Epidemic louse-borne relapsing fever now has disappeared with improvement in hygiene and the use of insecticides. It is an important disease only in north-eastern Africa, especially in the islands of Ethiopia and in South America. Endemic tick-borne relapsing fever has worldwide distribution and is endemic in western states of the United States.
B. recurrentis is found in the blood stream and also in the spleen,liver, lungs, kidneys, and bone marrow of the infected patients suffering from relapsing fever.
Humans are the only reservoir of epidemic relapsing fever. No extra-human reservoir is known. The infection is transmitted only from person to person. Persons suffering from relapsing fever are the source of infection. Human body louse (Pediculus humanuscorporis) is the vector of the disease. No extrahuman reservoir isknown. The Borrelia organisms are found only in the hemolymph of the lice. They are not excreted in saliva or excreta. Hence, the infection is transmitted by them being crushed and rubbed into the absorbed skin. They are not transmitted by bite of the lice. Endemic tick-borne relapsing fever is a zoonotic disease trans-mitted from animals to humans, rodents, small mammals.
Soft ticks (Ornithodoros species) are the main vectors of the disease. Unlike the louse-borne infection, B. recurrentis pro-duces a disseminated infection in ticks. However, the vector survives from the infection and becomes reservoir of the infec-tion by transovarian transmission. The Borrelia organisms are found in all parts of the body of tick; hence, they are secreted in the saliva and in excreta. Therefore, the infection is transmitted to human by the bite of the ticks. Several species of the genus Ornithodoros act as reservoir in different parts of the world.
Ornithodoros tholozani, Ornithodoros crossi, Ornithodoros lahorensis,and the fowl tick, Argas persicus are the vectors in India. Soft ticks are primarily nocturnal feeders. They feed while the host is sleeping and remain attached for only a few minutes. Hence, the bite of the tick goes usually unnoticed.