Entomological risk of African tick-bite fever (Rickettsia africae infection) in Eswatini

by Kimberly J. Ledger, Hanna Innocent, Sifiso M. Lukhele, Rayann Dorleans, Samantha M. Wisely

Rickettsia africae is a tick-borne bacterium that causes African tick-bite fever (ATBF) in humans. In southern Africa, the tick Amblyomma hebraeum serves as the primary vector and reservoir for R. africae and transmits the bacterium during any life stage. Previous research has shown that even when malaria has been dramatically reduced, unexplained acute febrile illnesses persist and may be explained by the serological evidence of rickettsiae in humans.

We collected 12,711 questing Amblyomma larvae across multiple land use types in a savanna landscape in Eswatini. Our results show that host-seeking Amblyomma larvae are abundant across both space and time, with no significant difference in density by land use or season. We investigated the entomological risk (density of infected larvae) of ATBF from A. hebraeum larvae by testing over 1,600 individual larvae for the presence of R. africae using a novel multiplex qPCR assay. We found an infection prevalence of 64.9% (95% CI: 62.1–67.6%) with no land use type significantly impacting prevalence during the dry season of 2018. The mean density of infected larvae was 57.3 individuals per 100m² (95% CI: 49–65 individuals per 100m²).

Collectively, our results demonstrate R. africae infected A. hebraeum larvae, the most common tick species and life stage to bite humans in southern Africa, are ubiquitous in the savanna landscape of this region. Increased awareness of rickettsial diseases is warranted for policymakers, scientists, clinicians, and patients. Early detection of disease via increased clinician awareness and rapid diagnostics will improve patient outcomes for travelers and residents of this region.