SARS-CoV-2, the virus that causes Covid-19, is zoonotic, meaning it spreads between animals and humans. It's possible the virus originated in bats, before infecting an intermediate animal host, which then spread the virus to humans. But since then, humans have transmitted the virus back to animals as well. So far, tigers, dogs, mink and domestic cats have contracted Covid-19 from humans, and a new study suggests that marine mammals could be added to that list.
The study, published in October in the journal Science of The Total Environment, found that 15 marine mammal species have the same receptor as humans in their cells that could make them vulnerable to contracting the virus, reports the Canadian Press. No confirmed Covid-19 cases have been found in marine mammals yet, but it's a possibility, reports Elizabeth Claire Alberts for Mongabay.
The species, including dolphins, beluga whales, sea otters and seals, have a specific receptor called ACE2 that allows SARS-CoV-2 to infect cells. The virus attacks the cells by binding to specific amino acids, or the building blocks of proteins. The ACE2 receptor is thought to be nearly universal in vertebrates.
"We do know that both dolphins and beluga whales have been infected with related gamma coronaviruses in the past," lead author Saby Mathavarajah, a biologist at Dalhousie University in Canada, tells Mongabay in an email. "Since most marine mammals are social, it is also possible for spread of coronaviruses between animals through close contact. So once one animal is infected, it could threaten entire populations."
In theory, marine mammals could contract the active virus through their mucus membranes, like their blowholes, eyes and mouths, but that would most likely happen if they come in contact with human handlers at a zoo or marine park. A tiger at the Bronx Zoo contracted the virus from a zookeeper. For this reason, the researchers suggest that people working with captive marine animals should also keep their distance from at-risk species.
The team also suggested marine mammals could be at-risk for disease transmission in areas with poor wastewater treatment practices—where contaminated sewage could pollute the animals' environment. One study the researchers cite to support their hypothesis found SARS-CoV-2 may remain stable in contaminated water for up to 25 days, but the study was published on a preprint server and hasn't yet been peer-reviewed.
David Larsen, an epidemiologist at Syracuse University, isn't so convinced by the wastewater hypothesis. The likelihood of marine mammals contracting the virus from wastewater is "possible, but very implausible," he tells Mongabay.
Most wastewater treatment processes significantly reduce or elliminate the presence of SARS-CoV-2 in sewage, the authors acknowledge in their paper.
"Our major concern is in developing nations, where there is already a disparity in public health and the wastewater treatment infrastructure required to handle the COVID-19 crisis," Mathavarajah says in the press release. "Monitoring susceptible species in these high-risk areas around the world will be pertinent for protecting wildlife during and post-pandemic."
However, past evidence to support this theory is scarce. When a group of free-ranging elephant seals contracted H1N1 off the coast of California in 2010, researchers suggested "feces discharged from the large number of shipping vessels" could be to blame. But they also pointed to seabirds, which commonly carry influenza viruses, as possible culprits, too.
"We don’t expect transmission to occur within water," Larsen says. "The idea of marine mammals getting COVID-19 is pretty far-fetched."