Monitoring vaccinated cattle for induction and longevity of persistent tick-transmissible infection: Implications for wider deployment of live vaccination against East Coast fever in Tanzania.

Abstract

The infection and treatment (ITM) procedure remains the only available method of immunization against Theileria parva infection. One constraint to deployment is the perception that the carrier state induced by ITM could result in enhanced disease problems. More than one million cattle have been ITM vaccinated in pastoralist systems in Tanzania over the last 2 decades. We present the results of a longitudinal study of six groups of cattle in Maasai villages in northern Tanzania exposed to natural tick challenge for between 2weeks and 14years post-vaccination. The p104 nested PCR revealed a higher frequency of T.parva carriers among vaccinates (30%) compared with controls (8%) (OR=4.89, p=.000), with the highest frequency of carriers found in calves vaccinated 6months previously, although carrier state was also detected in cattle vaccinated >10years prior to the study. Variable number tandem repeat genotype analysis revealed 6 MS7 alleles with sizes ranging from 150bp to 500bp, but only two alleles were detected in cattle vaccinated >4years earlier, relative to five alleles detected in recently vaccinated cattle and controls. In terms of heterozygosity, diversity was maximal in calves vaccinated within the last 2weeks (h=0.776) but lowest in cattle vaccinated 4years earlier (h=0.375). The analysis suggested close genetic relatedness of parasites in vaccinated and unvaccinated groups and up to 96% of variation was within rather than between the groups. These results confirm that ITM leads to a long-term T.parva carrier state in cattle and the detection of vaccine component VNTR in co-grazing unvaccinated cattle suggests potential vaccine transmission by ticks. However, vaccination stocks did not totally replace local genotypes, at least in cattle populations. These findings should mitigate concerns that ITM modifies T.parva field populations in a way that enhances disease in the medium term.