Project Manager: Dr Gert Venter

Worldwide, at least 66 different arbovirusses, 15 protozoans and 26 filarial wormshave been isolated from a variety of Culicoides biting midges (Diptera: Ceratopogonidae). Important livestock viruses transmitted by Culicoides species include African horse sickness virus (AHSV), bluetongue virus (BTV), epizootic hemorrhagic virus (EHDV), equine encephalosis virus (EEV), Akabane virus (AKAV) and the Palyam viruses. At least three of these, AHSV, BTV and EHDV, cause diseases of such international significance that they have been allocated Office International des Epizooties (OIE), the World Organisation for Animal Health list status. Diseases listed by the OIE have the potential to spread rapidly from one country to another, cause high mortality and morbidity in susceptible animals, and affect international trade in livestock and livestock products.

Losses due to arboviruses are evident at regional, national, and international levels, and cause severe hardship in rural communities that are dependent on ruminants for food and equines for motive power. Although these viruses may be found in many countries, specific pathogens may be absent from specific regions. Regional differences in the presence of these viruses may be due to differences in vector species and/or differences between populations of vectors. The reasons for the regional differences in the Orbiviruses, other than different insect species, are unknown. Bluetongue in particular has become established in all areas of the world where competent vector species occur and has vastly expanded its distribution since 1998 and outbreaks up to latitude 44°30’N have occurred, over 800 km further north than previously recorded. In August 2006 a sub-Saharan strain of BTV serotype 8 caused an unprecedented outbreak of BT among sheep and cattle in northern Europe. This virus not only overwintered in the region, but additionally spread rapidly across a large geographic area during subsequent years. These outbreaks of bluetongue were followed by the emergence and re-emergence of Schmallenberg virus in northern Europe. Our understanding of the epidemiological factors limiting both spread and transmission of these viruses remains poor.

The endemic presence of AHSV and other orbiviruses in southern Africa greatly impedes the movement of horses and other animals from South Africa to Europe and the rest of the world. These viruses are transmitted almost exclusively by certain species of Culicoides biting midges, the most important in South Africa being the Afro-Asiatic species Culicoides imicola. Transmission is only achieved when infected Culicoides females feed on a susceptible host. The geographical distribution and seasonal incidence of these orbiviruses is thus limited not only by the availability of the virus and susceptible vertebrate hosts, but also by the presence of competent arthropod vectors. The presence AHSV represents a major barrier to trade of domestic and wild livestock and to participation in sporting events for South Africa and adjacent countries. Outbreaks of AHS in the surveillance zone of the declared AHS free area in Cape Town, each time led to a two year embargo on the export of horses from South Africa costing the industry about R50 million (USD 8.2 million) per annum. Assessing the real cost is difficult. The cost of arboviral disease is due to direct costs that accrue to producers through disease, and indirect costs that comprise losses in economic benefits of trade and sporting events. Each country in Africa experiences a different effect from the orbiviruses. These national differences should be assessed.

There is increasing world interest in the wildlife resources of the region and interest in translocation of wildlife. The direct impact of disease in livestock cannot be quantified in purely monetary terms. In many societies in southern Africa livestock has a great social significance and exchange of livestock is a part of many family relationships. South Africa's current policy of reconstruction and development will lead to a change in livestock farming practices. Livestock ownership may not be matched by vaccine access and use; credit access may change irrigation patterns.

Studies regarding Culicoides midges at the ARC-OVI have shown AHS, BT, EE and EHD to potentially be multi-vectored diseases. This will increase the area and time periods at risk. It was shown that the live attenuated vaccine strains of both AHSV and BTV can replicate in Culicoides species. These results were now confirmed for BT in northern Europe and highlighted the need for the development of an inactivated vaccine for these diseases. High Culicoides abundance and therefore high risk of AHS occurring was shown in the AHS free and surveillance zones in the Western Cape. In addition it was shown that vector free periods are not present in most parts of South Africa and that relatively large populations of Culicoides midges may be found in winter in the Western Cape. Considering these uncertainties, coupled to the relatively high abundance of Culicoides midges in this area it is advisable that susceptible animals must be protected against these orbiviruses at all times.

It is important that the baseline understanding of the environmental characteristics man contributes to the epidemiology of orbiviruses be improved before development changes inadvertently create ideal habitats for vector proliferation. The ARC-OVR is the only organization in South Africa that has the capacity and infra-structure to accurately identify and determine the presence of the vectors and therefore the risk of the disease occurring in any given area. The ability to identify the vectors as well as the viruses (at the OIE reference centers located at the ARC-OVR) involved, renders the ARC-OVR one of the strongest research groups to determine risk assessment studies regarding these diseases. A strong molecular biology group at the ARC-OVR supplements studies regarding the AHSV and other orbiviruses.

The capacity and expertise are highlighted by the number of international visitors from various European countries and the involvement of personnel as supervisors on post graduate studies at several universities.

Research regarding Culicoides midges at the ARC-OVR is primarily depended on outside funding (e.g. European Union) and is therefore mostly opportunistic. Limited collections in the Eastern Cape showed that the numbers of C. imicola and therefore the risk of AHS may have increase in this area. In the absence of a long-term monitoring programme for Culicoides midges and the associated arboviruses on a countrywide scale it is, however, not possible to determine if this is a result of climate change. Climate change is considered one of the contributing factors for the present expansion of BT in Europe.

Light traps manufactured by the ARC were chosen by the OIE as the gold standard for the collection of Culicoides midges. These traps are exported to several countries in Europe. Research regarding the factors that will influence the efficacy of these traps are ongoing. These traps are also used to evaluate various repellants and/or attractants for Culicoides midges. To elucidate the epidemiology of these diseases is clearly a field that involves a good knowledge and infrastructure from Entomology, Virology and Molecular biology. It is clear that virus transmission and the factors that influence this consist of a complex interaction between vectors and the viruses they transmit.

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