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COVID-19 is circulating and evolving rapidly in white-tailed deer

Researchers at Ohio State University (OSU) have found that white-tailed deer are reservoirs for SARS-CoV-2 viruses.

The experts report that a significant number of white-tailed deer across Ohio have been infected with SARS-Cov-2 and that viral variants evolve about three times faster in deer than in humans. 

Focus of the study 

Between November 2021 and March 2022, the researchers collected 1,522 nasal swaps from free-ranging deer in 83 of Ohio’s 88 counties and tested them for the virus causing COVID-19. 

What the researchers discovered 

The analysis revealed that over 10 percent of the samples were positive, and at least one positive case was found in 59 percent of the counties in which testing was conducted. 

Surprisingly, genomic analysis showed that at least 30 infections in deer have spilled over from humans.

Interspecies transmission 

“We generally talk about interspecies transmission as a rare event, but this wasn’t a huge sampling, and we’re able to document 30 spillovers. It seems to be moving between people and animals quite easily,” said study co-senior author Andrew Bowman, an associate professor of Veterinary Preventive Medicine at OSU. 

“And the evidence is growing that humans can get it from deer – which isn’t radically surprising. It’s probably not a one-way pipeline.”

These findings suggest that white-tailed deer are a reservoir for SARS-CoV-2 which enables continuing mutation, and that the virus’s widespread circulation in deer could potentially lead to its spread to other wildlife and livestock.

Not just a localized problem 

In December 2021, Bowman and his colleagues first reported the detection of the virus in white-tailed deer in nine locations in Ohio. In the current study, they expanded monitoring in a variety of other locations. 

“We expanded across Ohio to see if this was a localized problem – and we find it in lots of places, so it’s not just a localized event,” Bowman explained. “Some of the thought back then was that maybe it’s just in urban deer because they’re in closer contact with people. But in rural parts of the state, we’re finding plenty of positive deer.”

Besides detecting active infections, the scientists also found a significant number of blood samples containing antibodies, suggesting that an estimated 23.5 percent of deer in Ohio had already been infected with the coronavirus.

Spillover events

Among the 80 whole-genome sequences obtained from the samples, the researchers identified the highly contagious and virulent delta variant (the predominant human strain in the United States in the early fall of 2021), which accounted for nearly 90 percent of the sequences, and alpha, the first variant of concern that was identified in humans in the spring of 2021. 

The investigation revealed that the genetic composition of delta variants in deer closely matched the dominant lineages found in humans during the same time period, pointing to several spillover events, and suggesting that deer-to-deer transmission followed in clusters, some of them spanning multiple counties.

“There’s probably a timing component to what we found – we were near the end of a delta peak in humans, and then we see a lot of delta in deer. But we were well past the last alpha detection in humans. So the idea that deer are holding onto lineages that have since gone extinct in humans is something we were worried about,” Bowman said.

According to the experts, vaccination is likely to protect people against severe diseases in case the virus will spill back into humans. For instance, an investigation of the effects of deer variants on Siberian hamsters (an animal model widely used in COVID-19 studies) provided clear evidence that vaccinated hamsters did not get as sick after infection as unvaccinated ones.

Study implications 

Unfortunately, the variants circulating in deer are expected to continue to change at a faster rate than that seen in humans. “Not only are deer getting infected with and maintaining SARS-CoV-2, but the rate of change is accelerated in deer – potentially away from what has infected humans,” Bowman reported.

Further research is needed to clarify how the virus is transmitted from humans to white-tailed deer and assess the likelihood of mutated variants to spill back into humans. 

Although no substantial outbreaks of deer-origin strains have occurred in humans until now, circulation among animals remains very likely. Moreover, since about 70 percent of free-ranging deer in Ohio have not been exposed to the virus yet, there is a large number of immunologically-naïve animals that the virus could spread through uninhibited.

“Having that animal host in play creates things we need to watch out for. If this trajectory continues for years and we have a virus that becomes deer-adapted, then does that become the pathway into other animal hosts, wildlife or domestic? We just don’t know,” Bowman concluded.

The study is published in the journal Nature Communications. 

By Andrei Ionescu, Staff Writer

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