Page 114 - VHSA - Onderstepoort 100 Years - Part 3

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ERSTEPOORT 100
dissemination by the insect vector, Culicoides midges, can take place.
In 1995 Barnard took over the responsibility for the OIE Reference Centre. In 1999 this function was transferred to Paweska and in 2002 to Gerdes.
• Bluetongue
Bluetongue was, for many years, believed to be a disease of sheep (especially imported breeds) confined to southern Africa and consequently most of the pioneer research work and the eventual development of a polyvalent live attenuated vaccine was done at Onderstepoort. Virological research on BT can be considered to have commenced in 1905 at Daspoort when Theiler proved the filterability of the causal agent. Theiler (1906) and later also du Toit used a relatively avirulent BT virus strain (which they believed became attenuated by serial passage in sheep) for the production of a ‘vaccine’ composed of the blood of infected sheep. Despite the variable results obtained this ‘vaccine’ was used until 1946.
In 1931 Neitz demonstrated that the blesbok (Damaliscus albifrons) was susceptible to experi-
mental infection with BT virus. Although the infection was subclinical, viraemia was present. This demonstration that antelope may harbour BT virus later led to the finding that practically all South African antelopes are susceptible to BT virus infection and that they may play a major role in the epidemiology of the disease.
Although Theiler and du Toit surmized the existence of a multiplicity of virus strains 204 (comparable to what was already known for AHS virus) it was not until 1944 that Neitz proved conclusively by cross-protection tests in sheep that several antigenically different viruses existed in nature and that BT virus could not be attenuated by serial passage in sheep. These discoveries revealed the serious shortcomings of the Theiler blood vaccine and stressed the need for other host systems for the isolation, propagation and attenuation of BT viruses. After several attempts J. Mason, J.D. Coles and Alexander eventually succeeded in cultivating BT virus in embryonated eggs but these isolates could not be maintained by serial passage in embryonated eggs. The eventual demonstration by Alexander in
1947 (assisted by his technician Rita Geyer) that the optimal temperature for BT vaccine isolation and propagation was 33.5°C and not 37°C proved to be of great significance. This resulted in the regular isolation and successful cultivation of BT virus in embryonated eggs and this significant finding led to the attenuation of a number of BT virus strains.
Indigenous sheep are more resistant to bluetongue virus than exogenous breeds
“In 1953 it was discovered that buffered-lactose peptone was an excellent stabilizer of the virus which also allowed freeze-drying. From then onwards vaccine could be produced all year round and stockpiled for periods of greater demand. By 1958 production had increased to more than 24 million doses (compared to only two million in 1946).”
In 1947 a trivalent and soon a quadrivalent avianized vaccine was developed by Alexander, Haig and Adelaar. Although the avianized BT vaccine was still pro- duced in sheep according to the methods of Theiler and du Toit, the post-vaccinal reactions in sheep were less severe whilst the resultant immunity was less variable. Due to the greater safety of the attenuated vaccine the demand for it increased. Great difficulties were experienced, however, in producing the avianized vaccine in sheep and the virus content of the vaccine could not readily be assayed. The vaccine was therefore produced in embryonated eggs but the macerated embryo material was suspended in normal sheep blood. For some reason this solved some of the problems of contamination and poor shelf life. Due to continual increase in demand it became a real problem to supply enough normal sheep blood. In about 1951 antibiotics be- came more freely available and the normal sheep blood was replaced with penicillin, streptomycin and crystal violet to control contamination. Annual production then increased to more than ten million doses (the increased demand was influenced by the dramatic increase in the price of wool at the time of the Korean War, which in turn resulted in an increase in the number and value of woolled sheep). In
PART 3
History of Individual Disciplines
1908-2008
Years