REGIONAL- The origins of the COVID-19 virus remain unclear, although the most promising theory points to a transfer of the virus from a species of bat in China to humans sometime in 2019.
Researchers determined quickly afterward that the virus was able to spread inter-species, as isolated cases of positive COVID tests in cats and dogs began to appear. Some pets showed COVID-like effects, but scientists were unable to conclude definitively at that time if the virus was the direct cause of the symptoms.
Scientists began to pay particular attention to the phenomenon of human-to-animal COVID transmission as more dogs and cats, as well as hamsters, ferrets, mink, otters, lions, tigers, leopards, hyenas, white-tailed deer and even manatees were diagnosed with the virus. This research continues to the present day, with $4.5 million awarded just last week to Penn State University to test for COVID-19 in 58 different species of wildlife animals. More than 20,000 samples will be collected from wildlife such as chipmunks, gray squirrels, raccoons, coyotes, moose, and three species of deer.
Today, the concern related to human-to-animal transmission is that wildlife could serve as a reservoir for COVID strains to survive, mutate, and re-infect humans. The risk is considered low for now but isn’t fully understood.
Deer studies
After federal and academic researchers discovered infections in captive white-tailed deer, they focused their attention on wild deer. A pilot blood sampling study in four states found that 40 percent of those samples contained COVID antibodies, indicating those deer had been exposed to the virus.
The first year of an extensive study of white-tailed deer in the U.S. reported in June by the U.S. Department of Agriculture sampled over 11,000 specimens and detected active COVID virus in 12.2 percent of them. One in three had antibodies from previous COVID exposure. The research showed that the virus was transmitted from humans to the deer, mutated, and was potentially transmitted back to humans.
Another study conducted by the University of Missouri in collaboration with Ohio State University and state and tribal wildlife agencies collected more than 9,000 respiratory samples from white-tailed deer in 27 states and found that COVID had been transmitted from humans to the deer 106 times, with three instances in which the virus was possibly transmitted back to humans.
“Deer regularly interact with humans and are commonly found in human environments — near our homes, pets, wastewater, and trash,” said University of Missouri professor Dr. Xiu-Feng Wan. “The potential for SARS-CoV-2, or any zoonotic disease, to persist and evolve in wildlife populations can pose unique public health risks.”
Of concern in the most recent deer research released by Ohio State researchers in August was the presence of the Alpha COVID variant in ten percent of the samples, eight-to-12 months after that variant’s dominance in humans.
“We have Alpha lineage persisting in deer for over a year,” researcher Andrew Bowman, DVM, PhD said in an article published by the University of Minnesota Center for Infectious Disease Research and Policy. “That’s potentially creating a reservoir of divergent strains that could come back into the human population. That is the thing we need to keep an eye on.”
Also, the rate of evolution of the virus leading to new variants was approximately three times higher in deer for the Alpha variant, and 2.7 times higher for the Delta variant, which was also found in sampling.
“When the virus jumps from one species to another, it is exposed to a new set of selective pressures. These pressures can favor the emergence of new variants that are better adapted to the new host,” said Jonathon Heale, a USDA staff biologist.
But white-tailed deer may pose little current risk of sparking a COVID outbreak in humans, as in the second year of the USDA study the prevalence of COVID in white-tailed deer dropped from 12.2 percent to 1.6 percent. Bowman suggested the drop could be related to the Omicron variant that caused the pandemic’s largest spike in cases in January 2022 and that has spawned subsequent variants driving caseloads through the present day.
“We’ve seen some early experimental work that deer seem to be less susceptible to Omicron lineages, and the virus doesn’t replicate as well as older lineages,” Bowman said.
But while the incidence of COVID in white-tailed deer has declined, the dynamics of transmission, incubation, and mutation present a cautionary tale of the potential public health threat reservoirs of animal COVID infections originating from human populations may pose in the future. For a virus most believe originated in wild animals, much is still to be learned about how animals may aid in preserving and propagating it back to humans once more.
“More research is needed to understand the full extent of the role that spillover events play in the evolution of SARS-CoV-2,” Heale said.