Submitted to: Journal of Medical Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/9/2009
Publication Date: 7/1/2009
Publication URL: ddr.nal.usda.gov/dspace/bitstream/10113/31394/1/IND44229129.pdf
Citation: Odongo, D., Ueti, M.W., Mwaura, S., Knowles Jr, D.P., Bishop, R., Scoles, G.A. 2009. Quantification of Theileria parva in Rhipicephalus appendiculatus (Acari: Ixodidae) Confirms Differences in Infection Between Selected Tick Strains. Journal of Medical Entomology. 46(4):888-894. Interpretive Summary: East Coast Fever (ECF) is an economically important disease of cattle in sub-Saharan Africa, and is one of the factors preventing development of an improved cattle industry in the region. ECF is caused by a protozoan parasite that is biologically transmitted by ticks; interventions that control ticks, or reduce their ability to act as vectors may lead to improved methods for controlling transmission. Developing an understanding of the way the tick vector and the parasite interact requires tools that allow us to assess the level of parasite replication in various tick tissues. The molecular techniques described in this paper allow us to sensitively detect the presence of the parasite (nPCR) and to estimate the numbers of organisms (qPCR). Using these techniques we studied two tick strains that are genetically different in their ability to act as hosts for the parasite. We found that the tick strain with the highest level of susceptibility consistently takes in more parasites in its blood meal and that this leads to both a higher proportion of infected ticks and a higher level of infection in those that get infected. We hope that these studies will help us discover the genes in the tick that are responsible for transmission. Once we have identified these genes they may be targets for anti-tick or transmission blocking vaccines.
Technical Abstract: Theileria parva is the etiologic agent of East Coast fever (ECF), an economically important disease of cattle in sub-Saharan Africa. This protozoan parasite is biologically transmitted by Rhipicephalus appendiculatus. An understanding of the details of the vector-parasite interaction may aid the development of improved methods for controlling transmission. We developed quantitative PCR (qPCR) and nested PCR (nPCR) assays targeting the T. parva-specific p104 gene to study T. parva pathogenesis in two strains of R. appendiculatus that had previously been selected to be relatively more (Kiambu) or less (Muguga) susceptible to infection. Nymphs from both strains were fed simultaneously on acutely infected calves. Immediately following engorgement qPCR confirmed that nymphal Kiambu ticks had significantly higher parasite loads at repletion than Muguga nymphs. By 12 days post engorgement parasites were below quantifiable levels but could be detected by nPCR in 83-87% (Muguga and Kiambu, respectively) of nymphs. After the molt, adult feeding on naïve cattle stimulated parasite replication in the salivary glands. PCR detected significantly more infected ticks than microscopy, and there was a significant difference between the two tick strains both in the proportion of ticks that develop salivary gland infections, and in the number of parasites within infected salivary glands. These data confirm that although both tick strains were competent vectors, Kiambu is both a significantly more susceptible and a more efficient host for T. parva than Muguga. The mechanisms that contribute to the levels of susceptibility and efficiency are unknown, however this study lays the groundwork for a comparison of the transcriptome of these tick strains, the next step towards discovering the genes involved in the tick-parasite interaction.