Does Tick-Borne Relapsing Fever Have an Animal Reservoir in East Africa?

Abstract

Tick-borne relapsing fevers (TBRF) are caused by infection with Borrelia spirochetes and transmitted to humans by ticks. All except East African TBRF, caused by Borrelia duttonii, are known zoonoses. This widespread, endemic and pathogenic infection has only been found in humans and the Ornithodoros sp. soft tick vectors. We investigated the role of domestic animals as possible reservoirs of infection in a TBRF endemic region. Tick infestations in households and pigpens were investigated in the villages near Mvumi hospital in central Tanzania. Blood from chickens and pigs was examined by PCR and flagellin gene sequencing was performed on any Borrelia sp. infections detected. A mark-recapture experiment investigated tick movement between pigpens and houses. The acceptability of chickens as tick hosts was also investigated. Tick infestation of the 122 houses investigated was high (47%). Pigpens also were tick infested (16%) and were more likely to be so if they were located close to tick infested households (p<0.001). PCR screening of peripheral blood found Borrelia infections in both chickens and pigs (11% and 8.9% respectively). Sequencing of a subset of positive samples revealed that the amplified Borrelia sp. flagellin gene fragments shared greatest homology with B. duttonii. In a mark-recapture experiment, ticks released in pigpens were recaptured inside human bedrooms. When offered chickens as hosts, over 20% of ticks fed. For the first time in East Africa, we record natural infections of Borrelia in domestic animals and show that tick populations may act as bridging vectors between animals and humans. These results, from villages where B. duttonii is already known to be prevalent and a major cause of illness in humans, and where it has been found at high levels in ticks, strongly support the case that it is a zoonosis. This increases understanding of the epidemiology and control of this important but neglected human disease.