<italic>Rhipicephalus appendiculatus</italic> ticks transmit <italic>Theileria parva</italic> from persistently infected cattle in the absence of detectable parasitemia: implications for East Coast fever epidemiology.

Background: East Coast fever (ECF) is a devastating disease of cattle and a significant constraint to improvement of livestock production in sub-Saharan Africa. The protozoan parasite causing ECF, <italic>Theileria parva</italic>, undergoes obligate sexual stage development in its tick vector <italic>Rhipicephalus appendiculatus</italic>. Tick-borne acquisition and transmission occurs transstadially; larval and nymphal ticks acquire infection while feeding and transmit to cattle when they feed after molting to the next stage. Much of the current knowledge relating to tick-borne acquisition and transmission of <italic>T. parva</italic> has been derived from studies performed during acute infections where parasitemia is high. In contrast, tick-borne transmission during the low-level persistent infections characteristic of endemic transmission cycles is rarely studied. Methods: Cattle were infected with one of two stocks of <italic>T. parva</italic> (Muguga or Marikebuni). Four months post-infection when parasites were no longer detectable in peripheral blood by PCR, 500 <italic>R. appendiculatus</italic> nymphs were fed to repletion on each of the cattle. After they molted to the adult stage, 20 or 200 ticks, respectively, were fed on two naïve cattle for each of the parasite stocks. After adult ticks fed to repletion, cattle were tested for <italic>T. parva</italic> infection by nested PCR and dot blot hybridization. Results: Once they had molted to adults the ticks that had fed as nymphs on Muguga and Marikebuni infected cattle successfully transmitted <italic>Theileria parva</italic> to all naïve cattle, even though <italic>T. parva</italic> infection was not detectable by nested PCR on salivary gland genomic DNA of a sample of individual ticks. However, a salivary gland homogenate from a single Marikebuni infected tick was able to infect primary bovine lymphocytes. Infection was detected by nested p104 PCR in 3 of 4 calves and detected in all 4 calves by <italic>T. parva</italic> 18S nested PCR/dot blot hybridization. Conclusion: We show that <italic>R. appendiculatus</italic> ticks are able to acquire <italic>T. parva</italic> parasites from infected cattle even in the absence of detectable parasitemia. Although infection was undetectable in a sample of individual ticks, cumulatively as few as 20 ticks were able to transmit <italic>T. parva</italic> to naïve cattle. These results have important implications for our understanding of <italic>T. parva</italic> transmission by <italic>R. appendiculatus</italic> in ECF endemic regions. [ABSTRACT FROM AUTHOR]