Reservoir:
Relapsing fever is a vector borne disease that can be acquired either by a louse or a tick. Relapsing fever, when acquired through a louse, occurs in epidemics while those acquired through ticks are sporadic. In case of louse-borne fever, the reservoir is the human while the primary reservoir hosts for the tick-borne relapsing fevers are rodents.

Vector and distribution:
The insect vector may be the soft ticks of the genus Ornithodoros or the body louse (Pediculus humanus). The louse-borne relapsing fevers are endemic only in parts of Africa and South America while the tick-borne, in the Americas, Africa, Asia, and Europe. The bacteria are transovarially passed to the next generation of ticks. The louse-borne disease is called epidemic relapsing fever because it can be rapidly disseminated under conditions of overcrowding and poor personal hygiene, such as during wars and natural disasters. The tick-borne relapsing fever is called endemic relapsing fever because it occurs when humans are exposed to infected ticks.

Mode of infection:
The louse is infected by feeding on a patient during the febrile stage. The spirochetes are not transmitted directly to man but when the louse is crushed, they are released and enter abraded skin or bites. Ticks acquire the spirochetes from rodents and humans are infected when spirochetes in the tick's saliva or coxal fluid (excreta) enter the skin as the tick bites. Congenital borreliosis has also been reported.

Pathogenesis:
One tip of the spirochete attaches to the host cell and some form of invasin apparently causes the host cell to release digestive enzymes that enable the spirochete with its corkscrewing motility to penetrate the host cell membrane.
The relapsing fever borreliae use antigenic variation as a mechanism to evade the host immune response and this antigenic variation results in a disease characterized by recurrent episodes of fever and spirochaetaemia.
The fever, which is marked by spirochetemia usually last for 3-5 days. As the body mounts a specific immune response, the spirochetes are cleared from the peripheral blood and the fever subsides. The patient may remain afebrile for few days to few weeks. A new antigenic variant develops in this period that gives rise to another episode of fever.

Clinical features:

• The incubation period ranges from 3 to 11 days (mean, 6 days). 

• The onset is sudden with chills, followed by high fever, tachycardia, severe headache, vomiting, muscle and joint pain, and often delirium.

• Late in the course of the fever, jaundice, hepatomegaly, splenomegaly, myocarditis, and cardiac failure may occur, especially in louse-borne disease. 

• The spirochetes appear in the blood during the febrile period and can be found in internal organs, especially the spleen and brain. 

• Hemorrhagic lesions may be seen in kidney and intestine. Brain and meninges may also be involved. 

• The duration of illness ranges from 4 to 10 days. 
  
  As the body mounts a specific humoral immune response, borreliae disappear from peripheral blood. This results in an afebrile period lasting few days to several weeks. An antigenic variant of this spirochete multiplies resulting in relapse.
  Relapse occurs with a sudden return of fever often with, all the former symptoms and signs. Jaundice and arthralgia are more common during relapse. The illness clears as before, but 2 to 10 similar febrile episodes may follow at intervals of 1 to 2 weeks. The episodes become progressively less severe, and recovery eventually occurs as the patient develops immunity.
  Myocarditis is the most common cause of death in fatal cases of relapsing fever.

Laboratory Diagnosis:
Diagnosis is suggested by the recurrent fever and confirmed by the appearance of spirochetes in the blood during a febrile episode. 

Specimen collected:
 Blood, CSF, synovial fluid

Microscopy:

• Darkfield examination to look for actively motile spirochetes
• Wright's or Giemsa stained thick and thin blood smears
• Acridine orange stain (fluorescent stain) for examining blood or tissue

Culture:
 Not cultivable on routinely used culture media.

Animal Inoculation:
In tick-borne infection, intraperitoneal injection of the patient's blood (1-2 ml) into a mouse or rat produces large numbers of spirochetes in the animal's tail blood within 3 to 5 days. 

The differential diagnosis includes malaria, dengue, yellow fever, leptospirosis, typhus, influenza, and the enteric fevers.

Treatment:
The commonly used antibiotics are Tetracycline, Doxycycline or erythromycin. Therapy should be started early during fever or during the afebrile stage, but should be avoided near the end of the episode because of the danger of Jarisch-Herxheimer reaction, which is characterised by abrupt chills, fever, headache, malaise, nausea and vomiting.