The white-tailed deer is a large reservoir of coronavirus infection

New research from the US has shown that white-tailed deer are infected with SARS-CoV-2, the virus that causes COVID-19 in humans. Antibodies were found in 40% of deer that were tested from January to March 2021 in Michigan, Pennsylvania, Illinois, and New York State. A second, unpublished study detected the virus in 80% of deer sampled in Iowa between November 2020 and January 2021.

Such high levels of infection led the researchers to conclude that deer are actively transmitting the virus to each other. The scientists also identified different variants of SARS-CoV-2, suggesting that there have been many infections between humans and deer.

The large numbers of white-tailed deer in North America and the fact that they often live close to people provide several opportunities for the disease to move between the two species. This can include wildlife management operations, field research, recreation, tourism, and hunting. In fact, hunters are likely one of the most obvious sources of possible reinfections, as they often handle dead animals. It has also been suggested that water sources contaminated with SARS-CoV-2 could provide a route of transmission, although this has yet to be proven.

Human-to-deer and deer-to-deer transmission is believed to be driving the rapid spread of the disease among white-tailed deer populations in the U.S. This is particularly evident during the early months of 2021 when infections by COVID increased in the human population. Previous studies have shown that SARS-CoV-2 can be transmitted from humans to domestic and captive animals, including cats, dogs, zoo animals, and especially farm minks. But, until now, the disease has not been shown to spread among wild species.

The white-tailed deer is the most abundant large mammal in North America with a range stretching from Canada to South America. The population of the United States alone is estimated to be 30 million animals. They are a social species that live in family groups of two to 12 individuals that can thrive in a variety of habitats, including urban parks and forests.

A white-tailed deer is found in the Fort Lee Historical Park in freezing temperatures, in front of the George Washington Bridge in Fort Lee, NJ, on January 8, 2015 (credit: REUTERS / MIKE SEGAR).

These aspects of their ecology and behavior have made them a species of particular concern when it comes to the spread of diseases, including bovine tuberculosis and chronic wasting. These pathogens have already had considerable effects on the health of domestic and wild animal populations around the world.

Findings from these latest studies have raised concerns that the white-tailed deer could be a reservoir for SARS-CoV-2. Not only could this easily infect large numbers of animals, but more worryingly, it could also spread to humans.

This type of infection cycle was documented in infected mink farm workers, ultimately leading the Danish government to euthanize its entire captive population of 17 million animals. It is important to note that there is currently no evidence of transmission of SARS-CoV-2 from white-tailed deer to humans. Initial experimental work also suggests that infected deer tend to be asymptomatic. Still, disease transmission in wildlife populations has considerable implications for human and animal health.

The possibility exists that a viral mutation in a reservoir host, such as the white-tailed deer, could lead to new variants of the disease. These variants can lead to higher infection rates, greater virulence (severity of symptoms), and be more effective in evading the human immune system. Similarly, any reinfection of wildlife reservoirs could also complicate our long-term efforts to combat and suppress the disease.

Influenza, which jumps easily between birds, humans, and other mammals (particularly pigs), presented similar problems. These multiple disease reservoirs can lead to the emergence of new strains against which humans have lower immunity, as was the case with swine flu in 2009.

It is important to note that there are limitations to these studies, both in terms of the methods used and the limited geographic range of research. The most recent and unpublished study used the latest genetic approaches to reliably detect SARS-CoV-2 in tissue samples, but it focused only on deer in Iowa. While antibody testing in the first study was performed on four states, but only show that the animal has been exposed to the virus. However, the combined findings have highlighted that transmission of SARS-CoV-2 is likely widespread in white-tailed deer.

There is much we still need to learn about the developing situation with COVID and deer. The most important topics to focus on include understanding how the virus is transmitted from humans to deer and determining the risk of spread to the human population. Research is urgently needed to assess the risk posed by this potential reservoir of SARS-CoV-2 to humans, as well as the possible spread of the virus to other wildlife species that deer interact with, such as predators and scavengers.

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