Rats living in the sewers of New York City carry SARS-CoV-2 — the virus that causes COVID-19 — a study has found. Researchers sampled nearly 80 wild rats and tested them for the virus, finding that more than 16 percent had contracted Covid infections. The finding raises concern that the rodents could form a reservoir for the disease, within which new strains that posed a risk to humans could develop.
Wild rats are common in urban areas in the United States — with experts having estimated that New York City alone plays home to eight million of the creatures.
The animals, the researchers note, have “ample opportunities” to interact with humans and exchange disease.
In fact, two previous studies based in Hong Kong and Belgium have both shown that wild rats in these locations have been exposed to an unknown strain of SARS-CoV-2.
Given this, in their study, Professor Wan and his colleagues set out to determine whether the COVID-19 virus had been transmitted from humans to the rat population of New York City.
Paper co-author and US Department of Agriculture (USDA) assistant director Dr Tom DeLiberto said that, in the autumn of 2021, the “USDA Animal and Plant Health Inspection service sampled Norway rats (Rattus norvegicus) in New York City to look for evidence of SARS-CoV-2 infection.
“Two trapping efforts were conducted during September and November with permission from the NYC Department of Parks and Recreation in and around locations surrounding wastewater systems.
“Most of the rats were trapped in city parks within Brooklyn, although some were captured near buildings outside of park boundaries.”
In total, the researchers collected and processed samples from 79 New York City rats. Analysis revealed that 13 of the rats — that is, 16.5 percent — tested positive for SARS-CoV-2.
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Paper author and virologist Professor Henry Wan of the University of Missouri said: “Our findings highlight the need for further monitoring of SARS-CoV-2 in rat populations for potential secondary zoonotic transmission to humans.
“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.”
Genomic sequencing of the viruses suggested a possible link to those that were circulating among humans during the early stages of the COVID-19 pandemic.
Prof. Wan said: “To the best of our knowledge, this one of the first studies to show SARS-CoV-2 variants can cause infections in the wild rat populations in a major US urban area.”
The team also conducted experiments to determine which variants of Covid are capable of infecting rats in the first place.
A so-called virus challenge study revealed that the Alpha, Delta and Omicron variants of SARS-CoV-2 are all capable of causing infection in “Sprague Dawley”-type albino lab rats.
The team found high viral replication levels in both the animals’ upper and lower respiratory tracts, as well as the induction of both innate and adaptive immune responses — although the susceptibility to infection was, they noted, varied depending on the particular variant.
Prof. Wan concluded: “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.
“SARS-CoV-2 virus presents a typical one-health challenge which requires collaborative, multisectoral and transdisciplinary approaches to fully understand such challenges.”
The full findings of the study were published in the journal mBio.
