COVID-19 has been devastating for people across the world, with more than 5 million confirmed cases globally. However, humans are not the only species that the SARS-CoV-2 virus can infect. In this Special Feature, we discuss recent research into animal populations that might harbor the virus.
Researchers believe that SARS-CoV-2 is a type of zoonosis — a disease that transfers from nonhuman animals to humans, often via an intermediary nonhuman animal with which humans have contact.
To date, scientists are not sure which species acted as the originator or the intermediary. However, all known human coronaviruses have their origins in nonhuman animals.
Experts believe that the meat markets of Wuhan, China, provided an opportunity for the SARS-CoV-2 virus to transfer from nonhuman animals to humans, as was the case with the original SARS-CoV virus in meat markets in China in 2002 and 2003.
As well as emerging from nonhuman animals, there is also evidence that SARS-CoV-2 has returned to other animal species.
The Centers for Disease Control and Prevention (CDC) note that the virus has infected pets, animals in zoos and sanctuaries, and mink on farms.
White-tailed deer infection
Multiple recently-released reports have documented that SARS-CoV-2 has now also spread to white-tailed deer in the United States.
In a brief report in the Proceedings of the National Academy of Sciences of the United States of America, researchers tested the blood of 624 deer from four U.S. states before and during the pandemic. They found that 40% of the samples taken since the pandemic began contained SARS-CoV-2 antibodies.
In a preprint study that is yet to undergo peer review, researchers report that they detected SARS-CoV-2 in 129 out of 360 deer in northeast Ohio, using a real-time reverse transcriptase-polymerase chain reaction (RT-PCR) test.
In another preprint study, researchers used RT-PCR tests on lymph node samples that they had taken from 283 captive and wild deer. One-third of the samples were positive for SARS-CoV-2.
A concern
The presence of SARS-CoV-2 in animal populations is a concern for scientists, as it raises the possibility that a new variant of the disease, which could potentially be more dangerous, could cross back into human populations.
Speaking with Medical News Today, Dr. Graeme Shannon — a lecturer in zoology at the School of Natural Sciences in Bangor University, Wales — said:
“Animal reservoirs have the potential to generate mutations that the human immune system has not come into contact with before. We see this regularly with influenzas that hop readily from birds and a number of mammals back into humans.”
“However, equally, the disease may infect wildlife and mutate but become less of a threat to humans as it adapts to the biology of the current host.”
“Certainly, the presence of multiple animal reservoirs on top of the high prevalence of the disease in humans would be cause for concern. This could complicate our attempts to suppress the disease. Indeed, we have already seen that infected captive mink were able to reinfect farmworkers,” said Dr. Shannon.
Origin of the infection?
Scientists are not yet sure how the deer became infected with SARS-CoV-2.
Prof. Vivek Kapur told MNT that there could be multiple ways in which the deer became infected but that direct hunting interactions were unlikely.
“While there are likely many sources with which the spillovers to deer from people may occur, including through contact with contaminated food — for example, a contaminated half-eaten apple thrown in the woods or contaminated bait or food left for deer in urban settings — [or a] contaminated environment — a discarded tissue, spit, or other bodily fluids from hunters or hikers in the forest — or even a yet undiscovered intermediary host such as deer mouse.”
Prof. Kapur is a professor of microbiology and infectious diseases and associate director of the Huck Institutes of the Life Sciences at Pennsylvania State University. He also co-authored one of the preprint studies mentioned earlier.
“We have no evidence that hunting interactions are the primary mode of transmission,” he explained. However, he also added that “hunting may contribute through the larger number of people on public lands where there are deer, and [it] also causes dispersal and mixing of deer that may enhance opportunities for transmission.”
Prof. Kapur also said that it is likely that the virus can infect other deer species.
“There is considerable evidence from natural or experimental infection with SARS-CoV-2 of many different animal species, and based on the structure of the ACE-2 receptor targeted by the viral spike protein, other cervid species are highly likely to be able to get infected.”
According to Dr. Shannon, the new reports are striking because they show that SARS-CoV-2 can circulate in wild animal populations.
“We are currently aware that SARS-CoV-2 can be transmitted to domestic animals such as cats and dogs, as well as captive species, particularly farmed mink. There are also reports of the virus in zoo animals.”
“I think what is really interesting about the latest studies from the U.S. is that there is now solid evidence that SARS-CoV-2 can be transmitted to and among free-ranging mammals.”
“Another point worth considering is that although these species are all mammals, they are quite phylogenetically distinct and come from a range of taxonomic families, demonstrating that the disease is not specific to one host — or even similarly related hosts.”
– Dr. Shannon
“This is likely further exacerbated by the prevalence of the disease in the human population, which presents multiple opportunities for SARS-CoV-2 to infect other animal species,” he added.
Other potential reservoirs?
Speaking with MNT, Dr. Eman Anis — assistant professor of pathobiology at the School of Veterinary Medicine at the University of Pennsylvania in Philadelphia — said there may be other animal species within which the virus could be circulating.
“We do not know the exact reservoir, or reservoirs, of SARS-CoV-2, so yes, there is a possibility that there are animal reservoirs that we may not be aware of.”
“To date, several animal species have been considered as potential reservoirs of the virus, such as civets and pangolins,” she explained. “White-tailed deer were not considered a potential reservoir of the virus until researchers detected the virus in deer populations across several states.”
“What we do know, however, is that any animal species that has the capacity to maintain the virus permanently and potentially spread it to humans or other domestic or wild animals could be a potential reservoir of SARS-CoV-2.”
“We will not be able to determine exact reservoirs and the true host range of SARS-CoV-2 until we conduct extensive surveillance on both domestic and wild animals to determine which species can permanently maintain the virus and spread the infection to other animals and humans.”
– Dr. Anis
Crucially, scientists are not yet sure whether SARS-CoV-2 circulating in deer populations can then return to humans.
Prof. Kapur told MNT that “we strongly recommend expanding surveillance for the virus in other peri-domestic and free-living species to better understand the risks associated with spillover of infections to other potential reservoir species and opportunities for spill back to humans.”
Further research needed
Dr. Roderick Gagne is an assistant professor of wildlife disease ecology at the School of Veterinary Medicine at the University of Pennsylvania. He told MNT that more research was necessary to build a better understanding of the risks that come with SARS-CoV-2 circulating in white-tailed deer.
“Because the virus has only recently been discovered in deer, no research has been done on the spillover risk to humans. We first must gain a better understanding of how common the virus is in deer, what variants are circulating in deer, and if deer can indefinitely maintain the virus and shed it in high titer/load.”
“Further research is required to compare the genetic relationships between SARS-CoV-2 recovered from deer and those recovered from humans at the same time.”
“In addition, epidemiological studies would need to evaluate the potential for deer to infect humans — for example, by monitoring those caring for captive deer,” said Dr. Gagne.
Speaking with MNT, Gail Keirn — public affairs specialist for the National Wildlife Research Center, U.S. Department of Agriculture Animal and Plant Health Inspection Service — also suggested that further research was essential to understand the significance of these initial reports.
“Research and surveillance are needed to determine 1) when and where white-tailed deer are being exposed to the virus, 2) if the virus is circulating in deer populations, 3) if new variants of the virus are emerging in deer, and 4) what is the risk, if any, to deer, other animals, and people,” said Keirn.
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