The Pacific Coast tick (Dermacentor occidentalis) is one of the most widely distributed and commonly encountered ticks along the western coastline of the United States. While it generally prefers feeding on livestock and larger wildlife, it is an opportunistic biter that readily attaches to humans and domestic pets [1]. Capable of transmitting several serious diseases, including Rocky Mountain spotted fever and tularemia, the Pacific Coast tick is a significant public health concern in its endemic range [1].
Identification and Biology
The Pacific Coast tick is a hard-bodied tick that shares a similar appearance with the American dog tick and the Rocky Mountain wood tick, making field identification challenging [1].
- Adult Females: They have a dark brown body with a mottled, off-white or grayish scutum (the shield behind the head) [1].
- Adult Males: Males are entirely dark brown but are covered in a mottled, “spotty” grayish-white pattern across their entire back [1].
- Size: They range from 3 to 15 millimeters in size, depending on their life stage and whether they are engorged with blood [1].
Like other Dermacentor species, the Pacific Coast tick follows a three-host life cycle, meaning it requires a different host for its larval, nymphal, and adult stages [1].
| Life Stage | Primary Hosts | Peak Activity |
|---|---|---|
| Larva | Small rodents (mice, voles) | Spring to Autumn |
| Nymph | Small to medium mammals (raccoons, squirrels) | Spring to Autumn |
| Adult | Deer, cattle, horses, dogs, humans | Spring |
The entire life cycle typically takes at least 54 days but can stretch up to two years depending on temperature and the availability of hosts [1]. Adult ticks are incredibly resilient and can survive for up to two years without a blood meal while waiting for a suitable host [1].
Habitat and Distribution
The Pacific Coast tick is found predominantly along the western coastline of North America. Its range extends from Oregon down through California and into northern Baja California, Mexico [1]. Within California, it is the most common tick found throughout the state, with the exception of the very dry regions of the central valley and the southeastern deserts [1].
This species prefers open habitats rather than dense, dark forests. They are most commonly found in shrub-lands, chaparral, open fields, tall grassy areas, and along hiking trails [1]. Adult ticks quest for hosts by climbing to the tips of grasses or low shrubs and waiting for a passing animal or human to brush against the vegetation [1].
Disease Transmission and Health Risks
While the Pacific Coast tick is not a vector for Lyme disease, it is capable of transmitting several other severe bacterial infections.
Rocky Mountain Spotted Fever (RMSF)
All life stages of the Pacific Coast tick can transmit Rickettsia rickettsii, the bacteria that causes Rocky Mountain spotted fever [1]. RMSF is a severe and potentially fatal disease characterized by a sudden high fever, severe headache, muscle pain, and a distinct spotted rash [2]. Prompt treatment with the antibiotic doxycycline is critical to prevent severe complications or death [2]. Recent studies suggest that roughly 11% of female Pacific Coast ticks may carry the bacteria [1].
364D Rickettsiosis (Pacific Coast Tick Fever)
This is an emerging spotted fever group rickettsiosis caused by Rickettsia philipii. It is transmitted by the bite of the Pacific Coast tick and causes symptoms similar to RMSF, including fever, headache, and a dark scab (eschar) at the site of the tick bite [1].
Tularemia
Also known as rabbit fever, tularemia is caused by the bacterium Francisella tularensis. The Pacific Coast tick can transmit this pathogen to humans, cats, and dogs [1]. Symptoms include a skin ulcer at the bite site and painfully swollen lymph nodes.
Tick Paralysis
The Pacific Coast tick has been implicated in causing tick paralysis in cattle, deer, and horses [1]. This condition is caused by a neurotoxin in the tick’s saliva that induces an ascending paralysis. While more common in animals, human cases can occur. Removing the tick usually results in a rapid and full recovery.
Prevention and Protection
Because the Pacific Coast tick prefers open, grassy areas and trails, hikers, outdoor workers, and pet owners must be vigilant.
- Trail Safety: Stay in the center of cleared trails. Avoid walking through tall grass, weeds, and brushy areas where ticks are likely questing.
- Property Management: If you live in an endemic area, keep your lawn mowed short and remove brush, tall weeds, and leaf litter from the edges of your property. This reduces the humidity ticks need to survive and removes habitats for the rodents that host immature ticks [1].
- Chemical Repellents: Treat outdoor clothing and gear with 0.5% permethrin. Apply EPA-registered insect repellents containing DEET, picaridin, or oil of lemon eucalyptus to exposed skin.
- Pet Care: Keep dogs and outdoor cats on year-round, veterinary-approved tick preventatives. Inspect pets thoroughly after they roam through tall grass or brush.
- Tick Checks: Perform a full-body tick check immediately after returning from outdoor activities.
If a tick is found attached, use fine-tipped tweezers to grasp it as close to the skin as possible and pull straight up with steady pressure. Clean the bite area with alcohol or soap and water.
References
[1] TickSafety.com: Pacific Coast Tick
[2] CDC: Rocky Mountain Spotted Fever