Tickborne Relapsing Fever
Most cases of tick borne relapsing fever (TBRF) are caused by infection with spirochetes of the Borrelia genus. In the United States, most cases are caused by infection with Borrelia hermsii in mountainous western states and Borrelia turicatae in southern states.
Borrelia is transmitted to humans following a bite with Ornithodoros hermsi ticks. Borrelia resides in the salivary gland of the soft tick and can be transmitted to humans within 30 seconds of initiation of a blood meal. Unlike hard ticks, they do not need to become embedded to transmit disease.
These soft ticks typically live in the nests of rodents such as ground squirrels, tree squirrels, and chipmunks in coniferous forests at elevations between 1,500 and 8,000 feet altitude. Soft ticks acquire TBRF Borrelia by feeding on infected rodents and remain infectious for life. Human infections occur in locations where rodents and humans are in close proximity, such as mountain cabins. TBRF is endemic in the western United States, predominately in mountainous regions.
As the name implies, clinical illness is characterized by relapsing bouts of fever, headache, and malaise. Febrile episodes characteristically last 3 to 5 days and relapse after 5 to 7 days of apparent recovery. Episodic fever is the result of antigenic variation in spirochete outer surface proteins, which temporarily allow the spirochete to evade the host immune response. TBRF is usually cured by antibiotic therapy. However, complications such as acute respiratory distress syndrome, myocarditis and meningoencephalitis can occur. Transplacental transmission can occur and pregnant women might be more susceptible to severe complications such as spontaneous abortion, preterm delivery, and perinatal mortality.
General laboratory studies are nonspecific. Mild to moderate normocytic anemia and thrombocytopenia can occur. Leukocyte counts are normal to slightly elevated. Leukopenia can occur during a febrile crisis. Aminotransferases and bilirubin may be elevated. Prothrombin and partial thromboplastin times may be moderately prolonged.
In patients with symptoms of meningoencephalitis, cerebrospinal fluid findings include lymphocytic pleocytosis, mildly elevated protein and normal glucose concentration.
A diagnosis of TBRF can be confirmed by observation of spirochetes in a peripheral blood smear taken during a febrile episode. Giemsa or Wright stains of thick and thin blood smears may reveal spirochetes if the concentration of microorganisms is greater than 105/mL. As many as 200 oil immersion fields may need to be viewed before deciding a smear is negative. The optimum time to obtain blood is between the fever's onset and its peak. Once the temperature is declining, or is back to the normal range in the absence of antipyretics, spirochetes usually cannot be visualized in blood.
Testing for serum antibodies is not valuable in the acute setting but might be useful for retrospective identification in convalescent patients.
References
MMWR January 30, 2015 / 64(03);58-60
Trevejo RT, Schriefer ME, Gage KL, et al. An interstate outbreak of tick-borne relapsing fever among vacationers at a Rocky Mountain cabin. Am J Trop Med Hyg 1998;58:743–7.
Burgdorfer W. The diagnosis of relapsing fevers. In: Johnson RC, ed. The biology of parasitic spirochetes. New York, NY: Academic Press; 1976:225–34.
