Relapsing Fever Caused by Borrelia Species

Organism

The genus Borrelia belongs to the family Spirochaetaceae. It consists of two groups: species that cause Lyme disease and species that cause relapsing fever ( Table 240.1 ). The relapsing fever spirochetes are genetically diverse pathogens that are broadly divided between the louse-borne Borrelia recurrentis and the tick-borne species. The TBRF agents are maintained by soft ticks of the family Argasidae. Some species such as Borrelia lonestari and Borrelia miyamotoi group genetically with the classic relapsing fever spirochetes but are spread by other tick species. B. lonestari is spread by the Lone Star tick, Amblyomma americanum. In one study it was associated with southern tick–associated rash illness by polymerase chain reaction (PCR) assay, but further studies were unable to confirm that result. B. miyamotoi, which is spread by Ixodes dammini or Ixodes scapularis, has been described in Japan, Europe, Russia, and the United States. Molloy and coworkers described 97 cases across the northeastern United States diagnosed by PCR (see Table 240.1 ).

The pathogens are spirochetes that are 8 to 30 µm long and 0.2 to 0.5 µm wide. The spirochetes have spontaneous antigenic variation of the outer membrane proteins called the variable major proteins ( vmp ). These proteins determine the serotypes and are the mechanism for the relapsing disease. Borrelia strains can grow in artificial media (Barbour-Stoenner-Kelly medium), although the diagnosis is usually confirmed by microscopic examination of a blood smear. The spirochetes are readily visualized with Wright or Giemsa stains, but species cannot be distinguished by the staining characteristics. The spirochetes can survive in the tick vectors for up to 12 years; ticks remain the optimal means of maintaining organisms.

Ticks are divided between the soft-bodied Argasidae ticks and the hard-bodied Ixodid ticks. Soft-bodied ticks of the Argasidae family are vectors for most TBRF and are strongly associated with rodents. Ticks become infected during a blood meal on a vertebrate with spirochetemia. Humans become infected by tick bites via salivary gland secretions. The hard-bodied ticks are the vectors for B. miyamotoi .

In the Rocky Mountain western states, relapsing fever is caused predominantly by Borrelia hermsii, but it can also be caused by Borrelia parkeri, Borrelia turicatae, and Borrelia mazzotti. Each TBRF Borrelia spp. is associated with one species of transmitting soft-bodied ticks of the Ornithodoros genus. In the western United States, B. hermsii is transmitted by Ornithodoros hermsi.

LBRF is caused only by B. recurrentis and is spread by the human body louse (Pediculus humanus). The spirochetes do not penetrate through the louse intestine, so the disease is spread by crushing or squeezing the louse, releasing the organisms on the skin.

Epidemiology and Transmission

TBRF and its host, the Ornithodoros tick, occur on every continent except Australia and Antarctica (see Table 240.1 ). Although the host and habitat of each Ornithodoros tick species may vary, they have some characteristics in common. The ticks are obligate blood feeders, and the spirochetes from a blood meal invade all tissues of the tick within hours. The spirochetes can persist for years in the tick salivary glands, facilitating the transmission during the short feeding period of 20 minutes. Infection of humans occurs when saliva or excrement is released during feeding. Animal reservoirs include chipmunks, squirrels, rabbits, rats, mice, owls, and lizards. The ticks inhabit rodent burrows, decaying wood, cabins, animal shelters, and caves.

Intrusion of humans into the tick's environment creates the opportunity for disease transmission. In the United States, the largest epidemic occurred in 1973 when 62 campers became ill after sleeping in log cabins in Arizona. In North America, TBRF occurs in the Rocky Mountain regions ( Fig. 240.1 ) of the western states at altitudes above 1500 feet, and most cases occur during the summer months, although cases have been reported in the winter. In Texas, most cases occur during the winter and are associated with spelunking. ) Outbreaks have been reported in Arizona, California, Colorado, Montana, New Mexico, Washington, and British Columbia; however, most illness is sporadic. Most patients have a history of exposure to a cabin or woodpile with rodent infestation. The ticks feed for less than 1 hour, and bites are painless, so many patients give no history of a tick bite. The ticks feed nocturnally. TBRF occurs more commonly in male patients, and most patients are younger than 20 years. TBRF has also been transmitted by transfusion, transplacentally, and by laboratory exposure.

Because TBRF occurs on almost every continent (see Table 240.1 ), the diagnosis should be considered in a febrile patient returning from Central America, Africa, the Middle East, or southern Europe. In West Africa, as the number of cases of malaria has decreased, the percentage of febrile patients with TBRF has increased to 7.3%. In some districts, cases of TBRF exceed malaria.

LBRF was epidemic in the early 20th century throughout Eurasia and Africa. The last major epidemic occurred during World War II in Africa and Europe when an estimated 50,000 people died of the infection under conditions of crowding, such as in displaced-person camps. Today it is largely confined to Ethiopia and the surrounding area, where an estimated 10,000 people a year have LBRF. LBRF has been suspected to cause illness in homeless people in France and has been described among refugees from northeast Africa to Europe.