Faculty of Veterinary Science
Department of Veterinary Tropical Diseases
Selected Highlights from Research Findings
A real-time polymerase chain reaction (PCR) assay based on species-specific sequences in the 18S ribosomal RNA gene has been developed for detection of Theileria parva infections in buffalo and cattle. The assay has proven to be more sensitive and specific than previous tests for T. parva and has been adopted by the ARC-OVI as a test of choice for diagnosis of T. parva infections. South African T. parva isolates have been characterized by PCR-RFLP analysis of the p67, p104 and polymorphic immunodominant molecule (PIM) genes in collaboration with the Institute of Tropical Medicine, Antwerp, Belgium. Phylogenetic analysis of sequence data obtained for the p67 and PIM genes is in progress. We believe that the outcome of this work will help us to establish whether South Africa is at risk of a recurrence of East Coast fever. Currently there is ongoing work on the characterization of Theileria sp. (buffalo) and other undescribed Theileria species present in African buffalo. In mixed infections with T. parva, the presence of Theileria sp. (buffalo) compromises the sensitivity of the real-time PCR assay in buffalo, and other previously uncharacterized Theileria species could further complicate the diagnosis of T. parva. Preliminary results indicate that there is some extent of genetic diversity in the 18S rRNA gene among isolates of Theileria sp. (buffalo). Using comparative genomics, we aim to identify a new gene target specific to T. parva which can be used for the development of a new real-time PCR assay that will be highly specific for the detection of T. parva in buffalo and cattle.
Contact person: Dr MC Oosthuizen.
Haemonchus is the most pathogenic nematode parasite in the small ruminant industry and resistant populations of the parasite occur widely disseminated in the world, with some of them resistant simultaneously to every applicable anthelmintic group. Based on the FAMACHAŠ method of clinical classification of anaemia, applicable to targeted selective treatment, the framework for a simulation model was developed that takes into account the climate and animal biological variables such as haemoglobin levels and body mass to evaluate the risk of haemonchosis in sheep during the actual worm season. The results indicated that under the conditions where the data were generated, the FAMACHAŠ system is sensitive enough, and adequately specific, to detect anaemic sheep despite some misclassification. The application of Receiver Operating Characteristic curve analysis to the FAMACHAŠ method to select FAMACHAŠ categories for treatment, was in agreement with the findings that misclassification on any farm would of necessity require that, unless the evaluators concerned are retrained, different treatment thresholds would have to be implemented to achieve the same test sensitivity on other farms. Although the application of the Receiver Operating Characteristic method requires the use of dedicated software, especially if large data sets are analysed, it was found to be an accurate and valid way of indicating FAMACHAŠ threshold categories for treatment on both farms, for a desired sensitivity. A previously published multiple regression model was modified to incorporate stochasticity in the FAMACHAŠ category proportions and the body mass of sheep, in order to simulate probable worm count. The fluctuations in simulated worm count adequately reflected the changing epidemiological situation of haemonchosis as indicated by temporal histograms of differential FAMACHAŠ proportions in flocks. The model was most sensitive to changes in FAMACHAŠ proportions in the sample, followed by increasing variability in body mass as a worm season progressed. Furthermore, for a given class of animal, a range of probable haemoglobin values could be associated with a pre-selected threshold worm burden. The model was sensitive to blanket drenching events, as a lower intensity of infection was predicted immediately after blanket drenching in all samples. The model is the prototype of an electronic predictive system that will enable farmers to make optimal decisions about when to treat their animals for Haemonchus, if at all, in order to delay the onset of resistance to anthelmintics.
Contact person: Dr JA Van Wyk.
Heartwater is a lethal disease of domestic and some wild ruminants which occur throughout sub-Saharan Africa. It is caused by an intra-cellular tick-borne organism, Ehrlichia ruminantium, and approximately 150 million domestic livestock are at risk throughout the endemic area. The annual economic costs of tick control, animal deaths and production losses amount to tens of millions of rands per annum in South Africa, and yet there is no effective vaccine against the disease. The National Research Foundation funds two heartwater research projects which aim to develop an effective recombinant vaccine; these are being undertaken as a collaboration between the Department of Veterinary Tropical Diseases (DVTD), the Onderstepoort Veterinary Institute (OVI), and the Bioinformatics Unit of the Department of Biochemistry, UP. One of these projects has identified potential vaccine candidate genes from the genome sequence of E. ruminantium, which are then expressed in vitro and screened to determine whether they stimulate immune responses in cells taken from animals immune to heartwater. Genes which give promising results in the process are then tested as an experimental vaccine formulation in sheep for their ability to stimulate protection against the disease. The other project aims to identify vaccine candidate genes by a different process. A virulent strain of E. ruminantium has been attenuated so that it no longer causes disease, although it still infects both animals and ticks. Expression profiles of the attenuated and virulent strains are being compared in order to identify genes which are implicated in infectivity and virulence, and these too will be tested in the experimental vaccine formulation described above. This work has produced numerous publications in the international scientific literature over several years and students have been and are being trained, at both MSc and PhD levels.
Contact person: Prof BA Allsopp.