One of the side effects of having a veterinarian who also studied public health is that a perfectly innocent walk through Aspen can turn into a mental game of “what zoonotic disease could that animal be carrying?”
See a rabbit dart across the trail? I think about tularemia. A deer mouse disappears behind a woodpile? Hantavirus. A mosquito buzzing around the porch? West Nile Virus.
Most of the time, there is absolutely no reason to panic. The wildlife around the Roaring Fork Valley is not plotting against us. But living so closely alongside wildlife does mean there are a few diseases worth knowing about, particularly as we spend more time hiking, camping, gardening and generally enjoying the outdoors.
Tularemia is a bacterial infection caused by Francisella tularensis. It is relatively uncommon, but Colorado is one of the states where it occurs, making it an interesting disease for those of us with plenty of rabbits and rodents in the backyard.
Rabbits and hares are particularly susceptible, along with rodents such as prairies dogs, voles and squirrels. Humans can become infected through a tick or deer fly bite, by handling an infected animal or, less commonly, by inhaling contaminated dust. That last route sounds bizarre until you remember how much time we spend mowing, landscaping and maintaining properties in the valley. Running a mower over an infected carcass can aerosolize bacteria and lead to pneumonic tularemia. I remember this particular scenario because it showed up as a question on my veterinary boards exam. Apparently, mowing over a dead rabbit was important enough for me to commit it to memory forever.
Symptoms depend on how the bacteria enter the body. A tick or deer fly bite can result in an ulcer at the bite site accompanied by swollen regional lymph nodes and fever. Inhalation can cause respiratory symptoms and pneumonia.
Cats are particularly susceptible to tularemia and can subsequently transmit the bacteria to humans. Of course, this is of little relevance to your indoor lap cat and primarily a risk to outdoor hunters. Dogs appear to be much more resistant, and cases are very rare. The takeaway? Don’t handle sick or dead wildlife with bare hands, and don’t mow over carcasses if you can avoid it.
Hantavirus is another zoonotic disease worthy of discussion. If you’ve ever opened an old cabin, shed or garage and discovered evidence that mice have been enjoying the space in your absence, hantavirus could be a concern. In Colorado, the deer mouse is the primary reservoir for the hantavirus that causes hantavirus pulmonary syndrome. The mice themselves don’t appear to be sick.
People become infected when particles contaminated with infected rodent urine, droppings or saliva become airborne and are inhaled. This is why sweeping or vacuuming a mouse-infested space can be a bad idea. Instead, ventilate the area when possible, wear appropriate respiratory protection and disinfect contaminated surfaces using bleach. Hantavirus infection is rare, including in Colorado, but Colorado has historically had more cases than most states. The illness can begin with flu-like symptoms before progressing rapidly to severe respiratory disease.
Interestingly, our dogs and cats are not considered important sources of human hantavirus infection. In fact, they don’t appear to be susceptible to the virus at all. However, data is too limited to conclude there is zero risk, so limiting rodent exposure is a good idea for both people and pets.
Finally, there is West Nile Virus, which feels particularly relevant as we move through late summer and fall in Colorado. West Nile virus cycles primarily between mosquitoes and birds. A mosquito becomes infected after feeding on an infected bird and can then transmit the virus when it bites a person, horse or other mammal.
Most infected people never develop significant illness. Those who do may experience fever, headache and body aches, while a small percentage develop serious neurologic disease such as meningitis or encephalitis. Fortunately, Aspen’s higher elevation keeps mosquito populations low, but risk is never zero. There is no specific antiviral treatment for West Nile in people, so prevention is important. Insect repellent, long sleeves, screens and eliminating standing water around your property can all reduce risk of bites.
Vector control programs have developed creative ways to reduce mosquito populations. Some programs introduce sterile male mosquitoes into the population to reduce reproduction, while others introduce mosquitoes carrying Wolbachia, a naturally occurring bacterium that can interfere with the ability of certain viruses to replicate inside mosquitoes.
No need to fear every rabbit, mouse or mosquito you encounter. But a little awareness of their potential disease risks can help keep us, and our four-legged companions, protected and healthy!
Dr. Kelly Voss is a veterinarian at Aspen Animal Hospital, where she practices small animal medicine and surgery. She has a background in emergency and critical care and is passionate about bringing high-quality veterinary care to the mountains. Have a question for the column? Email her at dr.kellyvoss@gmail.com.