RELAPSING FEVER DISEASE
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This is a disease caused by Spirochaeta spp., which is vectored by both ticks and lice (Phthiraptera), although other species have been suspected. Spirochaeta recurrentis (Lebert) may be the main incitant of the louse borne form (Matheson 1950), while in ticks Spirochaeta duttoni has been found to be the main incitant. The tick-borne typhus form and louse borne relapsing fever usually occur around the same time and in the same areas. However, sometimes their occurrences are staggered by several years. It has been noted that Tick-borne Relapsing Fever is the only important disease transmitted to humans by soft ticks (Table 1) (Service 2008). The body louse, Pediculus humanus corporis, has been found as the main vector in India where typhus does not occur (Mackie 1907) and subsequently at other world sites.
Symptoms include repeated bouts of fever, which last from 3-5 days. Apyrexia varies from 5-10 days.
The tick form of the disease, Spirochaeta duttoni, has been found to be vectored by Ornithodoros moubata (Murray) (Matheson 1950) or O. turcata (Weller & Graham 1930). Additional vector species have since been discovered (Table 2). Very young ticks are capable of transmission. Also, infection in the tick passes through the eggs even to the third generation. The ticks are believed to be main reservoirs of the disease, but other animals such as rodents may also be reservoirs.
Table 1. Distinction Between Hard & Soft Ticks (Derived from Service 2008)
Chung & Feng (1936) studying the disease life cycle in both body and head lice found that the vector obtains the spirochetes while drawing blood from a host. Most of the spirochetes are then digested and vanish after 6-8 hours. However, several manage to penetrate the intestinal wall and reach the coelomic fluid in about two hours. The spirochetes then multiply in the body cavity by transverse division and distribute to all parts of the body. They do not invade the tissues nor are they transmitted through the egg or feces. They will remain in the louse for its entire life of about 20 days. Humans are infected when the lice are crushed on the skin near abrasions.
There have been epidemics of relapsing fever worldwide, and it is always associated with either lice or ticks. Matheson (1950) details the areas and severity where epidemics have occurred over many years. Mortality rates have sometimes killed large numbers of people as s in West Africa where 80,000 died during a two-year epidemic.
Table 2. Occurrence of Relasping Fever
(Derived from Matheson 1950)
Control of louse populations is effective in eliminating the disease. Avoiding tick habitats and the diseases they carry is about the best way to avoid bites and infection. Traditionally various dips and sprays have been used for domestic animals. There are also vaccinations available for some diseases, and consulting a physician is advised for the latest treatments.
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