In a groundbreaking analysis of thousands of samples collected from wild deer, it has been suggested that Americans have transmitted COVID-19 to these animals hundreds of times. Furthermore, it has been discovered that mutated variants from deer have been caught and spread by humans at least three times.

The alarming findings came from the first year of a multiyear federal effort led by the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS) to study the virus as it spreads into American wildlife.

Published in the journal Nature Communications, the study focused on analyzing 8,830 samples collected from wild white-tailed deer across 26 states and Washington, D.C., between November 2021 and April 2022. The goal was to study the COVID variants that infected 282 deer. By comparing the virus sequences in deer to publicly reported samples from human infections worldwide, scientists were able to trace the likely spread of the variants between animals and humans.

The analysis identified a total of 109 "independent spillover events," linking viruses found in deer to predecessors that likely originated from previously infected humans. Some of the viruses are still mutating and spreading among deer, including the Alpha, Gamma and Delta variants of concern that caused a surge in deaths earlier in the pandemic. It is noteworthy that these lineages persisted even after the dominance of the Omicron variants that currently prevail nationwide, according to CBS News.

Interestingly, 18 samples did not have genetically close human SARS-CoV-2 sequences within the same state, making it challenging to track down the precursor variants in humans.

The research team emphasized that frequent introductions of new human viruses into free-ranging white-tailed deer continued to occur, with SARS-CoV-2 variants of concern capable of persisting in deer even after becoming rare in the human population. They identified three cases where the virus spilled over from humans to deer and then spilled back into humans, leading to infections.

Two of the spillback variants were found in North Carolina, while one was identified in Massachusetts. A subsequent investigation by the Centers for Disease Control and Prevention (CDC) located three individuals infected by a variant carrying the distinctive deer mutation, along with a few zoo lions infected by the same strain. Surprisingly, none of the infected individuals reported any close contact with either deer or the zoo.

APHIS researchers have been studying the potential of white-tailed deer, among other American wildlife species, to serve as long-term reservoirs for the virus, facilitating further mutations and spread among deer populations. A previous report from Canadian scientists identified a highly divergent lineage of SARS-CoV-2 that spread from deer to humans.

The implications of the virus spreading between humans and wildlife raise concerns among government scientists. Deer frequently interact with humans and are commonly found in human environments, such as near homes, pets, wastewater and trash.

In a news release, Professor Xiu-Feng Wan from the University of Missouri, one of the paper's authors, emphasized the risks associated with zoonotic diseases like SARS-CoV-2 and the need to minimize close contact with wildlife and their droppings.

The persistence and evolution of SARS-CoV-2, or any zoonotic disease in wildlife populations, present unique public health risks that demand further attention and preventive measures, as suggested in the study.