Tick-borne relapsing fever
What is relapsing fever?
Relapsing fever is caused by a group of bacteria called Borrelia. Borrelia are transmitted to humans by two vectors: ticks (of the genus Ornithodoros) and lice. Louse-borne relapsing fever will not be discussed further in this section.
Various species of Borrelia predominate in different geographic locations. Each Borrelia species are identified closely with their tick vector and they share parallel names. For example Borrelia hermsii are transmitted by Ornithodoros hermsii ticks. The ticks' hosts are wild rodents or other mammals.
Tick-borne relapsing fever (TBRF) is endemic in many countries. In developed countries humans acquire infection by contact with the ticks in caves, when crawling under houses in endemic areas, or in remote log cabins. There are approximately 25 cases of TBRF in the United States each year. In developing countries the ticks live in thatched roofs and mud walls and floors of houses. The incidence of TBRF can be more than 6% in parts of Africa.
Clinical features of tick-borne relapsing fever
TBRF is characterised by relapsing (i.e. recurring) episodes of fever, often accompanied by other symptoms. The symptoms of TBRF begin abruptly around 2 to 10 days after the tick bite. The patient experiences around 3 to 6 days of fever, headache, muscle and joint pain, shaking chills, and abdominal pain. The first fever episode ends in ‘crisis’, which consists of a markedly high temperature, raised heart rate, and raised blood pressure. After 10 to 30 minutes temperature and blood pressure fall dramatically, and the patient sweats profusely.
Following this the patient is symptom-free for around one week, before a recurrence of the fever and other symptoms. The patient experiences an average of three recurrent episodes throughout the course of the illness, although up to 10 recurrences can occur in patients who are untreated.
Other clinical features that may be present include: enlarged liver and spleen, jaundice, respiratory symptoms, and central nervous system involvement.
Long-term complications occasionally occur including: iritis (inflammation of the iris of the eye), depression, and heart failure. In an estimated 2% to 5% of patients the disease is fatal.
Skin manifestations of TBRF
Rash occurs in up to 50% of patients, depending on the particular species of Borrelia involved. A variety of skin lesions have been associated with TBRF:
- Most commonly the skin lesions are described as macules (flat discolourations), 1 to 2cm in size, itchy, with irregular borders, or circular, sharply demarcated, blanching, 18 to 24mm rose-coloured macules, resembling erythema multiforme.
- Papules (small lumps), petechiae (small red or purple spots due to bleeding into the skin), purpura (bleeding into the skin, includes petechiae and bruises), and facial flushing have been described. Petechiae may also occur on mucous membranes.
- The rash usually appears towards the end of the first febrile episode or during the symptom-free interval. The rash usually lasts 24 to 48 hours and may be present over the entire body or in localised areas. Lesions can occur on the face, trunk or extremities.
How is the diagnosis of relapsing fever made?
- TBRF is diagnosed by visualising Borrelia bacteria in blood smears taken while the patient has a fever.
- Borrelia may also be seen in smears of bone marrow or cerebrospinal fluid.
- Organisms are not found during the symptom-free period.
- Serologic testing (detection of antibodies against Borrelia in the blood) may be inconclusive as patients with TBRF may have false-positive serology for Lyme disease. Furthermore, serologic testing is not useful for making an immediate diagnosis, as it requires an acute sample to be taken within 7 days of symptom onset and a convalescent sample taken at least 21 days after symptoms start.
- Blood samples taken before antibiotics are given can be cultured using a special medium or by inoculating immature mice.
Treatment of tick-borne relapsing fever
- Antibiotics such as doxycycline, penicillin, erythromycin, chloramphenicol, and ceftriaxone are effective in treating TBRF.
- The Jarisch-Herxheimer reaction can occur within a few hours of giving antibiotics in up to 54% of patients. This reaction consists of chills, fever, raised heart rate, low blood pressure, and occasionally worsening of skin lesions. In some cases the Jarisch-Herxheimer reaction can be fatal.
Prevention of tick bites
- Avoidance of the Ornithodoros tick’s natural habitat is the best way to prevent TBRF.
- Avoid tick bites by using permethrin-impregnated clothing and DEET insect repellant.
- Rodent-proof buildings in infested areas and remove rodent nesting material from walls, ceilings, and floors.
- Post-exposure treatment with doxycycline appears to be effective.