Ticktransmitted rickettsioses

Rickettsia species within the so-called spotted fever group have been demonstrated among ticks in Europe, for example, R. conorii in Spain, France, and Italy and R. slovaca in eastern Europe. In 1979, Burgdorfer et al. demonstrated the presence of a rickettsia, later named R. helvetica, in Ixodes ricinus collected in Switzerland. Recent studies have demonstrated that R. helvetica is present in about 20% of nymphal (Figure 23.9) and adult I. ricinus in southern Sweden (Nilsson et al. 1998). Nilsson et al. (1999) have also demonstrated the presence of R. helvetica in heart muscle tissue and adjacent tissues in two men who had suddenly died during sporting activities. Even though it is not proven that R. helvetica was the cause of the death of these young men, this hypothesis needs to be tested. Rickettsia rickettsii, the etiological agent of Rocky Mountain spotted fever, is widely distributed in both North and Latin America. The vectors in eastern and western US are Dermacentor variabilis (the American dog tick) and D. andersoni (the Rocky Mountain wood tick), respectively (Burgdorfer 1975). The infection in humans appears now to be most prevalent in the eastern US. Untreated R. rickettsi infection is a serious disease. However, with the advent of antibiotics the mortality rate has now decreased considerably.

Rickettsia conorii is the etiological agent of Mediterranean tick typhus or boutonneuse fever in the Mediterranean area, in Israel and in southern and south-eastern Asia. It is so-called because it is associated with a button-like lesion at the site of the tick bite. The main vector is the brown dog tick,

Rhipicephalus sanguineus.

Rickettsia sibirica is distributed in northern Asia from the Far East westwards to Armenia. Dermacentor marginatus, D. silvarum, Hyalomma concinna, and R. sanguineus are the main vectors. The symptoms of R. sibirica infection resemble those of mild Rocky Mountain spotted fever with a rash, fever, and headache.

The rickettsioses mentioned are all transmitted via tick bite but some of the infections can presumably also be contracted, for instance, by picking infected ticks from dogs,

Figure 23.9 Nymph of the hard tick Ixodes ricinus. Natural size c.1.5 mm. (Photo: G. Wife and T. G. T. JaensonĀ©.)

crushing the ticks between the fingers, and then inadvertently contaminating wounds or mucous membranes with infective material. The Rickettsia organisms are usually transmitted, both trans-stadially and transovarially, in the vector populations.

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