Abstract:
Rickettsia africae is a gram-negative bacterium which causes African tick bite fever (ATBF) in humans. ATBF is a febrile disease mainly affecting travellers to Southern Africa. This bacterium is known to be transmitted by Amblyomma hebraeum and Amblyomma variegatum ticks. In Southern Africa, the principal vector is A. hebraeum. This project was performed in a rural community in Mpumalanga province and aimed at addressing knowledge gaps of R. africae infection in A. hebraeum ticks. Infection rates in adult ticks and larvae as well as transovarial transmission efficiency of R. africae from the tick to its offspring were determined. To accomplish this, 106 adult A. hebraeum ticks were collected from cattle from Utah A and 106 from Welverdiend A. Larvae (n=1060) were collected by dragging at each of the two targeted dip tanks. Engorged female A. hebraeum ticks (n=53) were also collected from cattle from each of the two dip tanks and they were incubated in a humidity tank to oviposit and egg masses were collected from each tick.
DNA was extracted from the engorged ticks and the egg masses as well as from the adult ticks and the larvae. After DNA quantification, a real-time quantitative PCR targeting Rickettsia gltA gene was performed to screen all samples for Rickettsia DNA. The gltA gene is common in all Rickettsia species. Samples positive for the gltA gene were subjected to conventional PCR targeting the ompA gene, which is specific for the Spotted Fever Group to which R. africae belongs.
The samples positive for amplicons of ompA gene were sequenced and all the sequenced samples were found to be 99.98% identical to R. africae sequences from GenBank. From the sampled adult ticks, 13.43% tested positive for R. africae and 13.20% of the larvae also tested positive. The infection rate for larvae and adult ticks collected from Utah A was 15.09% and those from Welverdiend A was 11.79%. From these results, there were no notable differences in the infection rates of ticks at different stages of development. R. africae infection rates for the two study sites were found to be similar which can be an indication of even distribution of R. africae in this area. Transovarial transmission was found to be 100% in engorged female ticks collected from Utah and 71.43% in ticks from Welverdiend, which indicates a high transmission rate of the pathogen to the offspring of the vector. The presence of R. africae in A. hebraeum ticks from this area is a cause of concern since there are chances of people getting ATBF after bites by these ticks.
