Relapsing fever is an illness caused by certain bacteria in the genus Borrelia. Because of their spiral-shape, these bacteria are known as spirochete. The bacteria are transmitted to humans by the bite of an infected soft tick of the genus Ornithodoros. Relapsing fever which is transmitted by ticks is known as Tick-borne relapsing fever (TBRF).
In the United States most TBRF has been found in limited areas of 11 western states (Arizona, California, Colorado, Idaho, Montana, Nevada, New Mexico, Texas, Utah, Washington, and Wyoming). Most people are infected while vacationing during the summer months and sleeping in rustic cabins in mountainous areas (elevations >8,000 feet). The soft tick Ornithodoros hermsii which is the tick most often associated with the transmission of the bacteria to humans is found living in rural areas that are usually mountainous and forested. The tick lives in dark, cool places where rodents nest, such as woodpiles outside buildings, in house crawl spaces, or between walls or beneath floorboards inside rustic cabins. The soft tick normally feeds on rodents, including deer mice, tree squirrels, chipmunks and other wild rodents. When these rodents are not readily available, the soft tick will feed on other mammals including humans. The ticks usually feed at night, typically while people are asleep, and they remain attached for only a short time (15 – 30 minutes). If the ticks are infected, they pass on the infection during the brief tick bite. The bites are painless and most people never realize they have been bitten.
Human cases of illness tend to peak in the warmer months, but they can occur year-around. A tick population often becomes established with rodents that inhabit rustic mountain cabins. If the rodents die off, leave, or hibernate, the ticks look for other hosts. In the winter, people will stay in these cabins and warm them up for a week. The rodents are not active, but the ticks get warmed up, and they become hungry and start moving around looking for a food source. A person’s breathing is basically a carbon dioxide generator. The ticks actually orient to a carbon dioxide gradient, as this is one of the ways ticks find their next blood meal, the person becomes the next food source for the tick.
Soft ticks of the family Argasidae differ in many ways from the so called hard ticks of the family Ixodidae which includes the more familiar dog tick and Rocky Mountain wood tick. In contrast to hard ticks, soft ticks take brief blood meals lasting less than a half hour, usually at night. Between meals the ticks live in the nesting materials of their host burrows. Individual ticks will take many such blood meals during each stage of their life cycle, including the development of eggs by adult females. Soft ticks can live up to 10 years and in certain parts of Russia the same tick has been found to live almost 20 years.
Ornithodoros hermsii (family Argasidae)
Most people who are infected with TBRF get sick about a week after they are bitten by the tick. The symptoms include repeated bouts of fever, chills, headache, muscle or joint aches, and nausea that last from 2 – 7 days, punctuated by periods of apparent wellness that last for about a week. The initial symptoms are the most severe, with sudden onset of high fever and severe headache. A spotted and/or itchy rash may sometimes occur during this first episode of illness. The number of episodes or “relapses” ranges from 2 – 10, and each is less severe than the previous one. Serious complications or death (fatality rate of 0 – 10%) can occur if the illness is not treated, especially in the very young, elderly, pregnant, or debilitated. Infection is usually cleared up easily with a 7 – 14 day regimen of antibiotic therapy.
There is no vaccine against TBRF. Therefore, help prevent TBRF by protecting yourself from exposure to soft ticks and rodents. To prevent infection:
* Avoid sleeping in rodent infested buildings.
* Limit tick bites by using insect repellent containing DEET (on skin or clothing) or permethrin (applied to clothing or equipment).
* Rodent-proof buildings.
* Identify and remove any rodent nesting material from walls, ceilings and floors.
* In combination with removing the rodent material, fumigate the building with preparations containing pyrethrins and permethrins. More than one treatment is often needed to effectively rid the building of the vectors, the soft-ticks. Always follow product instructions, and consider consulting a licensed pest control specialist.