Relapsing fever (RF) has been known since the time of Hippocrates and has occurred in epidemic, endemic or sporadic form throughout the  world. It is an arthropod- borne infection. They are of two types; louse- borne and tick- borne.

Borrelia is large, motile, irregular, spiral shaped organisms which occur as commensals on the buccal and genital mucosa.
The causative agent of epidemic or louse borne RF is Borrelia recurrentis, first observed in the blood of patients in Berlin in 1873.
It is an exclusive human pathogen, transmitted from one to another through body lice (Pediculus humanus corporis). Extra human reservoirs are not present.

Borrelia recurrentis is an irregular spirochete with pointed end body morphology. It possesses about 5 to 10 spiral coils. They microaerophilic in nature.

Grows well at 28 to 30^oC. Chorioallantoic membrane of chick embryos is good growth promoters of Borrelia.
The tick- borne Borreliae infection is caused by ticks that are residing in rodents, on which the ticks feed. Humans are infected only accidentally.
Over 10 species of Borrelia infects humans. B. dutonii, B. hermsii, B. parkeri, etc., are some among them. DNA homology evidences show that all of them belong to a single species, with separate host adaptation.

They have the ability to undergo antigenic variations, resulting in the occurrence of relapses in the disease. Ultimate recovery after several successive relapses may be due to the development of immunity to all the antigenic variants.
The antibodies developed during infection are difficult to be demonstrated, as there are technical difficulties in preparing satisfactory antigens for testing.

Pathogenesis of relapsing fever starts with sudden fever after an incubation period of 2- 10 days. During this period, Borrelia is abundant in the patient’s blood. The fever subsides in 3- 5 days. After a period of 4- 10 days, fever sets in again. Borrelia is non- demonstratable in the blood during this time. When the fever relapses, Borrelia starts to reappear. The disease ultimately subsides after 3- 10 relapses.

A drop of blood may be used for diagnosis or detecting Borrelia using dark field or phase contrast microscopy. Presence of spirochetes with lashing movement confirms its presence.  Animal inoculation using mice is more effective.
Poverty, overcrowding and lack of personal hygiene are the main reasons for louse infestation. The louse borne infections are more severe presented with jaundice, hemorrhage and fatality at times. The lice are not shed through excreta. The infection is not transmitted through the bite, but through the skin abrasions when they are crushed.

Tick- borne RF is considered as ‘place disease’, as the infection occurs only in those places where the ticks are inhabited. The pathogen remains in the body of the ticks for its whole life and is shed through saliva and feces. Humans are infected through bites or discharges.  
 
Prevention of infestation by louse by insecticides is needed to keep away louse- borne relapsing fever. Eradication of ticks are even more difficult. The ticks specifically and infested places have to be identified. They have to be avoided or eradicated.
No vaccines are available. Tetracyclines, chloramphenicol, penicillin and erythromycin are effective antibiotics.