Relapsing fever causes

Infection
In the United States, TBRF is caused by one of three Borrelia species: B. hermsii, B. parkerii, and B. turicatae. Most human illness is caused by B. hermsii.

The relapsing fever Borrelia spp are gram negative helical bacteria normally 0.2 to 0.5 microns in width and 5 to 20 microns in length. They are visible with light microscopy and have the cork-screw shape typical of all spirochetes (see picture). They have a unique process of DNA rearrangement in their linear DNA. Each time the DNA is read a different antigenic marker, also known as a variable major protein, is created, which allows the organism to evade the immune system and therefore cause recurrent patterns of fever and other symptoms.

Borrelia is transmitted to humans through the bite of infected soft ticks of the genus Ornithodoros. Soft ticks (family Argasidae) differ in many ways from the so-called hard ticks (family Ixodidae), including the more familiar dog tick and deer 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 in their host burrows. Individual ticks will take many such blood meal during each stage of their life cycles, including the development of eggs by adult females. The bites of soft ticks are usually painless and the persons who are bitten while asleep are usually unaware that they were bitten.

The individual Borrelia species that cause TBRF are usually associated with specific tick vectors. For instance, B. hermsii is transmitted to humans by O. hermsi ticks, while B. parkerii is transmitted by O. parkeri and B. turicatae is transmitted by O. turicata. Each tick has a preferred environment and preferred set of hosts. O. hermsi tends to be found at higher altitudes (1500 – 8000 feet) where it is associated primarily with ground or tree squirrels and chipmunks. O. parkeri occurs at lower altitudes, where they inhabit caves and the burrows of ground squirrels and prairie dogs, as well as those of burrowing owls. O. turicata occurs in caves and ground squirrel or prairie dog burrows in the plains regions of the Southwest, feeding off these animals and occasionally burrowing owls or other burrow- or cave-dwelling animals.

In the tick, Borrelia can be found in all the tissues including salivary glands and ovaries of certain subspecies of ticks (Schwan and Piesman 2002). Infected Ornithodoros ticks can transmit relapsing fever spirochetes to humans through their saliva while feeding. O. turicatae and O. parkeri ticks also can transmit spirochetes through secretion of infectious fluids from their coxal glands, which are excretory organs located at the base of the ticks' legs. O. hermsi also secretes infectious coxal fluid but in such small amounts that it dries very quickly after being secreted and, therefore, poses little or no threat to its hosts. Since the infection is also in the ovaries of certain ticks, such as O. hermsi, they can transmit their infections over many generations from female ticks to their offspring. Soft ticks can live up to 10 years. In certain parts of the Russia the same tick has been found to live almost 20 years.

Louse-borne relapsing fever
Borrelia recurrentis is the only agent of louse-borne disease. Pediculus humanus, is the specific vector. Louse-borne relapsing fever is more severe than the tick-borne variety.

Louse-borne relapsing fever occurs in epidemics amid poor living conditions, famine and war in the developing world; it is currently prevalent in Ethiopia and Sudan.

Mortality rate is 1% with treatment; 30-70% without treatment. Poor prognostic signs include severe jaundice, severe change in mental status, severe bleeding, and prolonged QT interval on ECG.

Borrelia species causing relapsing fever include Borrelia recurrentis, caused by human body louse. No animal reservoir exists. Lice that feed on infected humans acquire the Borrelia organisms that then multiply in the gut of the louse. When an infected louse feeds on an uninfected human, the organism gains access when the victim crushes the louse or scratches the area where the louse is feeding. B. recurrentis infects the person via mucous membranes and then invades the bloodstream.

Tick-borne Relapsing Fever
Other relapsing infections are acquired from other Borrelia species, such as Borrelia hermsii or Borrelia parkeri, which can be spread from rodents, and serve as a reservoir for the infection, via a tick vector. Borelia hermsii and Borrelia recurrentis cause very similar diseases although the disease associated with Borrelia hermsii has more relapses and is responsible for more fatalities, while the disease caused by B. recurrentis has longer febrile and afebrile intervals and a longer incubation period.

Tick-borne relapsing fever is found primarily in Africa, Spain, Saudi Arabia, Asia, and certain areas in the Western U.S. and Canada. Most cases occur in the summer months and are associated in particular with sleeping in rustic cabins in mountainous areas of the Western United States. There are approximately 25 cases of TBRF in the United States each year.