FRIDAY, March 10, 2023 (HealthDay News) -- Rats can become infected with the virus that causes COVID-19, according to a new study that found many rodents in New York City's sewer system and elsewhere had been exposed.

Rats collected in the study tested positive for alpha, delta and omicron variants of the SARS-CoV-2 virus.

The findings were published March 9 in mBio, an open-access journal of the American Society for Microbiology.

“Our findings highlight the need for further monitoring of SARS-CoV-2 in rat populations for potential secondary zoonotic transmission to humans,” said principal researcher Henry Wan, director of the University of Missouri Center for Influenza and Emerging Infectious Diseases.

“Overall, our work in this space shows that animals can play a role in pandemics that impact humans, and it’s important that we continue to increase our understanding so we can protect both human and animal health, Wan said in a journal news release.

New York City has about 8 million rats, and they have ample opportunities to interact with people, the researchers said.

The study set out to determine whether the virus had been transmitted from humans to rats in New York City, which variant or variants had been passed and whether they could cause infection in rats.

“In Fall of 2021, U.S. Department of Agriculture Animal and Plant Health Inspection Service [APHIS] sampled Norway rats [Rattus norvegicus] in New York City to look for evidence of SARS-CoV-2 infection,” study co-author Dr. Tom DeLiberto said in the release. He's the SARS-CoV-2 coordinator at USDA APHIS Wildlife Services.

With permission from New York City officials, the group trapped rats in and around locations surrounding wastewater systems. Wastewater has been an important indicator of virus levels in communities.

“Most of the rats were trapped in city parks within Brooklyn, although some were captured near buildings outside of park boundaries,” DeLiberto said.

In all, biologists found that 13 of 79 rats (16.5%), tested positive for SARS-CoV-2.

“To the best of our knowledge, this is one of the first studies to show SARS-CoV-2 variants can cause infections in the wild rat populations in a major U.S. urban area,” Wan said.

In a lab study, researchers determined that alpha, delta and omicron variants can cause infections in wild-type Sprague Dawley rats.

This included high levels of virus replication in the rats' respiratory tracts that caused immune responses. Susceptibility to infection varied by variant, the authors said.

“Our findings highlight the need for further monitoring of SARS-CoV-2 in rat populations to determine if the virus is circulating in the animals and evolving into new strains that could pose a risk to humans,” Wan said. “SARS-CoV-2 virus presents a typical one-health challenge which requires collaborative, multisectoral and transdisciplinary approaches to fully understand such challenges.”

Previous studies had suggested rats were exposed to COVID in Hong Kong and Belgium, but the variant involved was unknown.

More information

The U.S. Centers for Disease Control and Prevention has more on COVID-19 and animals.

SOURCE: mBio, American Society for Microbiology, news release, March 9, 2023