Submitted to: Parasites & Vectors
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 19, 2010
Publication Date: November 19, 2010
Citation: Bastos, R., Ueti, M.W., Knowles Jr, D.P., Scoles, G.A. 2010. The Rhipicephalus (Boophilus) microplus Bm86 gene plays a critical role in the fitness of ticks fed on cattle during acute Babesia bovis infection. Parasites & Vectors. 3:111. Interpretive Summary: Although vaccines to prevent or reduce infestation of cattle with the cattle tick, Rhipicephalus (Boophilus) microplus, have had some efficacy, they are not commercially available in the US. These vaccines are based on Bm86, a protein that is a normal part of the gut of R. microplus. When cattle are induced to produce an immune response against this protein the feeding success of ticks is reduced. However, in spite of the fact that this protein was described many years ago, there is very little data describing the tick tissues expressing Bm86, or its function. In this study we show that the Bm86 gene is expressed in larvae, nymphs, adult males, and gut and ovaries of partially engorged adult female ticks. Targeted silencing of Bm86 expression resulted in an increased level of mortality when female ticks were fed to repletion on a calf that was acutely infected with Babesia bovis, a disease of cattle that is common in many parts of the world where this tick species occurs. Silencing of the Bm86 gene significantly decreased the reproductive fitness of ticks fed on a B. bovis-infected calf, suggesting that Bm86 might be a good target for a vaccine intended to reduce transmission of this pathogen.
Technical Abstract: Background: Rhipicephalus (Boophilus) microplus is an economically important tick of cattle involved in the transmission of Babesia bovis, the etiological agent of bovine babesiosis. Commercial anti-tick vaccines based on the R. microplus Bm86 glycoprotein have shown some effect in controlling tick infestation; however their efficacy as a stand-alone solution for tick control has been questioned. Understanding the role of the Bm86 gene product in tick biology is critical to identifying additional methods to utilize Bm86 to reduce R. microplus infestation and babesia transmission. Additionally, the role played by Bm86 in R. microplus fitness during B. bovis infection is unknown. Results: Here we describe in two independent experiments that RNA interference-mediated silencing of Bm86 decreased the fitness of R. microplus females fed on cattle during acute B. bovis infection. Notably, Bm86 silencing decreased the number and survival of engorged females, and decreased the weight of egg masses. However, gene silencing had no significant effect on the efficiency of transovarial transmission of B. bovis from surviving female ticks to their larval offspring. The results also show that Bm86 is expressed, in addition to gut cells, in larvae, nymphs, adult males and ovaries of partially engorged adult R. microplus females, and its expression was significantly down-regulated in ovaries of ticks fed on B. bovis-infected cattle. Conclusion: The R. microplus Bm86 gene plays a critical role during tick feeding and after repletion during blood digestion in ticks fed on cattle during acute B. bovis infection. Therefore, the data indirectly support the rationale for using Bm86-based vaccines, perhaps in combination with acaricides, to control tick infestation particularly in B. bovis endemic areas.