ABSTRACT

Borrelia is a genus of spirochetes belonging to the Spirochaetaceae. Morphologically, the typical organism is composed of five to ten loosely wound, irregular coils measuring 10 to 35 m in length and 0.3 to 0.5 m in width. Although susceptible to desiccation and to many chemical agents, the organisms are able to survive in citrated blood for as long as three months at 2C to 3C. Disease caused by these spirochetes may be divided into two forms:

1. An epidemic form, in which the body louse, Pediculus humanus corporis, is usually the principal vector

2. An endemic form, in which infection is mostly transmitted through ticks of the Ornithodoros genus

Louse-borne relapsing fever is rare and occurs primarily by importation. Tick-borne disease usually occurs in the spring and summer, primarily in the western mountainous states in the United States. The clustering of cases is not unusual.