When most people think of ticks, they picture “hard ticks” with a tough, shield-like plate on their backs that attach to a host for days at a time. However, there is an entirely different family of ticks known as “soft ticks” (family Argasidae), primarily of the genus Ornithodoros. These ticks look, act, and feed very differently from their hard-shelled cousins, and they are the exclusive vectors for a severe illness known as Soft Tick Relapsing Fever (STRF) [1].
Identification and Biology
Soft ticks are easily distinguished from hard ticks by their appearance and feeding habits.
- Appearance: They lack the hard scutum (shield) found on hard ticks. Instead, their bodies are covered in a leathery, wrinkled, and textured cuticle [1]. They are often described as looking like a small raisin or a wrinkled piece of leather.
- Mouthparts: Unlike hard ticks, whose mouthparts protrude visibly from the front of their bodies, the mouthparts of a soft tick are located underneath its body and are not visible from above [1].
- Feeding Behavior: This is the most critical difference. Hard ticks attach to a host and feed continuously for several days. Soft ticks are fast feeders. They emerge at night, attach to a sleeping host, feed rapidly for 15 to 30 minutes, and then drop off and return to their hiding places [1]. The bite is painless, and most victims never even know they were bitten [1].
Soft ticks are exceptionally long-lived. Individual ticks can live for decades, taking multiple blood meals during each stage of their life cycle [1]. Furthermore, some species can pass disease-causing bacteria directly through their eggs to their offspring, meaning a tick population can remain infectious for generations without ever feeding on an infected host [1].
Habitat and Distribution
Soft ticks do not wait in tall grass or leaf litter for a host to walk by. Instead, they live in the nests and burrows of their preferred hosts, primarily rodents like mice, chipmunks, and squirrels [1]. In the United States, soft ticks are primarily found in the mountainous areas of the western states (such as Colorado, California, Washington, and Idaho) and in parts of Texas [1].
Human exposure almost always occurs in one of two specific environments. The most common scenario involves people staying overnight in rodent-infested mountain cabins. The ticks live in the walls, attics, or floorboards where rodents nest. At night, the ticks emerge, feed on sleeping humans, and retreat before morning [1]. Less frequently, people become infected while exploring caves that house rodents or bats, particularly in Texas [1].
Disease Transmission: Soft Tick Relapsing Fever (STRF)
Soft ticks in the U.S. are the vectors for Borrelia hermsii and Borrelia turicatae, the bacteria that cause Soft Tick Relapsing Fever (STRF) [1]. Symptoms typically begin 4 to 21 days after the brief, unnoticed tick bite. The disease is characterized by a very specific and recognizable pattern: a sudden onset of high fever, severe headache, muscle aches, chills, and sometimes nausea or joint pain; this fever episode lasts for about 3 days; the fever abruptly breaks and the patient feels relatively normal for about 7 days; and then the fever and symptoms return (relapse) [1].
Without antibiotic treatment, this cycle of fever and remission can repeat multiple times. During the fever episodes, symptoms can be severe, including drenching sweats, shaking, and dangerously low blood pressure [1]. STRF is particularly dangerous for pregnant women, as it can affect the fetus and cause pregnancy loss or premature birth [1].
Prevention and Eradication
Because soft ticks feed quickly at night and hide in the walls, standard tick prevention methods (like wearing long pants on a hike) are ineffective against them. Prevention requires managing the indoor environment.
- Avoid Rodent-Infested Buildings: Do not sleep in cabins or rustic buildings that show signs of rodent activity, such as droppings, shredded nesting material, or a stale odor [1].
- Bed Placement: If staying in a rustic cabin, move the bed away from the walls and ensure bed sheets do not touch the floor. Never sleep directly on the floor [1].
- Cave Safety: If exploring caves in endemic areas, wear long sleeves and pants, and avoid direct skin contact with the cave floor or animal burrows [1].
- Professional Pest Control: Because soft ticks can live for decades, a cabin that becomes infested can remain a health hazard for years. Do not attempt to remove rodent nests yourself, as this can expose you to other fatal diseases like Hantavirus [1]. Consult a licensed pest control professional to safely remove all rodent nests and treat the building’s cracks and crevices with appropriate pesticides.