To the Editor:
One of the more common tick-borne infections seen in travelers returning from sub-Saharan Africa is caused by Rickettsia africae, which is the etiologic agent of African tick-bite fever (ATBF), the most common tick-borne bacterial zoonosis.1 There are 2 known tick vectors of disease: Amblyomma variegatum, found in sub-Saharan Africa and the West Indies, and Amblyomma hebraeum, found specifically in southern Africa.2,3
Unlike other disease-carrying ticks that passively wait on vegetation to be picked up by a host, A hebraeum uniquely attract other nearby ticks to the host. Studies have shown that male ticks feeding on a nonhuman host (usually cattle) can emit an aggression-attachment pheromone that attracts other ticks to the host. The presence of the pheromone allows unfed ticks to actively discriminate between hosts on which these parasites have fed successfully (ie, suitable hosts) and those on which they have not.4
The aggressive hunting nature of A hebraeum explains the clinical presentation of multiple eschars and why large groups of exposed travelers—such as soldiers, leisure safari tourists, game hunters, and foreign-aid workers—are affected.2
Another southern African spotted fever group, Rickettsia conorii is the causative agent of Mediterranean spotted fever (MSF). Ticks carrying R conorii exhibit a much less aggressive hunting stylethan A hebraeum; consequently, infected patients present with a single eschar site.5
Rickettsia africae is estimated to have very high prevalence (95.2%) in Amblyomma ticks and a fairly high prevalence (approximately 4.0% to 8.6%) in travelers coming from rural southern Africa,6,7 with an incubation period of 5 to 10 days after inoculation by an infected tick.8 Signs include fever, a generalized maculopapular or papulovesicular rash, and regional lymphadenopathy; symptoms include fatigue, headache, and myalgia.
The inoculation eschar—single or multiple—commonly presents on the legs and is accompanied by tender lymphadenopathy of draining nodes1,8 More severe findings, such as myocarditis and subacute neuropathy, have been reported in elderly patients.9
A 77-year-old man presented with a pruritic maculopapular and papulovesicular rash distributed over the upper and lower extremities of 3 weeks’ duration. The patient reported having been on a 12-day mission trip to Limpopo, South Africa, where he was constructing and installing safe toilets to replace dangerous toilet pits. He believed he had been bitten by 2 ticks, after which he noted a dark purple and black patch on the left lower leg by the third day of the trip. He developed a sudden persistent pruritic rash, first on the lower extremities and then spreading to the upper extremities. The patient was seen by an American physician in South Africa who gave him a 7-day course of oral doxycycline monohydrate 100 mg twice daily. He then returned to the United States.
FIGURE 1. Erythematous plaque and central eschar on the medial aspect of the lower left leg.
Sixteen days after being bitten by the ticks, the patient was examined in our dermatology office. Physical examination revealed an erythematous plaque with a central eschar over the medial aspect of the left leg (Figure 1) and multiple 3- to 6-mm, erythematous, dome-shaped papules scattered over the dorsal aspects of the feet and ankles (Figure 2). The examination was otherwise normal. Blood was drawn the same day for laboratory analysis; no abnormalities of platelets, red blood cells, or white blood cells were found. Results of a chemistry panel and liver enzyme tests were within reference range.
FIGURE 2. Multiple 3- to 6-mm, erythematous, dome-shaped papules with central puncta scattered over the ankle and dorsal aspect of the foot.