Up to 40 percent of white-tailed deer living wild in parts of the northeast US appear to have been exposed to the coronavirus behind COVID-19, according to a preliminary new antibody survey by the US Department of Agriculture’s Animal and Plant Health Inspection Service (USDA-APHIS).
Luckily, none of the deer appear to be visibly suffering from the disease, but it's concerning as the first evidence for widespread exposure to SARS-CoV-2 in wild animals potentially from human-to-animal transmission. It raises worries of another common animal that could potentially act as a reservoir for SARS-CoV-2, ready to spill back into humans and spark new flare-ups of the disease.
The preliminary study, which has not yet been peer-reviewed, was published on the pre-print site bioRxiv on July 29.
Scientists analyzed 624 serum samples collected both before and after COVID-19 took hold from wild deer living in Illinois, New York, Michigan, and Pennsylvania. Of the samples collected in 2021, 152 samples (40 percent) appeared to contain antibodies against SARS-CoV-2, indicating the wild deer had been exposed to the virus. State by state, the prevalence of positive cases was 67 percent of samples in Michigan, 31 percent in New York, 44 percent in Pennsylvania, and 7 percent in Illinois.
The researchers also found three positive samples from January 2020, very early in the pandemic. Altogether, around one-third of samples from 2020 and 2021 had antibodies for SARS-CoV-2. They also found one positive sample from 2019. Currently, there’s no concrete evidence that the virus was in the US at this time and it's possible this case could simply be a false-positive result.
All of these results need further confirmation, but it appears that a surprising number of wild deer had some exposure to SARS-CoV-2. Even if a few false positives have crept into the results, the likelihood of over 150 errors seems remarkably slim.
"The finding that wild white-tailed deer have been exposed to SARS-CoV-2 is not unexpected given that white-tailed deer are susceptible to the virus, are abundant in the United States, often come into close contact with people, and that the CDC estimates that more than 114 million Americans have been infected with SARS-CoV-2," the USDA-APHIS said in a statement given to IFLScience.
How this exposure occurred remains unclear. The study suggests it's possible the deer came into contact with the virus directly through humans engaging in hunting, field research, conservation work, wildlife tourism, and so on. Alternatively, contaminated water sources could be a potential transmission route. Another likely candidate is minks that have escaped from farms where there have been COVID-19 outbreaks.
Deer are far from the only animal that can be infected with SARS-CoV-2. There have been reported infections in a range of captive animals and pets, including dogs, cats, tigers, gorillas, and minks, to name just a few. Outbreaks among farmed and domestic animals have caused some worries, although the outbreaks can be kept in check through both vaccination and culling. When it comes to wild animals, like deer, it can be much harder to control.
While the risk of deer-to-human transmission is assumed to be low, it's possible that deer — as well as other wild and domesticated animals — could silently harbor the virus and spark outbreaks of COVID-19 in the future. For now, however, more research is needed until anything of this can be confirmed.
"Currently, there is no evidence that animals play a significant role in spreading SARS-CoV-2, the virus that causes COVID-19, to people. The risk of animals, including deer, spreading COVID-19 to people is low," the USDA added.
Updated 04/08/2021: This article has been updated to include a statement from the USDA.