COVID in Deer: What Is the Risk to Humans?

The virus that causes COVID-19, SARS-CoV-2, has been identified in wild white-tailed deer populations across numerous American states. Studies confirm the presence of active infections and antibodies, indicating past exposure. A U.S. Department of Agriculture (USDA) study found that 40% of deer sampled in four states had antibodies. In Iowa, the infection rate was even higher, with up to 80% of deer in some areas testing positive for the virus during the winter of 2020-2021. This discovery has prompted scientific inquiry into how the virus behaves in this species.

Virus Transmission in Deer Populations

The initial jump of SARS-CoV-2 from humans to deer, known as spillover, has occurred numerous times. Genomic analysis identified at least 30 separate instances of human-to-deer transmission in Ohio alone. Scientists suggest this could happen through environmental contamination, such as deer encountering human waste or contaminated water sources. The virus’s prevalence is higher in deer populations near regions with more crop coverage and proximity to urban centers, suggesting a link to human activity.

Once the virus enters a deer population, it spreads efficiently from animal to animal. Experimental studies show that infected deer shed the virus in their nasal secretions and feces, allowing for transmission through indirect contact. The social nature of white-tailed deer, which often live in groups and groom each other, facilitates this rapid spread.

The susceptibility of deer to the virus is linked to their ACE2 receptors, the proteins on cell surfaces that the virus uses for entry. These receptors are structurally similar to human ACE2 receptors, making infection easier. Though infected, the deer do not show obvious signs of illness. Experimental infections resulted in no overt symptoms, although a brief, transient fever was sometimes observed.

Deer as a Potential Viral Reservoir

The widespread circulation of SARS-CoV-2 has established white-tailed deer as a viral reservoir. A reservoir is an animal population where a pathogen can survive long-term, providing it with a continuous foothold in nature. This allows the virus to persist and evolve independently from the strains circulating in the human population. The World Health Organization (WHO) has cited white-tailed deer as a newly formed wildlife reservoir for the virus.

As the virus replicates within deer, it undergoes genetic mutations, leading to new variants. Research has shown that the virus evolves approximately three times faster in deer than in humans. This accelerated evolution means that deer-adapted variants could acquire unique mutations. For example, scientists found that older variants, such as Alpha, persisted in deer populations long after they stopped being the dominant strain in people.

Spillback Risk to Humans and Other Species

The circulation of SARS-CoV-2 in deer raises the question of “spillback,” which is the transmission of the virus from an animal population back to humans. While the initial transmission was from humans to deer, evidence now suggests that deer-to-human transmission has likely occurred. One analysis identified at least three instances where mutated variants from deer may have spread back to people.

Despite these findings, public health and animal health agencies assess that animals, including deer, do not play a significant role in spreading the virus to humans. The risk of an individual contracting COVID-19 from a deer is considered low. There have been no confirmed cases of a person getting sick from preparing or eating wild game meat. The primary mode of SARS-CoV-2 transmission remains person-to-person.

However, the existence of a large wildlife reservoir means the potential for spillback events, while currently low, cannot be dismissed. The ongoing evolution of the virus in deer could produce a variant that is more easily transmitted back to humans.

Surveillance and Safety Recommendations

Government agencies have established surveillance programs for SARS-CoV-2 in deer. The USDA’s Animal and Plant Health Inspection Service (APHIS) leads a multi-year effort, collaborating with state and tribal wildlife agencies to monitor the virus in deer and other cervids like mule deer and elk. This involves testing samples from hunter-harvested animals and roadkill to track the virus and identify variants.

These surveillance efforts aim to understand the scope of the infection and assess potential risks to animal and public health. By monitoring the virus’s evolution in deer, scientists can identify changes that might make it more of a threat. The American Rescue Plan has provided funding for this extensive monitoring.

For individuals who have close contact with deer, such as hunters and wildlife biologists, safety precautions are recommended. Standard best practices for handling wildlife should be followed, including wearing gloves when processing animals and practicing good hygiene by washing hands and equipment thoroughly. Enjoying wildlife from a distance is advised, as wild animals can carry diseases without appearing sick.