Earlier this year, researchers found that several wild deer in Michigan had antibodies indicating the animals had been exposed to SARS-CoV-2, the virus that causes COVID-19. It has been a major cause for concern, as a large number of susceptible animals could act as a reservoir allowing the virus to spread back to humans.
But at the time, doubts prevailed. The study looked at only a small sample of deer populations in one state — we didn’t know how the animals were exposed, and we didn’t know if the virus was actually spreading among wild deer. Since then, a few blanks have been filled in. Crucially, deer-to-deer transmission has been observed in captivity. On Monday, a new preliminary paper answered some other questions, showing that the infection is widespread in a second case, driven by its spread from humans and deer-to-deer transmission.
In general, the news is not particularly good, although we still do not understand the risks it may pose to humans.
There are still many unknowns about the spread of SARS-CoV-2 to deer, and its transmission is among them. We know that transmission to deer is possible, but we don’t have a strong sense of how often the virus jumps from humans to deer or whether it happens directly or via an intermediary species. Once infected, deer show some obvious symptoms, but the virus has a similar time course: it can be detected after four days and remains detectable for three weeks after infection.
The team included members from the Iowa Department of Natural Resources who were inspired by the findings in Michigan to see what was going on in their state. They’ve partnered with a variety of academic researchers to test both wild deer and deer in game reserves, many of them killed by poachers or cars. Their study began in April of 2020, but most of the 283 samples came after the hunting season began in September; Things concluded in early January 2021.
To some extent, the sampling process coincided with the initial trajectory of the epidemic in Iowa, which saw its first spike in cases in April of 2020 before peaking at the end of the year. Finally, 94 of the deer (33 percent) tested positive over the course of the study. But this number obscures some obvious changes over time.
In deer, at least, the virus was not detected at all until after the hunting season began, although the sampling rate in this period was too low to see how significant this finding was. But the first positive deer was detected soon after the hunting season began, and numbers rose dramatically as cases in humans rose. By December, more than 80 percent of the deer tested tested positive for the virus.
How did this get here?
Sampling rates during the months of the hunting season have remained relatively constant, so the increase in cases appears to be real. But it is not possible to know how much of this contribution came from having more hunters among the deer and how much from having more people around with the virus. Hunting can also change the deer’s behavior in a way that makes it easier for them to spread it among themselves.
To help figure out what might be going on, the research team obtained genomic sequences from the virus in all of their samples, allowing them to trace individual lineages of the virus and compare viruses found in deer to those in surrounding humans.
The research team found dozens of different strains of SARS-CoV-2 in the deer community. The most common was also the most common of the strains circulating in humans in Iowa at the same time. Furthermore, there were some similarities between the deer subspecies and humans, but the authors caution that the results represent a small sample from both groups, because genome sequencing for humans was not common at the time either.