Incongruent effects of two isolates of Rickettsia conorii on the survival of Rhipicephalus sanguineus ticks

Abstract

Rickettsia conorii, the etiologic agent of Mediterranean spotted fever is widely distributed in Southern Europe, the Middle East, Africa, India and the Caspian region. In the Mediterranean region, the brown dog tick, Rhipicephalus sanguineus, is the recognized vector of R. conorii. To study tick-pathogen relationships and pathogenesis of infection caused in model animals by the bite of an infected tick, we attempted to establish a laboratory colony of Rh. sanguineus persistently infected with R. conorii. Rhipicephalus sanguineus ticks of North American and Mediterranean origin were exposed to R. conorii isolates of African (R. conorii conorii strain Malish) and Mediterranean (R. conorii israelensis strain ISTT) origin. Feeding of ticks upon infected mice and dogs, intra-hemocoel inoculation, and submersion in suspensions of purified rickettsiae were used to introduce the pathogen into uninfected ticks. Feeding success, molting success and the longevity of molted ticks were measured to assess the effects of R. conorii on the survival of Rh. sanguineus. In concordance with previously published results, Rh. sanguineus larvae and nymphs from both North American and Mediterranean colonies exposed to R. conorii conorii Malish experienced high mortality during feeding and molting or immediately after. The prevalence of infection in surviving ticks did not exceed 5%. On the other hand, exposure to ISTT strain had lesser effect on tick survival and resulted in 35-66% prevalence of infection. Rh. sanguineus of Mediterranean origin were more susceptible to infection with either strain of R. conorii than those from North America. Previous experimental studies had demonstrated transovarial and transstadial transmission of R. conorii in Rh. sanguineus; however, our data suggest that different strains of R. conorii may employ different means of maintenance in nature. The vertebrate host may be a more important reservoir than previously thought, or co-feeding transmission between different generations of ticks may obviate or lessen the requirement for transovarial maintenance of R. conorii.

Co-feeding as a route for transmission of Rickettsia conorii israelensis between Rhipicephalus sanguineus ticks

“…We chose this particular isolate because it had shown to be infectious to Rh. sanguineus and to cause only low mortality in ticks, whereas infection with R. conorii strain Malish was detrimental to infected ticks (Levin et al 2009). The agent was grown in Vero E6 cells at 32°C in antibiotic-free Minimal Essential Medium supplemented with 2% fetal calf serum and 2 mg/ml L-glutamine.…”

“…Rhipicephalus sanguineus ticks for this study were derived from colonies of North American and Mediterranean origins maintained in our laboratory as previously described (Levin et al 2009;Troughton and Levin 2007). The North American colony originated from adult ticks collected in Oklahoma, USA in 2001, and the Mediterranean colony originated from adult ticks collected in Israel in 2005.…”

“…Cohorts of infected adult ticks were produced by initial feeding of uninfected larvae from both colonies upon dogs inoculated intravenously with R. conorii israelensis, and following feeding of infected nymphs upon a naïve rabbit as previously described (Levin et al 2009). The resulting prevalence of infection was 50.0 ± 22.48 and 30.0 ± 20.61% in adult ticks from North American and Mediterranean colonies, respectively.…”

Update on Tick-Borne Rickettsioses around the World: a Geographic Approach

“…conorii and its tick host was called into question based on the lethal effect of this species of Rickettsia on Rhipicephalus sanguineus, the brown dog tick, in experimental models of infection (20,21). The reasons for this reduction in fitness were shown to be unrelated to the geographical origin of the ticks (22) and to the inoculation methods (20,23). However, when naturally infected Rhipicephalus sanguineus individuals were used (24), larvae, nymphs, and adults were maintained under laboratory conditions over several generations with a transovarial transmission (TOT) rate (the percentage of infected females that pass microorganisms to their progeny) that reached 100% and a filial infection rate (FIR) (the percentage of infected progeny derived from an infected female) of nearly 99% (25).…”

Ehrlichia chaffeensis Infections in Drosophila melanogaster

“…In the same study, fewer larvae developed from infected ticks. Decreases in molting success and increased death were also observed in a study in which Rhipicephalus sanguineus ticks were experimentally infected with Rickettsia conorii (36).…”