CNN
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The millions of Norway rats that live alongside New Yorkers are among the animals that can catch the virus that causes Covid-19, a new study says. However, reports of the virus spreading from any types of animals to humans remain rare.

Pets like cats, dogs and hamsters; zoo animals such as big cats, primates and hippos; farmed mink; and wildlife such as deer and anteaters are among the animals in which Covid-19 infections have been reported. For the study, published Thursday in the American Academy of Microbiology’s journal mBio, the researchers captured 79 rats from three sites in Brooklyn in fall 2021 and tested them for exposure to SARS-CoV-2.

“Most of the rats were trapped in city parks within Brooklyn, although some were captured near buildings outside of park boundaries,” study co-author Dr. Tom DeLiberto, SARS-CoV-2 coordinator with the US Department of Agriculture’s Animal and Plant Health Inspection Service, said in a news release.

Thirteen, or 16.5%, of the 79 rats were found to have IgG or IgM antibodies against the virus, suggesting a previous infection with SARS-CoV-2.

“A number of studies have suggested that fragments of SARS-CoV-2 genomes were identified in sewage water systems, and that the prevalence of SARS-CoV-2 in sewage water systems coincides with outbreaks in resident human populations,” the researchers note. “However, no evidence has shown that SARS-CoV-2 viruses in sewage water are infectious, suggesting that sewage rats may have been exposed to the virus through airborne transmission, e.g. overlapping living spaces with humans or indirect transmission from unknown fomites, e.g. contaminated food waste.”

Two rats had positive blood tests as well as carrying viral RNA, “implying that previously exposed seropositive animals may still contract and shed SARS-Co-V-2,” the study says.

Genomic analyses showed that the viruses infecting the rats were associated with the B-lineage strain that was dominant in the city early in the Covid-19 pandemic.

To look deeper, the researchers also conducted a virus challenge study and found that the Alpha, Delta and Omicron variants of the coronavirus can infect lab rats.

Other research has found that rats in Hong Kong may have been exposed to the coronavirus, but the new study is thought to be one of the first to show that variants can infect urban rats, said Dr. Henry Wan, the study’s principal investigator and director of the Center for Influenza and Emerging Infectious Diseases at the University of Missouri, in the release.

The findings show the need for continued monitoring of rat populations to watch for the evolution of new strains of the virus, Wan says, “and it’s important that we continue to increase our understanding so we can protect both human and animal health.”

Although cases in which humans spread the virus to animals have been well-documented, reports of infected mammals spreading the virus to humans through close contact are rare, according to the US Centers for Disease Control and Prevention. “There is no evidence that animals play a significant role” in spreading the virus to people, the agency says.

The findings won’t change what people do, and humans remain far more likely to catch Covid-19 from other humans than from animals, said Dr. J. Scott Weese, director of the Centre for Public Health and Zoonoses at the University of Guelph in Canada, who was not involved in the new research.

However, the study is a good reminder that the virus continues to circulate, he said, “and it’s also a reminder for the future, and that we need to be approaching things in the broader context, animal and human health all together.”

“This study, I think, just reinforced that, yeah, we did a good job of passing it on to other species, and they may be maintaining it. That’s what we need to see over time: Does it actually stay circulating in the rat, and does it change over time?”

In the meantime, Weese said, “there are lots of reasons to stay away from wild rats. The various things you can get from them, just add [Covid-19] to the list of reasons you probably shouldn’t be handling a rat.”

When the Norway rats were collected in fall 2021, the US was on the down slope of the Delta wave, with more than 1 million Covid-19 cases each week in September and more than 500,000 each week in November, CDC data shows.

Now, with the Omicron variant dominating, there were 226,618 weekly cases as of March 1.