Earlier this year, researchers found that many wild deer in Michigan had covid-19 antibodies, suggesting that the animals had been exposed to SARS-COV-2 (the virus that causes covid-19). This is an important reason to be concerned, as the large number of susceptible animals can serve as a reservoir for the virus to be re-transmitted to humans.
At the time, however, uncertainties abounded. The study looked at only a small sample of a state's deer herd, and the researchers didn't know how the animals were exposed or whether the virus actually spread in wild deer. Since then, some gaps have been filled. Importantly, researchers have observed transmission from deer to deer in captivity. On Monday, a preprint of a new paper answered some of the more questions, suggesting that infection is widespread in the second state, driven by its spread from humans and from deer to deer.
Overall, the news isn't particularly good, though scientists still don't understand the risks it could pose to humans.
There are still many unknowns about the spread of SARS-cov-2 to deer and its spread among deer. Scientists know that it is possible to transmit to deer, but it is not clear how often the virus spreads from humans to deer, or whether it spreads directly or through intermediate species. Once infected, deer have few obvious symptoms, but the virus has a similar time course: it is detectable after four days and still within three weeks of infection.
The team, which included members of the Iowa Department of Natural Resources, was inspired by the Michigan study to see how things were going in their state. They collaborated with a variety of academic researchers to test wild deer and deer in wildlife sanctuaries, many of which were killed by hunters or cars. Their study began in April 2020, but most of the 283 samples emerged after the start of the hunting season in September; things ended in early January 2021.
In part, sampling is similar to the initial course of the Iowa pandemic, where it first saw a surge in cases in April 2020 and then peaked at the end of the year. Overall, 94 deer (33 percent) tested positive over the course of the study. But this number masks some of the noticeable changes over time.
At least in deer, the virus was not detected until after the start of the hunting season, although the sampling rate during this period was too low to determine how significant the finding would be. But the first positive deer were detected shortly after the hunting season began, and the numbers increased dramatically as cases in the human population surged. By December, more than 80 percent of deer tested were positive for the virus.
How did this come about?
Sampling rates remain relatively stable during the months of hunting season, so the surge in cases appears to be real. But researchers can't say how much of the contribution came from hunter activities in deer herds and how much from the presence of humans infected with the virus in large numbers. Hunting may also change deer behavior, making it easier to spread the virus between them.
To help figure out what might happen, the team obtained the genome sequence of the virus from all of their samples, allowing them to trace individual strains of the virus and compare the virus in the deer's body to that of surrounding humans.
The team found more than a dozen different SARS-COV-2 strains in the deer herd. The most common of these are also the most common of the strains of virus circulating in Iowa humans during the same period. Among other things, there are some similarities between deer and human strains, but the authors caution that the results represent only a small sample of the two populations, as human genome sequencing was not common at the time.
In any case, it is clear that deer from the same location are almost always infected by the same variant, and in some cases, the genome of the virus is the same. The findings provide strong evidence that the virus is spreading in deer herds in the wild. The fact that different variants appeared in different locations suggests that these cases are the product of inter-species infection events. As a result, the virus appears to be able to spread regularly from humans to deer. The relatively short lag between the surge in human cases and the high positive levels of deer seems to make intermediate carrier species less likely.
While the virus doesn't cause much noticeable symptoms to deer, they become contagious within four days of infection, and the virus can still be detected within three weeks of infection.
What does this mean?
Researchers have not yet determined whether the virus can be transferred from deer to humans. Hunters appear to be at the greatest risk, but there is no indication that their health is being tracked after they are involved in sampling.
That is, deer can provide an important host for viruses. The species studied here is estimated at 25 million in North America, many of which have ungulates found in densely populated areas of humans. The two populations also interact frequently due to hunting and traffic accidents.
If there is an opportunity to pass it back on to humans, it carries some risks. Deer are clearly not affected by any public health measures used to limit the spread of the virus, so it is likely that covid will spread uncontrolled among deer herds, which may provide a pathway for the virus to re-enter human populations with little infection.
In addition, the virus may evolve in its new host, with unpredictable consequences for humans. Or, the way the virus evolved may exacerbate its spread or the symptoms it causes in human hosts. Finally, any of these changes may alter their immune status, limiting the protection provided by the vaccine or previous infection.
All of this means that researchers need to understand the risk of this form of interspecies infection as soon as possible.