The inability of tick to maintain pathogenic and nonpathogenic species of spotted fever group rickettsiae via transovarial transmission in nature has been well described. However, the rickettsia-specific basis and mechanism by which this interference occurs is not known. To determine if a similar phenomenon is observed between nonpathogenic species of rickettsiae a challenge bioassay was developed. Using a capillary feeding method Rickettsia montanensis- and Rickettsia rhipicephali-infected lines of Dermacentor variabilis were established, and R. montanensis-infected ticks were exposed to R. rhipicephali, and vice versa, in reciprocal challenge experiments. Species-specific primers were used in a PCR to monitor rickettsial infection in ticks for two generations. While both cohorts of ticks were refractory to their respective challenge rickettsiae, the lack of detectable levels of infection in the R. rhipicephali-infected cohort enabled these ticks to acquire R. montanensis and transmit the rickettsiae to their progeny. The resistance to co-infection and apparent host-specific nature of infection has led to an examination of tick-rickettsiae interactions at the molecular level. PCR-based techniques, including differential display and subtractive hybridization, have been used to identify candidate molecules that may play a role in rickettsial maintenance, as well as transovarial transmission interference. The data indicate that a number of tick-derived molecules are differentially expressed in the ovaries of D. variabilis, as well as in other tissues, in response to rickettsial infection.
Species 1: Acari Ixodidae Dermacentor variabilis
Species 2: Rickettsiales Rickettsiaceae Rickettsia montanensis
Species 3: Rickettsiales Rickettsiaceae Rickettsia rhipicephali
Keywords: disease transmission, Rickettsiae
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