I’ve been a laggard with blog posts lately. It’s a bit crazy and it’s been easier to get things out quickly via Twitter (@weese_scott). However, one interesting topic I wanted to get back to is SARS-CoV-2 in deer. It’s caused a big stink in some areas and, like a lot of things involving this virus and animals, we don’t really know the big picture implications yet. I wrote about SARS-CoV-2 in deer a few months ago, but some new data have come out recently that raise even more issues.
Earlier this year, there was an experimental study that showed white tailed deer are susceptible to SARS-CoV-2 and, importantly, are able to spread it deer-to-deer. At the time that put deer onto a fairly large and growing list of species that are susceptible this virus, but there are some specific concerns with a wildlife species like deer. We really don’t want SARS-CoV-2 to become established in wildlife, as that creates the potential for a reservoir of infection for people and other species, and the potential for emergence of new mutants (variants of concern) that could cause issues if they spread back into people (spillback). One key thing that’s needed for an animal reservoir is a large enough group of susceptible individuals to allow continued circulation of the virus, and there are in fact large relatively mobile populations of deer across wide ranges in North America.
However, “susceptible” and “relevant” aren’t the same, and we didn’t know whether there was much chance deer were actually going to be exposed to SARS-CoV-2, and how likely infection in deer would be in the real world. The answer surprised us (or at least me).
SARS-CoV-2 in deer went from a theoretical risk to a real world issue when the USDA released a study reporting high rates of seropositivity (antibodies against the virus in blood) in deer in various parts of the US. Overall, 40% of tested deer from 4 US states had evidence of previous infection. A subset of samples was also tested with the standard virus neutralization test, with good agreement. That, and the lack of positive results in samples collected pre-pandemic (see figure to right) suggested the results were reliable.
Some recent pre-print studies have taken this issue a step further.
Hale et al. (2021) reported detection of SARS-CoV-2 by PCR in 36% (129/360) of deer sampled in northeast Ohio between January and March 2021. Interestingly, at least three different lineages (strains) of virus were identified, meaning at least three different introductions into the deer population. As expected, they were lineages common in people in the same areas, and people were the most likely source of the introductions.
Kuchipudi et al. (2021) performed PCR testing on retropharyngeal lymph nodes (i.e. the lymph nodes at the back of the throat) from wild and captive deer in Iowa. SARS-CoV-2 RNA was identified in 33% of samples from April to December 2020, and a whopping 83% of samples from November 23, 2020 to January 10, 2021. Twelve different lineages were identified. None of 17 samples collected earlier (April-August 2020) were positive.
Does this mean people can get infected with SARS-CoV-2 from deer?
- We don’t know.
Does this mean the SARS-CoV-2 virus is actively circulating in deer?
- We don’t know.
- High rates of seropositivity could mean that the virus gets into a deer population, spreads quickly, infects a lot of deer but then burns out and disappears. That would be good. However, it could also mean that the virus circulates through the population as it continues to find new susceptible deer at a slower pace. This is a big question that needs to be answered.
Does this mean the SARS-CoV-2 virus is likely in deer populations elsewhere?
- We don’t know. (Sensing a pattern here?)
- It seems likely and it’s being actively investigated. So far it’s only been found in deer in the US.
Are “deer variants” of SARS-CoV-2 being created?
- We don’t know. None have been found to date.
- The likelihood of “deer variants” emerging really depends on the amount of virus circulation in the deer population. Mutations occur during virus replication. The more transmission, the more replication and the greater the risk of variants emerging.
What’s the risk to the general public from SARS-CoV-2 in deer?
- Pretty low. (I could say “we don’t know” but I’ll mix it up.)
- Few people have close contact with deer (although every time I say or write that, I get flooded with emailed pictures showing deer hanging out on front lawns, porches, lounging in the yard with dogs, etc. so it may be more common for some than most).
- It’s direct contact or close range aerosol transmission that I’m worried about, and since deer are outdoors in well ventilated spaces, simply being in the same area as deer shouldn’t pose any risk.
What’s the risk to hunters from SARS-CoV-2 in deer?
- That’s a big question. Hunters will have the closest direct contact with deer and have the potential for exposure to respiratory aerosols in wounded animals, as well as close contact with carcasses. We have no idea what the risks might be.
- To be prudent, we should assume there is some risk from this kind of close contact associated with hunting in areas where this virus might be circulating in deer.
Should hunters do anything different?
- It’s hard to say, but a little bit of practical prevention makes sense. It has been recommended that hunters wear a mask when handling deer carcasses . That’s reasonable.
- However, since dead deer don’t breathe, we’re not worried about exhaled respiratory aerosols when people handle carcasses. We’re mostly concerned about splashes or direct contact with respiratory tissues or secretions (+/- feces). So, wearing a mask when dressing a carcass, for example, still makes sense, to protect from splashes and prevent hand-to-mouth contact (which occurs more often than most people realize) when a person’s hands may be contaminated.
- Keep in mind that if we’re worried about splashes, eye protection also makes sense.
- Being particularly careful around wounded deer, avoiding close contact and using a mask and eye protection if such an animal must be approached would also be reasonable.
Does venison pose a risk?
- Presumably not.
- SARS-CoV-2 doesn’t survive long outside the body, and high levels of virus wouldn’t be expected in deer meat. Contamination of meat from feces or respiratory secretions is always possible, but good hygiene can reduce that. It’s not really different from how you’d prevent contamination with typical foodborne pathogens. We should handle meat (from any animal) like is has various infectious agents in/on it, use good hygiene and cook it properly. Any SARS-CoV-2 contamination would be effectively controlled through those same measures.
How do we figure out how SARS-CoV-2 gets from people to deer?
- That’s a huge challenge. Figuring out how it gets into deer will be very tough and I’m not sure we’ll have a good answer soon.
- Is it from rare human-deer interactions that involve close contact but leave a live deer? (i.e. not hunting)
- Is it from deer coming in contact with contaminated items like garbage from infected people?
- Unlikely, since things like that aren’t thought to be a realistic source of human infection and deer aren’t classical dumpster divers.
- Is it from deer coming in contact with human wastewater?
- Probably not. We can use wastewater for surveillance but we typically only detect bits of viral RNA, not viable virus. We don’t have evidence that infectious virus is released in wastewater.
- Could there be an intermediate animal host that carries the virus from humans to deer?
- Cats would be a leading candidate, if they have contact with infected people in a household but are allowed outside and then have contact with deer. It seems like a stretch based on cat behaviour, but can’t be ruled out.
Where do we go from here?
We need more surveillance, especially from other regions. It’s critical to determine if the virus is actually undergoing sustained transmission in deer populations, and if so where. If it is circulating on an ongoing basis in deer, we’ll need surveillance to look for emergence of significant variants. Hunting history and deer contact history should be considered when investigating new cases of COVID-19 in people, especially when there’s no other clear source. If the virus is circulating in deer, we’d also want more wildlife surveillance of other potentially susceptible species, to see if it’s spreading into other populations that could also become reservoirs.
Closing notes (the usual)
- The best way to keep this virus out of deer is to control it in people.
- The best way for a hunter to reduce their risk of getting COVID-19 is vaccination.