, seals conceivably could have borne the virus from the Atlantic to the Pacific, Dr. Goldstein said.
Melting sea ice is a viable explanation for the spread of viruses — but not the only one, said Charles Innis, a veterinarian and director of animal health at the New England Aquarium in Boston.
“A skeptic could make arguments that maybe this virus could be transmitted through an intermediate host, like a bird that can fly long-distance,” said Dr. Innis, who was not involved in the new study. “Or maybe it’s being transmitted in the ballast water of ships or something like that.”
Even the illegal pet or wildlife trade, or tainted meat shipped from one coast to another, might spread a virus, he added.
Dr. Goldstein and her team also looked at antibodies to the virus in the animals. There was no evidence of antibodies in tests conducted before the year 2000.
By 2002, though, the new study found “quite a difference” in antibody levels in Steller sea lions, Dr. Goldstein said, suggesting the animals had active infections or had recovered from them.
Phocine distemper virus is quite lethal among harbor seals in the Atlantic. Hundreds of harbor seals and gray seals were found dead in 2018 along the New England coast, from Massachusetts to Maine, because of infections with distemper and the flu.
But harp seals seem to be better able to survive phocine distemper, Dr. Goldstein said, and may serve as its reservoir — the ecological niche in which the infection persists. Outbreaks may begin when a sick harp seal comes into contact with a gray seal.
The outbreaks seem to arrive in cycles, Dr. Goldstein said, because the animals build immunity to the infection. Every five to 10 years, as new seals and otters are born and overall immunity wanes, the population becomes susceptible again and another outbreak occurs.
The new study identified a second wave of viral antibodies in 2009 in several seal species, including ice seals, northern fur seals and Steller sea lions. The current study ended in 2016, so it’s not clear if the virus has been spreading since then, Dr. Goldstein said.
But she worries that another cycle of infection may not be far off. “These channels in the ice seem to be open every year, so these rare events might become more common,” she said.