Ehrlichia ruminantium in Amblyomma hebraeum ticks and their infestation on goats in the Mnisi area (Mpumalanga), South Africa: first year analysis

Abstract

The obligate intracellular gram-negative rickettsia Ehrlichia ruminantium is the cause of heartwater. Ticks of the genus Amblyomma are vectors of this disease in small and large ruminants in South Africa. The main vector tick is Amblyomma hebraeum. This tick has a three-host life cycle consisting of adults, nymphs and larvae. The relative abundance and seasonal dynamics of these ticks and the prevalence of infection with E. ruminantium are important factors to take into perspective when studying the epidemiology of heartwater. This study looked at these factors in four sampling periods over a period of one year in the Mnisi area (Mpumalanga), South Africa. It showed that there is a significant difference in the median of the mean burden of ticks per goat in June/July 2013, November 2013, March/April 2014 and June/July 2014. Adult ticks were predominant in summer (November 2013). Nymphs were the highest representatives of the different stages throughout the year. E. ruminantium-infected nymphs were found during the whole year with no significant difference between the four sampling periods. The infection rate of the nymphs varied from 12% to 47% and the adult infection rate varied between the 10% to 25%. Larvae were not infected. These data offer more insight in the epidemiology of heartwater in the area. The significant difference in tick abundance over the year is important for subsequent control methods, as is the observation that the infection rate is not different between the periods. These results show that the goats are continuously challenged throughout the year with E. ruminantium and that their highest burden is in winter. However to get a better understanding of the dynamics of this three-host tick and its influence on the dynamics it is advised to have a closer look at the larval stage. This has two reasons. First, the larval stage contributes to the infection rate of the next instar and secondly this stage could give more insight in the infection rate with E. ruminantium in goats. Therefore, further research is needed to continue the monitoring of the abundance and seasonal dynamics of A. hebraeum on the goats in Mnisi (Mpumalanga), South Africa.