Caroline Zeiss doesn’t know how Connecticut’s wild white-tailed deer and white-footed mice became infected with COVID-19.
“This is the million-dollar question,” she said. “No one knows.”
What Zeiss knows is that mice, deer, mink, cats, ferrets and other species are not as likely to be infected by COVID-19 as they once were.
Zeiss is a veterinarian and professor of comparative medicine at Yale School of Medicine. She has been working with Guillermo Risatti, a professor in the UConn Department of Pathology and Veterinary Medicine and director of the UConn Veterinary Medical Diagnostic Laboratory, to track the incidence of COVID-19 in wild and domestic animals in Connecticut.
Their findings, published late last year in the journal Nature, aim to better understand how the virus spreads between human and animal groups, with a view to learning how to better predict and control this spread. They said they learned that while COVID-19 was circulating in wildlife during the height of the pandemic, later variants, such as the omicron strain, were less well suited to animal hosts.
Many viruses can and do infect humans and animals, a process called zoonotic transmission. Zeiss calls them “generalist viruses.” For example, there is a variant of influenza called “avian influenza,” which Zeiss says is “probably the largest pandemic in animal history and is still ongoing.”
Coronaviruses also frequently jump from animal species to humans. Despite controversy, the current scientific consensus is that SARS-CoV-2, commonly known as COVID-19, originated in a Chinese bat population and was transmitted to humans at the Huanan Seafood Market in Wuhan, China.
“We’ve found a whole family of coronaviruses in animals and humans, some of which are very host-specific because they’ve evolved,” Chase said. “I think SARS-CoV-2 is evolving to become a human-specific virus.”
Noah’s Ark of COVID-19
White-footed mice are the most common rodent found in Connecticut, according to Zeiss. But how these mice become infected with COVID-19 and spread it to each other and other species remains a mystery.
“This state is literally infested with rats,” Chase said. “There are about 20 million rats in the state, but they occupy very small territories, so they don’t spread it beyond the 0.25 square kilometers they live in.”
In 2022, two white-tailed deer in New Jersey tested positive for the coronavirus, and Connecticut began testing the state’s deer herd for COVID-19 soon after.
“We know rats and deer are involved in the spread of other diseases, including Lyme disease,” Chase said.
As director of the Connecticut Veterinary Medical Diagnostic Laboratory, Risatti regularly tests species for disease.
“We take in animals because most of the time, people want to know the cause of death,” he said.
Ultimately, Zeiss and Rissati tested 889 animals from 28 different species, including domestic dogs and cats, various rats, bobcats, ferrets, fishermen, weasels, pigs, sheep, raccoons, and even a wallaby who tested negative for the coronavirus.
They say animals often experience the same symptoms as humans when infected with COVID-19, although they are usually less severe and rarely fatal. It may vary depending on the species. Minks, for example, are vulnerable to COVID-19 infection, they said.
In most cases, though, “they don’t die,” Chase said. “In the laboratory, rodents may lose some weight.”
result
Previous studies have detected active disease in wild and domestic animal populations, and there is evidence that the virus had previously infected some animals but was no longer active.
Zeiss and Rissati’s findings were different.
“We did not detect active SARS-CoV-2 infection in any species,” they said, although the researchers did identify some species that had been previously infected with COVID-19.
The virus has also declined in humans, but has not disappeared. According to the Connecticut Department of Public Health, more than 30,000 cases of COVID-19 have been confirmed in Connecticut over the past year. During the same period, 179 people have died from COVID-19 in the state.
Zeiss and Rissati concluded that the virus had evolved to infect humans as its preferred host, causing other diseases in the animals.
“The apparent decrease in infection or exposure rates in animals may reflect the reduced affinity of later SARS-CoV-2 variants for non-human hosts,” their study said.
“I remember when the pandemic started, our reaction was, ‘What is this coronavirus? This is really weird. This is really weird,'” Zeiss said. “Now we’re seeing it behave like every other coronavirus, emerging as a new virus and infecting several species and then colonizing one species.”
This article was originally published on Connecticut researchers now know why coronavirus no longer infects animals.
