The infectious organisms in relapsing fever are spirochetes of the genus Borrelia. The infection has two forms: tick-borne and louse-borne. The main reservoir for tick-borne relapsing fever is rodents, which serve as the source of infection for ticks. Tick-borne relapsing fever may be transmitted transovarially from one generation of ticks to the next. Humans can be infected by tick bites or by rubbing crushed tick tissues or feces into the bite wound. Tick-borne relapsing fever is endemic, but is not transmitted from person to person. The distribution and seasonal incidence of the disease are determined by the ecology of the ticks in different areas. Different species (or strain) names have been given to Borrelia in different parts of the world where the organisms are transmitted by different ticks. There are over 25 species of Borrelia associated with tick-borne relapsing fever, including B miyamotoi, which was first identified as a cause of human disease in Russia in 2011 and is widely found throughout Asia, Europe, and North America. In the United States, infected ticks are found throughout the West, especially in mountainous areas, but clinical cases are uncommon in humans.
The louse-borne form is primarily seen in the developing world, and humans are the only reservoir. Large epidemics may occur in louse-infested populations, and transmission is favored by crowding, malnutrition, and cold climate.
There is an abrupt onset of fever, chills, tachycardia, nausea and vomiting, arthralgia, and severe headache. Hepatomegaly and splenomegaly may develop, as well as various types of rashes (macular, papular, petechial) that usually occur at the end of a febrile episode. Delirium occurs with high fever, and there may be various neurologic and psychological abnormalities. The attack terminates, usually abruptly, after 3–10 days. After an interval of 1–2 weeks, relapse occurs, but often it is somewhat milder. Three to ten relapses may occur before recovery in tick-borne disease, whereas louse-borne disease is associated with only one or two relapses.
During episodes of fever, large spirochetes are seen in thick and thin blood smears stained with Wright or Giemsa stain. The organisms can be cultured in special media but rapidly lose pathogenicity. The spirochetes can multiply in injected rats or mice and can be seen in their blood.
A variety of anti-Borrelia antibodies develop during the illness; sometimes the Weil–Felix test for rickettsioses and nontreponemal serologic tests for syphilis may also be falsely positive. Infection can cause false-positive indirect fluorescent antibody and Western blot tests for Borrelia burgdorferi, causing some cases to be misdiagnosed as Lyme disease. PCR assays have been developed but are not widely available. CSF abnormalities occur in patients with meningeal involvement. Mild anemia and thrombocytopenia are common, but the white blood cell count tends to be normal.