Why Are There So Few Rickettsia conorii conorii-Infected Rhipicephalus sanguineus Ticks in the Wild?

Cristina Socolovschi,Idir Bitam,Didier Raoult,Philippe Parola,Thi Phong Huynh,Jean Gaudart

doi:10.1371/journal.pntd.0001697

Abstract

Background Rickettsia conorii conorii is the etiological agent of Mediterranean spotted fever, which is transmitted by the brown dog tick, Rhipicephalus sanguineus. The relationship between the Rickettsia and its tick vector are still poorly understood one century after the first description of this disease.Methodology/Principal FindingsAn entomological survey was organized in Algeria to collect ticks from the houses of patients with spotted fever signs. Colonies of R. conorii conorii-infected and non-infected ticks were established under laboratory conditions. Gimenez staining and electron microscopy on the ovaries of infected ticks indicated heavy rickettsial infection. The transovarial transmission of R. conorii conorii in naturally infected Rh. sanguineus ticks was 100% at eleven generations, and the filial infection rate was up to 99% according to molecular analyses. No differences in life cycle duration were observed between infected and non-infected ticks held at 25°C, but the average weight of engorged females and eggs was significantly lower in infected ticks than in non-infected ticks. The eggs, larvae and unfed nymphs of infected and non-infected ticks could not tolerate low (4°C) or high (37°C) temperatures or long starvation periods. R. conorii conorii-infected engorged nymphs that were exposed to a low or high temperature for one month experienced higher mortality when they were transferred to 25°C than non-infected ticks after similar exposure. High mortality was observed in infected adults that were maintained for one month at a low or high temperature after tick-feeding on rabbits.Conclusion/SignificanceThese preliminary results suggest that infected quiescent ticks may not survive the winter and may help explain the low prevalence of infected Rh. sanguineus in nature. Further investigations on the influence of extrinsic factors on diapaused R. conorii-infected and non-infected ticks are required.

Highlights

Rickettsia conorii conorii is the etiological agent of Mediterranean spotted fever (MSF), one of the oldest recognized vector-borne infectious diseases [1]
The bacterium Rickettsia conorii conorii is the etiological agent of Mediterranean spotted fever (MSF), which is a lifethreatening infectious disease that is transmitted by Rhipicephalus sanguineus, the brown dog tick
We collected naturally infected ticks from the houses of people afflicted by MSF in Algeria

Summary

Introduction

Rickettsia conorii conorii is the etiological agent of Mediterranean spotted fever (MSF), one of the oldest recognized vector-borne infectious diseases [1]. Rh. sanguineus rarely feeds on humans, in temperate countries, it seems to have a greater human affinity in warmer temperatures [4] This tick is highly adapted to warm climates and thrives in dog kennels and human homes. Rickettsia conorii conorii is the etiological agent of Mediterranean spotted fever, which is transmitted by the brown dog tick, Rhipicephalus sanguineus. The relationship between the Rickettsia and its tick vector are still poorly understood one century after the first description of this disease

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