Human ehrlichiosis natural history, complications and prognosis
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]
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Natural History
Studies throughout the world have shown that many cases of human illness are caused by zoonotic pathogens that are maintained by animal hosts in their natural cycles. Many zoonoses require a vector (e.g., mosquito, tick, mite) to be transmitted from the animal host to the human host. The ehrlichial disease cycle includes four components: the pathogen, the animal reservoir species, the vectors, and the human host. For the types of human ehrlichiosis in the United States, the arthropod vectors are ticks.
Only one of the three tick families, Ixodidae (hard ticks), is associated with ehrlichiae. These ticks have four stages in their life cycle: egg, larva, nymph, and adult. After the eggs hatch, each stage must feed once to develop into the next stage. Larvae are uninfected with ehrlichiae when they begin to look for a blood-meal. Ticks become infected with ehrlichiae while feeding on blood from the host in either the larval or nymphal stage. After the tick develops into the next stage, the ehrlichiae may be transmitted to the following host during the feeding process. Both male and female ticks may bite humans but it is the females that are responsible for most transmission. In the United States, it appears that both the nymphal and adult stages are responsible for transmission of ehrlichiae, but one stage may be more important for each Ehrlichia species.
Can A Person Get Ehrlichiosis More Than Once?
Very little is known about immunity to ehrlichial infections. Although it has been proposed that infection with ehrlichiae confers long-term protection against reinfection, there have been occassional reports of laboratory-confirmed reinfection. Short-term protection has been described in animals infected with some Ehrlichia species and this protection wanes after about 1 year. Clearly, more studies are needed to determine the extent and duration of protection against reinfection in humans.