Question
Relapsing fever
Answer
Relapsing fever is a spirochetal disease caused by various Borrelia species, characterized by recurring febrile episodes separated by afebrile intervals — a distinctive clinical pattern arising from the organism’s ability to evade the host antibody response through antigenic variation.
Types (by mode of transmission):
- Louse-borne (epidemic) relapsing fever — caused by Borrelia recurrentis, transmitted by the human body louse (Pediculus humanus); humans are the only reservoir; associated with crowding, poor hygiene, and war/famine conditions; louse-borne disease tends to be more severe.
- Tick-borne (endemic) relapsing fever — caused by several other Borrelia species (e.g., B. duttonii, B. hermsii), transmitted by soft ticks of the genus Ornithodoros, with rodents as the animal reservoir.
Pathogenesis: after the initial febrile episode (during which the organism is present in high numbers in the blood), the host mounts an antibody response against the dominant surface antigen (a variable major protein), clearing most organisms and terminating the fever — however, a small subpopulation of spirochetes undergoes programmed antigenic variation, switching to a different variable surface protein not recognized by the existing antibody, allowing them to multiply again and produce a relapse of fever; this cycle can repeat several times, with each relapse typically progressively milder than the last, as the immune response broadens.
Clinical features: recurring episodes of high fever, headache, myalgia, and sometimes hepatosplenomegaly and jaundice, each episode lasting several days and separated by afebrile intervals of similar duration.
Laboratory diagnosis: demonstration of spirochetes in a peripheral blood smear (Giemsa or Wright stain) taken during a febrile episode, when spirochetaemia is highest — dark-field microscopy of fresh blood can also demonstrate motile spirochetes.
Treatment: tetracycline or doxycycline (erythromycin as an alternative, e.g., in pregnancy); treatment can precipitate a Jarisch-Herxheimer reaction (an acute febrile reaction from rapid spirochete lysis releasing pyrogenic products), requiring careful monitoring.