Rustrela virus is known to cause “staggering disease” in domestic cats, but the discovery of an infected mountain lion in Colorado indicates that wildlife officials also may need to consider the impact on wild feline populations. (victorschendelphotography.com)
November 18, 2024
By Dr. Dave Samuel
Viruses aren’t just a problem for humans; they cause issues for wildlife, too — including the game species we pursue as bowhunters. While it seems viruses are more common in wildlife today than they were years ago, it’s also true that modern science pays a lot more attention to them as we seek to better understand these illnesses and how to respond from a management perspective. This month, I thought it would be interesting to share updates on four viruses that impact wildlife (and possibly humans), as well as discuss the potential impact on hunters.
Rustrela Virus A report recently published in the journal Emerging Infectious Diseases cited an unusual incident. A female yearling mountain lion was found in Douglas County, Colo. The lion could not move its hind legs and was not mobile in any way. It responded to stimuli but could not eat. The animal was euthanized by wildlife officials and studied by Karen Fox with the Colorado State University College of Veterinary Medicine and Biomedical Sciences. They considered lots of possible causes and did various tests to determine the cause. They found the presence of a RusV variant virus, a virus that causes “staggering disease” in domestic cats.
Little is known about the occurrence of rustrela virus in domestic cats in the United States, but RusV has been shown to be the “causative agent of staggering disease in domestic cats from Austria, Sweden and Germany, as well as in lions from zoological collections in Germany.” No other lions in Colorado have been reported with staggering disease symptoms. For the lion that was found, they do not know the source of the rustrela virus. Dr. Fox’s report suggests looking for a local reservoir host “because rodents of the genus Apodemus have been identified as RusV reservoir hosts in Europe…” even though they don’t experience any negative effects. Fox also noted there are many RusV hosts in Germany, including a donkey, mustelids (weasels, badgers, otters, martens), rodents and opossums.
The Colorado public is being asked to report any mountain lions seen staggering or having trouble walking to Colorado Parks and Wildlife officials. Maybe this was just a one-time occurrence and no cause for concern, but the last sentence in Fox’s report is a bit unnerving. “RusV should be considered as a possible cause for neurologic diseases in all mammal species in North America.” Obviously, this incident means wildlife officials need to keep a close watch on any staggering disease incidents in any mammal.
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SARS-CoV-2 COVID was the virus-caused disease that stopped the world in its tracks several years ago, resulting in a global pandemic that caused serious illness and death to millions of people. We all experienced the effects of COVID first-hand, and I personally lost several bowhunting friends to the virus.
The virus that caused the COVID pandemic is called severe acute respiratory syndrome coronavirus 2, more commonly known as SARS-CoV-2. Now we know this virus has bi-directional transmission between people and animals. People can give it to animals, and animals can give it to us. Studies done in 2023 show that deer are one of the animals that can get COVID from us, and it is likely that deer can get COVID from other mammals. One study done in Virginia showed that deer mice, opossums, raccoons, groundhogs and rabbits all had high rates of COVID infection. That study showed that the COVID infection rate in individuals from these species living along hiking trails frequented by humans was three times higher than in individuals living farther from humans. Researchers believe the virus infects those species through food discarded along the hiking trails. Deer and other animals coming into contact with these species also can get the virus. Of course, once a few deer have SARS-CoV-2, they can spread it to other deer.
The big question is whether we can get COVID from deer. As we’ve learned, the COVID virus develops new variants in humans that require new vaccinations. That probably will never end. The same happens to the virus in deer, as new variants have been found in deer in many states. Can humans get one of those virus variants from whitetails? Most scientists speculate that we can’t, but ongoing research is looking at this possibility. The exact reasons why deer don’t seem to infect humans with COVID aren’t clear, but at least for now it appears we are safe from this deer-to-human transmission.
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EHD I wrote at length about Epizootic Hemorrhagic Disease virus (EHD) in my July column. EHD is having an increasing impact on our hunting world, killing whitetails, mule deer and antelope. We know that EHD is transmitted by midges, tiny biting flies commonly known as “no-see-ums.” Seven days after infection, a deer can get a swollen head, neck, tongue, or eyelid. Animals become weak and lose their fear of humans. They then die quickly. The bad news is that there is no cure. Other bad news is that the warmer winters we are now seeing means that more midges survive to infect deer in late summer. Thus, EHD is becoming more common in more northerly areas where it was once rare. Also, there are new variants of the EHD virus that come into the country all the time. This means that even though some deer are immune to EHD, they are not immune to a new variant.
It appears new variants of EHD will continue to evolve. Thus, eliminating EHD just won’t happen. The good news is that outbreaks occur locally and rarely are widespread. Some deer get EHD and survive. Thus, given time, deer populations in an area will recover. Even after really severe outbreaks when the majority of a local deer population is wiped out, recovery can occur within five years.
HPAI Although the Highly Pathogenic Avian Influenza (HPAI) virus doesn’t really impact our bowhunting world, it does affect hunting in several ways. As background, this avian influenza virus has devastated the poultry business, but since 2022 many terrestrial and migratory bird species have been found with HPAI. Wild birds can carry the avian influenza virus and show no signs of the disease, but they can carry the disease to other birds, including poultry farms, when they migrate. In the United States, HPAI has been reported in all 50 states, but exposure to hunted birds such as waterfowl rarely affects humans. However, new research shows it can be picked up by retrieving hunting dogs that come in contact with infected birds.
After the 2022 waterfowl hunting season in the state of Washington, a blood survey was done on 194 hunting dogs. Antibodies to avian influenza were detected in 18 dogs that had hunted in areas were the virus had been found in wild ducks. Thankfully, none of these dogs showed any symptoms of the virus. The authors concluded that the HPAI strains that emerged in North America in 2022 were poorly adapted to dogs. Other studies have shown the avian influenza virus remains infectious in the environment for several years, so avian influenza is probably here to stay. If a new variant evolves that does impact dogs, then hunters who use dogs could have big problems.
