Could Marine Mammals Contract COVID-19?
Scientists say it’s possible.
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Some marine mammals carry gene mutations that could make them more susceptible than humans to SARS-CoV-2, the coronavirus at the heart of the ongoing pandemic. If these marine mammals get infected, the results could be devastating—more than half of the species predicted to be susceptible to SARS-CoV-2 are already at risk of extinction.
Several mammals, including cats, dogs, and minks, have been detected with COVID-19, and in April, eight tigers and lions at the Bronx Zoo in New York became sick. Graham Dellaire, a pathologist at Dalhousie University in Nova Scotia who is studying how the virus affects other animals, and his colleagues have shown in their latest research, which is still under review, that many marine mammals may be at risk, too.
So far, the threat is theoretical—no marine mammals are known to have contracted the virus—but it’s based on research that looks at the genetic susceptibility of various marine mammals to infection by SARS-CoV-2.
To determine that vulnerability, scientists are studying how amenable an animal’s angiotensin-converting enzyme 2 (ACE2) receptors—found on cells in the lungs, heart, kidneys, and elsewhere—are to the virus’s spike protein. Dellaire likens this spike protein to the teeth on a key, and the animal’s ACE2 receptors to the grooves in the lock. How well they line up determines how easily the virus can penetrate those cells and cause infection. In previous research, for instance, Dellaire showed that a single genetic variation makes cats more susceptible than dogs.
Some marine mammals have ACE2 receptors that bind even more easily with SARS-CoV-2’s spike protein than human receptors. This creates a risky situation, Dellaire says: “If binding efficiency is high, you don’t need a high viral load to become infected.”
Unlike house cats, or tigers and lions in a zoo, wild marine mammals aren’t often in close contact with people. But their high susceptibility to the SARS-CoV-2 virus means that marine mammals may be at risk from even small amounts of the virus.
Given that potential, Dellaire and his colleagues analyzed ways marine mammals might come into contact with the virus, identifying the greatest potential threat as contaminated, untreated wastewater.
In their paper, the scientists wanted to present a potential case study of how and where wild marine animals might be most at risk of catching COVID-19 because of untreated wastewater spillover. They focused on Alaska, juxtaposing the locations and types of more rudimentary wastewater treatment facilities with known habitats of marine mammals.
“We looked at coastal regions that have either primary or lagoon-based treatment, which is the worst level of treatment for waste,” Dellaire says. “If any of those leak, or you have a big rainfall, that raw sewage just goes right into the ocean.” The analysis shows that in Alaska, beluga whales, harbor seals, and sea otters face the greatest potential danger of SARS-CoV-2 infection.
This pathway would not be unprecedented. Previous research, says Tracey Goldstein, a veterinary pathologist at the University of California, Davis, has revealed cases where wastewater runoff has led to marine mammals becoming sick with terrestrial pathogens. Toxoplasma gondii, for example, a protozoan parasite that can be shed by feral cats, mountain lions, foxes, and bobcats, has gone on to infect sea otters in California. And 10 years ago, a group of elephant seals showed antibodies to the H1N1 flu. Since H1N1 is a human virus, Goldstein says one hypothesis for how they contracted it is that they may have encountered wastewater discharged from ships.
Though Dellaire’s research focused on the United States, other new research—such as the recent discovery of the SARS-CoV-2 virus in a river in Ecuador—extends the possibility of infection to untreated wastewater in other countries.
A July review study, however, downplays the risk that the virus is spreading to marine mammals. Ian Hewson, a biological oceanographer at Cornell University in New York, says that while there’s no question that SARS-CoV-2 virus particles have been found in wastewater, they should experience considerable decay and loss of infectivity because of dilution and their exposure to sunlight in the ocean.
“The big question is whether [wastewater runoff into the ocean is] a viable route for transmission … and my best guess is that it’s not,” Hewson says. Though, he adds that SARS-CoV-2 has so far defied expectations and proven to be far more stable in the environment than scientists expected.
Goldstein agrees with Hewson that the risk for most marine animals is probably not that high. Though she, too, adds the caveat that SARS-CoV-2 is “just so efficient at surviving” that the risk cannot be completely ruled out.
If that pathway to infection is open, Dellaire says he is particularly concerned about the possibility of SARS-CoV-2 infecting the baiji, a nearly extinct freshwater dolphin that lives in the Yangtze River in China. The river runs through Wuhan, the original epicenter of the coronavirus pandemic. “I guarantee you,” Dellaire says, “some virus made it into the Yangtze.”
Overall, he says, he hopes the study motivates coastal cities with susceptible marine mammals to shore up their wastewater treatment plants to keep the virus out of rivers and oceans.
“What we’re saying is if you have local populations of these marine mammals that you care about, that are endangered, you want to make sure that the wastewater is not going to get into [natural waterways] in the first place.”