Why are there so few Rickettsia conorii conorii-infected Rhipicephalus sanguineus ticks in the wild?
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.


RESULTS
An 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.


CONCLUSIONS
These 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.
Faculté de Médecine, Université de la Méditerranée, Unité de Recherche en Maladies Infectieuses et Tropicales Emergentes (URMITE), UMR CNRS-IRD 6236, WHO Collaborative Center for Rickettsial Diseases and Other Arthropod-Borne Bacterial Diseases, Marseille, France.
Cristina
C
Socolovschi
Jean
J
Gaudart
Idir
I
Bitam
Thi Phong
TP
Huynh
Didier
D
Raoult
Philippe
P
Parola
D000462:Algeria; D000818:Animals; D005260:Female; D007814:Larva / Q000382:microbiology / Q000502:physiology; D008853:Microscopy; D010053:Ovary / Q000382:microbiology / Q000473:pathology; D048508:Rhipicephalus sanguineus / Q000382:microbiology* / Q000502:physiology; D020114:Rickettsia conorii / Q000302:isolation & purification*; D016019:Survival Analysis; D013696:Temperature