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Pet hamsters likely transferred the Delta variant of SARS-CoV-2 to humans in Hong Kong, according to a genetic analysis that detected the virus in several animals in a pet shop there. The researchers suspect the virus may have jumped from humans to hamsters and then back to humans, causing 50 people, some of whom had visited the pet shop, to become infected so far.

Although the research, published on January 28 as a Lancet preprint, has yet to be peer-reviewed, it is the first scientific evidence that hamsters can transmit SARS-CoV-2 to humans, reports Nature. So far, they’re the only animals besides mink to be known to do so—though, according to Nature, hamsters are highly susceptible to SARS-CoV-2 infection and are commonly used in the lab to study COVID-19.

See “Denmark to Cull 17 Million Mink Amid SARS-CoV-2 Mutation Concerns

The findings suggest that the pet trade may be a route for COVID-19 transmission, co-author Leo Poon, a virologist at the University of Hong Kong, tells Nature. But “to be fair to the hamsters,” people are still more likely to be infected by other people, he says.

Researchers were clued into the hamsters when a 23-year-old pet shop employee tested positive for the Delta variant of the virus on January 15. Prior to that, thanks to Hong Kong’s “zero-tolerance” COVID-19 policy, the last community case of Delta had been reported in early October.

Public health officials soon tested hundreds of animals at the pet shop and the warehouse supplying it. They detected SARS-CoV-2 viral RNA or antibodies against the virus in 15 of 28 Syrian hamsters (Mesocricetus auratus), but none in any other rodents, including dwarf hamsters and mice. The animals exhibited no overt signs of infection, the authors write.

The researchers then genomically sequenced the samples they collected from 12 hamsters and the first three people that tested positive for COVID-19, including the pet-shop worker and a visitor to the shop. In all of the samples, the researchers detected a version of Delta that had not previously been detected in Hong Kong, implicating the pets as the source of the human infections. Furthermore, the first and second patients’ sequences differed enough to suggest two separate human infection events. The third patient—the spouse of the second—hadn’t visited the store but had a virus identical to spouse’s, suggesting the viruses transmitted from hamsters to humans retained the ability to be transmitted person-to-person. 

See “The Rise of COVID-19 Vaccines in Animals

The team also noticed diversity in the hamster sequences, which along with their similarity to sequences from Europe (where the animals were imported from), led them to conclude that the animals had likely been infected back in November before they arrived in Hong Kong. The virus, they speculate, had then had time to spread among the animals and accumulate mutations. 

Arinjay Banerjee, a virologist at the University of Saskatchewan in Saskatoon, Canada, tells Nature that he disagrees with that conclusion, instead suggesting that the hamsters may have been infected by a person in Hong Kong and may not ultimately be responsible for importing the virus into the country. “There are so many people handling hamsters during the process of them being transported,” he says.  

He also tells Nature that the risk of contracting the virus from hamsters “still seems low,” but is “something to be aware of.” Because of Hong Kong's zero-tolerance policies, when officials detected the virus in the pet hamsters, some 2,000 of the animals were culled, reports The Washington Post