Effect of East Coast Fever Marikebuni Vaccine on Feeding and Reproductive Success of Rhipicephalus Appendiculatus

Abstract

East Coast Fever is a tick-borne disease of economic importance in bovines caused by Theileria parva, a protozoan parasite transmitted transstadially by a three-host tick, Rhipicephalus appendiculatus. The control of ECF requires integrated pest management, including vaccine development. Theileria parva Marikebuni vaccine is a live parasite vaccine used to immunize cattle against ECF. At present, there is no information on the effect of this vaccine on the feeding and reproductive success of R. appendiculatus. This study investigated the interaction between the T. parva vector and its immunized host,cattle. Three groups of ECF naïve calves aged between 3-12 months were selected for the study, each having six calves. Calves in the first group, the “immunized group”, received 1 ml of Marikebuni vaccine and a long-acting oxytetracycline blocking agent at 30ml/kg body weight. The 2nd group, “Control Group 1” received uninfected tick material and the blocking agent while the 3rd group, “Control Group 2” received the uninfected tick material only. Ticks were counted and weighed before and after application to calves. Incubation of ticks was done at 27-28 ⁰C optimum temperature and 80% -85% humidity. The number of ticks that fed successfully, the mean blood meal weight, and the number of successfully molted nymphs were analyzed to indicate the feeding success of R. appendiculatus. The mean egg mass weight, number of egg batches and clutch sizes of eggs indicated oviposition success. The number of live larvae hatched determined the egg viability. Data were compared using linear regression, an ANOVA test, and a binary logistic regression model using R statistical software Version 4.1.3 (2022-03-10), at a p<0.05 significance level. The outcomes of this study showed that the number of fed nymph ticks (OR = 0.996; p-value = 0.29); nymph blood meal weight (OR = 0.712; p-value = 0.34); number of successfully molted nymphs (OR = 1.004, p value = 0.36); number of successfully fed adult female ticks (OR = 1.05; p-value = 0.93); and the adult female tick blood meal weight (OR = 0.32, p value = 0.48; F = 3.26, p-value = 0.11)did not differ significantly between R. appendiculatus that fed on immunized calves and those that fed on controls. The egg mass weight differed significantly between the ticks that fed on immunized calves and controls (F = 7.993; p-value = 0.023), and a significant pairwise average difference of 1.34 was detected between the ticks that fed on immunized and control group 2 calves and between those that fed on control groups 1and 2 (adjusted p-value =0.03). Other parameters determining oviposition success did not differ significantly between the ticks fed on the immunized group and the controls: clutch sizes of eggs (OR = 1.0, P-value = 0.40); number of egg batches (OR = 0.08, p-value = 0.35). The number of live larvae that hatched successfully did not differ significantly between the immunized and control groups (OR = 0.99; p-value = 0.33). In conclusion, the blood feeding success of Rhipicephalus appendiculatus was not significantly altered by the vaccine, despite the observed differences (p >0.05). The oviposition success of the tick vector was altered by the vaccine, reducing the egg mass weight in ticks that fed on the immunized group significantly (p<0.05). The viability of eggs from R. appendiculatus ticks was not significantly impacted by the vaccine, despite the observed differences (p > 0.05). The findings of this work shed light on the efficacy of the Marikebuni vaccine against R. appendiculatus. Significantly reduced egg mass weight on ticks fed on cattle immunized with Marikebuni vaccine shows that the vaccine indirectly leads to some degree of vector control. The production and distribution of this vaccine in ECF endemic regions of sub-Saharan Africa should therefore be increased.