Faculty of Veterinary Science
Department of Veterinary Tropical Diseases
Selected Highlights from Research Findings
Bovine tuberculosis is a chronic disease of livestock which can be transmitted to a wide range of wildlife species some of which can maintain the disease with far reaching ecological, economic and veterinary public health impacts. The availability of reliable diagnostic tests to identify infected wild animals and populations is one of the cornerstones of managing the disease at the wildlife/livestock interface. Our current research aims in this project are to optimise and validate diagnostic assays, which have been developed for livestock, for affected wildlife hosts. Examples where we have achieved this objective (and published the results internationally) include the optimisation of the interferon gamma assay for the diagnosis of bovine tuberculosis in African buffalo and the intradermal tuberculin skin test in lions. More recently we developed new interferon gamma assays for the detection of this disease in rhinoceroses and lions. Promising preliminary results have provided a ‘proof of principle’ of the test system in blood samples from these species and a full validation will be pursued in collaboration with conservation authorities such as SANParks.
Contact person: Dr AL Michel.
A molecular epidemiological survey of the parasites that cause equine piroplasmosis revealed the existence of as many as 25 distinct 18S rRNA sequences for T. equi, and six different B. caballi 18S sequences, in horses and zebra in South Africa. A TaqMan quantitative real-time PCR (qPCR) assay targeting the 18S rRNA gene was developed for the detection of B. caballi, and a recently developed T. equi-specific qPCR assay was evaluated for its ability to detect T. equi 18S rRNA variants identified in South Africa. Both tests were sensitive and specific and could detect the parasites in field samples.
A B. caballi-specific competitive enzyme-linked immunosorbent assay (cELISA) failed to detect B. caballi antibody in the sera of infected horses from South Africa. The rhoptry-associated protein gene (rap-1) that codes for the antigen used in the cELISA was therefore characterized from South African B. caballi isolates. Significant sequence heterogeneity in the rap-1 gene sequences was found. Marked amino acid sequence differences in the carboxy-terminal region, and therefore the probable absence of the monoclonal antibody binding site, explains the failure of the cELISA to detect antibody to B. caballi in South African samples.
The existence of genetic and serological heterogeneity in T. equi and B. caballi parasites in South Africa explains the failure of many molecular and serological tests to detect these parasites in field samples in this country. Our results add substantially to the existing knowledge of equine piroplasmosis and will be invaluable in the development of further molecular or serological diagnostic assays.
Contact person: Dr NE Collins.
Lumpy skin disease, caused by the lumpy skin disease virus (LSDV), a poxvirus, is an economically important cattle disease in Africa and sporadically the Middle East. In general, poxviruses have been shown to enter the host through the skin or respiratory tract. Direct contact between infected and susceptible animals is considered to be a relatively inefficient route of transmission of LSDV. Biting flies and mosquitoes are suspected to play a role in mechanical transmission; however, no studies on the potential role of ixodid (hard) ticks in the transmission of LSDV have been carried out. This is a collaborative project between the Institute for Animal Health, Pirbright, UK and the DVTD, Faculty of Veterinary Science, UP where the role of ixodid (hard) ticks in the transmission of the virus is investigated. Three common African tick species (genera Rhipicephalus, Amblyomma and Rhipicephalus (Boophilus)) in different life cycle stages were fed on LSDV infected animals during the viraemic stage and on skin lesions. Partially fed male ticks were transferred to the skin of non-infected “recipient” animals. Females were allowed to lay eggs and nymphs were allowed to develop. Ticks and tick eggs were tested using polymerase chain reaction (PCR) and virus isolation (VI). Skin and blood samples from “recipient” cattle were tested using PCR and VI. This is the first report showing molecular evidence of transmission of LSDV by ixodid ticks. Viraemia was detected in one R. appendiculatus recipient animal. Evidence of transstadial and transovarial transmission of LSDV by R. (B.) decoloratus ticks, and mechanical or intrastadial transmission by R. appendiculatus and A. hebraeum ticks were shown.
Contact person: Prof EH Venter.
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