Borrelia – Relapsing Fevers

Several Borrelia sp. Are capable of causing epidemic or tick borne relapsing fever. These sp. Include B. duttoni, B. hermsii, B. parkeri and B. turicatae. Borrelia sp. Transmitted to humans by soft bodied ornithodorus ticks. The natural hosts for these organisms include rodents and other small mammals on which the ticks normally feed.

B. recurrentis is an obligate human pathogens who is responsible for the epidemic or louse – borne relapsing fever. It is transmitted from person to person by the body louse, Pediculus humanus.

The spirochetes causing the two forms of relapsing fever differ in their mode of growth in the arthropod vector, and this influence the way human infection is initiated.

B. recurrentis grows in the hemolymph of the louse but does not invade tissues. As a result the excrement of the louse is non-infectious and the bacterium is not transferred transovarially to the progeny.

Human infection occurs when bacteria released from crushed lice gain entry to tissues through damaged or intact skin or mucous membranes. Transovarial transmission to the tick progeny maintains the spirochete in the tick population.

In both forms of relapsing fever, acute symptoms, including high fever, rigors, headache, myalgia, arthralgia, photophobia and cough, develop about one week after infection. A skin rash may occur and there is central nervous system involvement in up to 30 % of cases. During the acute phase there may be up to 10^5 spirochetes per cubic millimeter of blood. The primary illness resolves within 3 – 6 days, and terminates abruptly with hypotension and shock, which may be fatal. Relapse of fever occurs 7 – 10 days later, and several relapses may take place.