The Rocky Mountain wood tick (Dermacentor andersoni) is a prominent parasite in the western United States and southwestern Canada. Found primarily at higher elevations, this tick is a significant vector for several severe diseases, including Colorado tick fever and Rocky Mountain spotted fever, and is a leading cause of tick paralysis [1]. Understanding its habitat, life cycle, and the risks it poses is crucial for anyone living in or visiting the mountainous regions of the West.
Identification and Biology
The Rocky Mountain wood tick is very similar in appearance to the American dog tick (Dermacentor variabilis), and distinguishing between the two often requires microscopic examination [1]. However, their geographic ranges rarely overlap, which aids in field identification.
- Adult Females: They have a dark brown body with a prominent, silvery-gray or whitish scutum (shield) located just behind the head [1]. When fully engorged, they can expand to half an inch in width and take on a grayish color.
- Adult Males: Males are entirely covered in a mottled, grayish-white pattern over a brown background [1].
- Immature Stages: Larvae have six legs, while nymphs have eight. Both stages are small and rarely bite humans, preferring smaller hosts.
The Rocky Mountain wood tick has a complex life cycle that typically takes two to three years to complete [1].
| Life Stage | Primary Hosts | Peak Activity |
|---|---|---|
| Larva | Small rodents (voles, mice, chipmunks) | Summer |
| Nymph | Small to medium mammals (marmots, rabbits) | Summer |
| Adult | Large mammals (deer, elk, livestock, humans) | Spring to Early Summer (April – July) |
A notable characteristic of adult Rocky Mountain wood ticks is their extreme longevity; they can survive for up to 600 days without a blood meal while waiting for a host [1].
Habitat and Distribution
As its name suggests, this tick is endemic to the Rocky Mountain states and the northern intermountain region. Its range extends from the western portions of Nebraska and the Dakotas westward to the Cascade Mountains, and from northern Arizona and New Mexico up into British Columbia and Alberta, Canada [1].
The Rocky Mountain wood tick prefers subalpine elevations, typically between 4,000 and 10,000 feet [1]. They are most commonly found in shrublands, lightly wooded areas, open grasslands, and along trails [1]. Adult ticks quest for hosts by climbing to the tips of knee-high vegetation and waiting for a large mammal to brush past.
Disease Transmission and Health Risks
The Rocky Mountain wood tick is responsible for transmitting several serious pathogens and causing a unique, potentially fatal toxic reaction.
Colorado Tick Fever (CTFV)
Colorado tick fever is a viral illness transmitted almost exclusively by the Rocky Mountain wood tick [1]. Symptoms usually begin 1 to 14 days after a bite and include a sudden high fever, chills, severe headache, muscle aches, and extreme fatigue. A defining characteristic of CTFV is a “biphasic” fever: the fever lasts for a few days, disappears, and then returns for another few days. While rarely fatal, the fatigue and weakness can last for weeks or even months. There is no specific treatment for CTFV.
Rocky Mountain Spotted Fever (RMSF)
Despite its name, RMSF is more common in the eastern U.S., but it was first identified in the Rocky Mountains, where Dermacentor andersoni is the primary vector [2]. Caused by the bacterium Rickettsia rickettsii, RMSF is a severe and potentially fatal disease [2]. Symptoms include high fever, severe headache, and a characteristic spotted rash. Prompt treatment with the antibiotic doxycycline is critical to prevent severe organ damage or death [2].
Tick Paralysis
The Rocky Mountain wood tick is one of the leading causes of tick paralysis in North America [1]. This condition is caused by a neurotoxin secreted in the tick’s saliva during feeding, usually by an adult female that has been attached for several days [1]. It causes an ascending paralysis that begins in the legs and moves upward. If the paralysis reaches the respiratory muscles, it can be fatal. However, if the tick is found and completely removed, the symptoms usually reverse within 24 to 72 hours, and the patient recovers fully [1].
Tularemia
Also known as rabbit fever, tularemia is a bacterial infection (Francisella tularensis) that can be transmitted by the bite of an infected Rocky Mountain wood tick [1]. It causes a skin ulcer at the bite site and swollen lymph nodes, and requires antibiotic treatment.
Prevention and Protection
When hiking, camping, or working in the mountainous regions of the western U.S., specific precautions are necessary to prevent bites from the Rocky Mountain wood tick.
- Avoid Questing Areas: Stay in the center of trails and avoid brushing against knee-high grass and shrubs where adult ticks wait for hosts.
- Dress Appropriately: Wear light-colored clothing, long pants, and tuck pant legs into socks.
- Chemical Repellents: Treat clothing and gear with 0.5% permethrin, and apply EPA-registered repellents (like DEET or picaridin) to exposed skin.
- Frequent Checks: Because tick paralysis requires the tick to be attached for several days to secrete enough toxin, performing thorough daily tick checks is highly effective at preventing this condition. Pay special attention to the scalp and hairline, as adult ticks often crawl upward to attach.
If a tick is found, remove it immediately using fine-tipped tweezers. Grasp the tick close to the skin and pull straight up with steady pressure to ensure the mouthparts are not left behind.
References
[1] TickEncounter: Rocky Mountain Wood Tick
[2] CDC: Rocky Mountain Spotted Fever