Short Report: First Outbreak of Atlantic Rainforest Spotted Fever in North-Eastern of Brazil-Clinical Features and...

Since 2010, six cases of Atlantic rainforest spotted fever (ARSF) have been confirmed and reported in Brazil, a tick-borne disease caused by Rickettsia parkeri strain Atlantic rainforest and associated with the tick Amblyomma ovale. So far, none of the cases have been associated with death. One of the cases occurred in 2018 in the municipality of Ilhéus, Bahia state, that is in Atlantic Forest Biome. In this study, we report the first confirmed outbreak of ARSF in Brazil, with three new cases associated with tick bites when entering Atlantic Forest areas in the same municipality, from March 11 to April 4, 2025.

Scab samples from inoculation eschars of three patients presenting post-tick-bite symptoms were collected for molecular analysis targeting a fragment of the ompA gene of R. parkeri, followed by DNA sequencing. Paired serum samples (acute and convalescent phases) were evaluated by immunofluorescence assay (IFA) against R. parkeri antigens. Additionally, small mammals were captured using Sherman and Tomahawk traps during the same period in the exposure sites to evaluate tick infestation dynamics and species.

All patients developed symptoms 7-10 days after the bite, including inoculation eschar with progressive enlargement, exanthema, regional lymphadenopathy, fever, myalgia, and prostration; all responded to doxycycline treatment. Molecular analysis confirmed the presence of R. parkeri strain Atlantic rainforest DNA in all scab samples. Seroconversion (IgG) to R. parkeri was demonstrated in two patients. A total of 10 immature A. ovale ticks (larvae and nymphs) were collected parasitizing the wild cricetid rodents Hylaeamys seuanezi  (three individuals) and  Thaptomys nigrita (one individual) in the outbreak area.

This report demonstrates three new confirmed cases and the first reported cluster of ARSF in Brazil, confirmed by laboratorial criteria, including molecular analyses, demonstrating the potential of the southern region of Bahia for transmission of R. parkeri, thus making it an endemic area for the disease.