| Summary: | <p>Abstract</p> <p>Background</p> <p>The cattle tick <it>Rhipicephalus (Boophilus) microplus </it>is involved in the transmission of the protozoan <it>Babesia bovis</it>, the etiological agent of bovine babesiosis. Interactions between ticks and protozoa are poorly understood and the investigation of tick genes that affect tick fitness and protozoan infection can set the stage for dissecting the molecular interactions between the two species.</p> <p>Results</p> <p>In this study, RNA interference was used to silence <it>R. microplus </it>genes that had been previously shown to be up-regulated in response to <it>B. bovis </it>infection. The silencing of a putative immunophilin gene (<it>Imnp</it>) in female ticks fed on a calf acutely infected with <it>B. bovis </it>decreased the hatching rate and survival of larval progeny. Interestingly, <it>Imnp </it>was up-regulated significantly in ovaries of <it>R. microplus </it>in response to <it>B. bovis </it>infection and its silencing in female ticks significantly increased the infection rate of the protozoan in larval progeny. The results also showed that the silencing of a putative Kunitz-type serine protease inhibitor (<it>Spi</it>) gene and a putative lipocalin (<it>Lpc</it>) gene decreased the fitness of <it>R. microplus </it>females, but had no significant effect on the infection rate of <it>B. bovis </it>in larval progeny.</p> <p>Conclusion</p> <p>The silencing of the <it>Imnp</it>, <it>Spi </it>or <it>Lpc </it>genes decreased the fitness of <it>R. microplus </it>females fed on a calf during acute <it>B. bovis </it>infection. The <it>Imnp </it>gene data suggest that this putative immunophilin gene is involved in the defense system of <it>R. microplus </it>against <it>B. bovis </it>and may play a role in controlling the protozoan infection in tick ovaries and larval progeny.</p>
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