GD09 Spotted fever after safari: a case of African tick bite fever highlighting the value of eschar swabs for the molecular detection of rickettsial disease

Abstract African tick bite fever (ATBF) is an emerging rickettsial disease of the spotted fever group increasingly seen in international travellers returning from sub-Saharan Africa. It is caused by Rickettsia africae, an intracellular Gram-negative bacillus, and transmitted by Amblyomma ticks, which feed on cattle and wild game. Risk factors for transmission include safari tourism, game hunting and travel to endemic areas in sub-Saharan Africa and the West Indies. The exact prevalence of seropositive individuals in endemic areas is unknown but may be as high as 80%. The estimated prevalence among travellers is 4.0–8.6%. ATBF is characterized by an acute, influenza-like illness with fever, headache and myalgia. The incubation period is 6–7 days. Typical cutaneous findings include solitary or multiple inoculation eschars with associated regional lymphadenopathy. A generalized vesicular or maculopapular rash can also be seen. It is typically a mild illness; however, rare complications include subacute neuropathy and myocarditis. ATBF is primarily a clinical diagnosis and empirical treatment should not be delayed. In cases of diagnostic uncertainty, detection of rickettsia from serology or tissue can be useful for confirmation. While eschar biopsies have traditionally been used, eschar swab and/or crust samples can also be used for the molecular detection of rickettsia. We present a case of rickettsiosis, likely caused by R. africae, in a man who presented with a 4-day history of fevers, headache and myalgia following recent travel to South Africa for safari. On examination he had a solitary inoculation eschar at his left upper thigh with associated tender left-inguinal lymphadenopathy. He was febrile but otherwise vitally stable. Investigations were notable for raised C-reactive protein and mild lymphopenia. He was commenced on empirical doxycycline and ciprofloxacin to cover for eschar-associated diseases, and a dermatology opinion was sought with a view to performing a diagnostic biopsy. As per specialist advice, eschar swab and crust samples were recommended instead with paired serology. Rickettsiae serology was negative; however, eschar polymerase chain reaction was positive for Rickettsia species. The cycle threshold value was too high for the exact species to be sequenced; however, given the travel history and clinical findings, a presumptive diagnosis of ATBF was made. The patient completed a 14-day course of doxycycline with resolution of symptoms. Our case highlights the value of using noninvasive eschar samples to support the diagnosis of ATBF. It is a simple, quick method that can be performed in a variety of clinical settings compared with traditional eschar biopsy.