borelliosis

Relapsing fever is an insect-transmitted disease characterized by recurrent fevers with spirochetemia. Epidemic relapsing fever is caused by body louse-transmitted Borrelia recurrentis, which infects only humans.

B. recurrentis, which is associated with overcrowding due to poverty or war, caused multiple large epidemics in Africa, Eastern Europe, and Russia in the first half of the twentieth century, infecting 15 million people and killing 5 million, and is still a problem in some developing countries. Endemic relapsing fever is caused by several Borrelia species, which are transmitted from small animals to humans by Ornithodorus (soft-bodied) ticks.

In both louse- and tick-transmitted borreliosis, there is a 1- to 2-week incubation period after the bite as the spirochetes multiply in the blood. Clinical infection is heralded by shaking chills, fever, headache, and fatigue, followed by disseminated intravascular coagulation and multiorgan failure. Spirochetes are temporarily cleared from the blood by anti-Borrelia antibodies, which target a single major surface protein called the variable major protein.

After a few days, bacteria bearing a different surface antigen emerge and reach high densities in the blood, and symptoms return until a second set of host antibodies clears these organisms.

The lessening severity of successive attacks of relapsing fever and its spontaneous cure in many untreated patients have been attributed to the limited genetic repertoire of Borrelia, enabling the host to build up cross-reactive as well as clone-specific antibodies. Antibiotic treatment of Borrelia infections may cause a massive release of endotoxin, resulting in the production of cytokines that cause fever with rigors, fall in blood pressure, and leukopenia (the Jarisch-Herxheimer reaction).

Morphology. Diagnosis of relapsing fever can be made by identification of spirochetes in blood smears obtained during febrile periods. In fatal louse-borne disease, the spleen is moderately enlarged (300 to 400 gm) and contains focal necroses and miliary collections of leukocytes, including neutrophils, and numerous borreliae.

There is congestion and hypercellularity of the red pulp with erythrophagocytosis. The liver may also be enlarged and congested with prominent Kupffer cells and septic foci. Scattered hemorrhages resulting from DIC may be found in serosal and mucosal surfaces, skin, and viscera. Pulmonary bacterial superinfection is a frequent complication.