Ornithodoros hermsi is commonly known as the tick-borne relapsing fever (TBRF) vector or soft tick. It is a species of argasid soft tick found primarily in the western United States and Canada.
Ornithodoros hermsi is an important vector for several diseases, most notably tick-borne relapsing fever caused by the spirochete bacteria Borrelia hermsii. This tick can transmit B. hermsii and other Borrelia species to humans through its bite, leading to recurrent fever and other flu-like symptoms. TBRF has occurred in sporadic outbreaks in North America associated with camping, sleeping in cabins, and other outdoor activities. Understanding the biology and ecology of O. hermsi is critical for monitoring and controlling the incidence of this disease.
Taxonomy and Physical Description
Ornithodoros hermsi belongs to the genus Ornithodoros, family Argasidae, order Parasitiformes. Argasid ticks have a leathery flexible cuticle, allowing expansion when ingesting a blood meal. Unlike hard ticks, they do not have a hard protective scutum on their dorsal surface. O. hermsi are oval in shape and brownish-black in color, with adults reaching approximately 5 mm in length.
Key Identifying Features
- Soft, leathery body lacking a dorsal scutum
- Oval body shape
- Adult size around 5 mm
- Brownish-black coloration
- Dorsoventrally flattened unfed nymphs and adults
- Presence of a dorsal plate on larva
- Gnathosoma (head area) visible from above
Geographic Distribution and Habitat
O. hermsi is native to western North America. Its geographic range extends from southern British Columbia in Canada south through Washington, Oregon, and California. This species is associated with the coniferous forests found throughout this region.
These ticks inhabit rodent nests and other animal burrows. Important animal hosts include tree squirrels (Sciurus and Tamiasciurus species), chipmunks (Tamias species), and Ornate shrews (Sorex ornatus). O. hermsi ticks feed quickly on hosts during the night and then retreat back into nests and burrows during the day.
Key Locations
- Southern British Columbia
- Washington
- Oregon
- Northern California
Life Cycle and Feeding Behaviors
O. hermsi has a three-host life cycle consisting of larva, nymph, and adult. Larvae and nymphs feed briefly on hosts before dropping off and molting to the next stage in the safety of nests and burrows. Adult females feed repeatedly to obtain large blood meals for egg production.
These ticks are rapid nocturnal feeders, attaching to hosts for less than 30 minutes to obtain a blood meal. They use specialized mouthparts to saw into skin and anchor themselves. Saliva injected during feeding contains anticoagulants and anesthetics. O. hermsi does not remain attached to hosts after feeding.
Key Behaviors
- 3-host life cycle with larvae, nymphs, and adults
- Larvae and nymphs feed briefly on hosts
- Adult females take repeated blood meals
- Nocturnal feeding for less than 30 minutes
- Do not remain attached to hosts after feeding
Medical and Veterinary Importance
O. hermsi is an important vector of tick-borne relapsing fever in humans caused by Borrelia spp. bacteria. In North America, this disease is most often caused by B. hermsii transmitted by the bite of O. hermsi ticks. Symptoms include recurring fever, headache, muscle and joint pain, and nausea.
These ticks can also transmit other Borrelia spp. and disease agents to humans and animals. However, they are not known vectors of other major tick-borne diseases like Lyme disease, anaplasmosis, or Rocky Mountain spotted fever.
Key Facts
- Primary vector of tick-borne relapsing fever caused by Borrelia hermsii
- Transmit infection through rapid nocturnal bites
- Not known to vector other major tick-borne diseases
- Also transmit other Borrelia spp. and disease agents
Monitoring and Control Methods
Surveillance of O. hermsi relies on collecting ticks from forests and examining them for Borrelia spp. Public health education on recognizing habitats, performing tick checks after outdoor activities, and wearing protective clothing can help reduce human exposures.
Rodent nest sanitation, use of tick-killing chemicals in nesting areas, and controlling rodent populations may reduce tick abundance. No vaccines are currently available for TBRF or pathogens transmitted by O. hermsi.
Key Prevention and Control Measures
- Tick surveillance and testing for pathogens
- Public education on avoiding tick habitats and bites
- Rodent nest sanitation measures
- Acaricide application in nesting areas
- Rodent population control
- No available vaccines for TBRF
Conclusion
Ornithodoros hermsi, also known as the tick-borne relapsing fever vector, is a significant public health threat in the western United States and Canada. These soft ticks transmit Borrelia bacteria that cause recurrent fevers in humans through their rapid nocturnal bites. Monitoring O. hermsi populations, educating the public on prevention, and controlling rodent reservoir hosts are important measures for reducing the incidence of tick-borne relapsing fever caused by these ticks.