COVID-19 Virus Is Rapidly Evolving in White-Tailed Deer
By OHIO STATE UNIVERSITY
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| Photo by Will Collette |
Scientists collected 1,522 nasal swabs from
free-ranging deer in 83 of the state’s 88 counties between November 2021 and
March 2022. More than 10% of the samples were positive for the SARS-CoV-2 virus, and at least one positive case was
found in 59% of the counties in which testing took place.
Genomic Analysis and Findings
Genomic analysis showed that at least 30
infections in deer had been introduced by humans – a figure that surprised the
research team.
“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 Andrew Bowman, associate professor of veterinary preventive
medicine at The Ohio State University and co-senior author of the study.
“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.”
The combined findings suggest that the
white-tailed deer species is a reservoir for
SARS-CoV-2 that enables continuing mutation, and that the virus’s circulation
in deer could lead to its spread to other wildlife and livestock.
The study is published on August 28,
2023 in the journal Nature Communications.
Previous Observations and Expansions
Bowman and colleagues previously reported detection
of SARS-CoV-2 infections in white-tailed deer in nine Ohio locations in
December 2021, and are continuing to monitor deer for infection by more recent
variants.
“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 said. “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.”
Beyond the detection of active infections,
researchers also found through blood samples containing antibodies – indicating
previous exposure to the virus – that an estimated 23.5% of deer in Ohio had
been infected at one time or another.
Variant Analysis
The 80 whole-genome sequences obtained from
the collected samples represented groups of various viral variants: the highly
contagious delta variant, the predominant human strain in the United States in
the early fall of 2021 that accounted for almost 90% of the sequences, and
alpha, the first named variant of concern that had circulated in humans in the
spring of 2021.
The analysis revealed that the genetic
composition of delta variants in deer matched dominant lineages found in humans
at the time, pointing to the spillover events, and that deer-to-deer
transmission followed in clusters, some 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,” Bowman said. “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.”
The study did suggest that COVID-19
vaccination is likely to help protect people against severe disease in the
event of a spillover back to humans. An analysis of the effects of deer
variants on Siberian hamsters, an animal model for SARS-CoV-2 studies, showed
that vaccinated hamsters did not get as sick from infection as unvaccinated
animals.
Rapid Evolution in Deer
Disturbingly, the variants circulating in
deer are expected to continue to change. An investigation of the mutations
found in the samples provided evidence of more rapid evolution of both alpha
and delta variants in deer compared to 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 said.
How the virus is transmitted from humans to
white-tailed deer remains a mystery. And so far, even with about 30 million
free-ranging deer in the U.S., no substantial outbreaks of deer-origin strains
have occurred in humans.
Potential Implications
Circulation among animals, however, remains highly
likely. Bowman noted that about 70% of free-ranging deer in Ohio have not been
infected or exposed to the virus, “so that’s a large body of naive animals that
the virus could spread through rather uninhibited.”
“Having that animal host in play creates
things we need to watch out for,” he said. “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.”
Reference: “Accelerated evolution of
SARS-CoV-2 in free-ranging white-tailed deer” 28 August 2023, Nature Communications.
DOI:
10.1038/s41467-023-40706-y
Martha Nelson of the National Library of
Medicine was co-corresponding author of the study. Ohio State co-authors Dillon
McBride, Steven Overend, Devra Huey, Amanda Williams, Seth Faith and Jacqueline
Nolting worked on the study with co-authors from St. Jude Children’s Research Hospital;
the University of California, Los Angeles; the National Research Centre in
Giza, Egypt; PathAI Diagnostics; the Ohio Department of Natural Resources; the
U.S. Department of Agriculture; Columbus and Franklin County Metroparks; and
the Rega Institute for Medical Research in Belgium.
This work was supported by the National
Institute of Allergy and Infectious Diseases and Ohio State’s Infectious
Diseases Institute.