|Cell Wall: Gram Negative||Shape: Spirochete|
- Infections with certain Borrelia species cause the disease Relapsing Fever. Epidemic disease is caused by B. recurrentis while sporadic cases are found endemically in certain countries and are caused by non-recurrentis Borrelia species. For ease of the discussion, we largely focus on the B. recurrentis species in the discussions below.
- Epidemic Disease: Due to human-to-human spread via body lice that transmit organisms during blood meals. Endemic Disease: In certain parts of the world rodents are the natural hosts for Borrelia species and transmission to humans occurs via the bite of Ticks which have previously taken a blood meal from infected animals.
- B. recurrentis disseminates throughout the body hematogenously following inoculation into the host. Organisms multiply to enormous levels, precipitating fever and associated symptoms described below. Development of a successful humoral immunity is largely able clear the infection using antibodies to B. recurrentis surface antigens, heralding a waning of the fever. However, B. recurrentis displays antigenic variation, meaning that in every population some organisms switch their antigenic surface structure and in doing so become resistant to the host's antibodies. These resistant organisms are then able to multiply to enormous levels, re-initiating the fever and associated symptoms. These cycles of organismal proliferation, successful immune response, antigenic variation, and organismal re-proliferation explain the clinical phenomenon of Relapsing Fever.
- Relapsing Fever is characterized by the rapid onset of constitutional symptoms such as high fever, headache, myalgia, and chills. These symptoms last for 3-6 days and suddenly wane, corresponding with the development of protective humoral antibodies to the organism. An afebrile period normally lasts about a week after which the above constitutional symptoms suddenly reappear, corresponding with the reappearance of organisms in the blood and organs.
- Doxycycline or erythromycin are effective; however, their administration may result in the Jarisch-Herxheimer Reaction.