Page 115 - VHSA - Onderstepoort 100 Years - Part 3

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).
The next major achievement was the cultivation of egg- adapted BT virus in lamb kidney cell cultures by Haig, D.G. McKercher (a Californian guest worker at Onderstepoort) and Alexander in 1956. This paved the way for future characterization of the virus, the development of sophisti- cated serological tests as well as the development and pro- duction of vaccine. D. Shone, a visiting scientist from Salisbury, now Harare, Haig and McKercher found that the cell culture propagated virus proved to be a highly satisfactory antigen for use in the complement-fixation test, a valu-
ONDERSTEPOORT 100
(from 1969–1975) some of the poorly immunogenic strains were omitted, the polyvalent vaccine then containing only nine serotypes. In 1975 all 14 egg-attenuated vaccine strains were plaque purified by Weiss and van Rooyen (with assistance from Erasmus). By analogy to what had earlier been done with the AHS vaccine, the 14 BT vaccine strains were combined into two vaccines, each containing seven strains. As with AHS these had to be administered at an interval of at least 3weeks. Unfortunately one of the vaccines was issued as the ‘standard BT vaccine’ whilst the second was called the ‘supplementary BT vaccine’. The two vaccines were not issued as a unit and it was left to the individual farmer to decide on the use of the second vaccine. Not quite unexpectedly only about ten per cent of farmers used the supplementary vaccine as well.
In 1976 serious outbreaks of BT were reported, affecting both vaccinated and unvaccinated sheep. After intensive investigation Erasmus isolated two new BT virus serotypes (Types 18 and 19) from field samples. Of the two, Type 19 was by far the most prevalent and further characterization indicated only a minor antigenic relationship to other BT strains, thus explaining the fact that previously vaccinated sheep appeared completely susceptible. These findings as well as the fact that sheep in large areas of the country had not been infected (but could soon be) by this serotype made its attenuation and inclusion into the polyvalent vaccine imperative. Attenuation of BT viruses in embryonated eggs required an average of 70-100 serial passages. In an attempt
to expedite the matter Erasmus applied 205 the plaque selection technique which
worked so well for AHS virus. Unfortunately
no genetic markers could be found that correlated with low pathogenicity as was
the case with AHS. Since the Type 19 isolate had by then already been passaged for 29 generations in eggs, it was argued that the proportion of avirulent to virulent virus particles should have changed sufficiently to allow the selection of avirulent virus by plaque selection. Serial dilutions of virus were seeded onto monolayers of Vero cells and ten plaques were selected at random at the dilution end-point. These were grown up on BHK21 cells and screened in BT susceptible sheep for immunogenicity and lack of pathogenicity. Three of the most promising candidates were then tested more extensively and eventually the best performing one was selected as a potential vaccine candidate and subjected to testing in large numbers of sheep under
laboratory and field conditions.
The development of the Type 19 vaccine strain was
completed in 1979 and the urgency to incorporate it into the BT vaccine meant that the latter would now be composed of 15 vaccine strains. Research done by Erasmus proved that,
able group specific test. Howell next ex- ploited the fact that the cytopathic effect induced by BT virus in cell cultures could be inhibited by homologous antiserum. This important observation led to the de- velopment of a very practical and relatively inexpensive method for the serotyping of BT viruses. By 1960 Howell had described 12 distinct serotypes and in 1970 he described an additional four, bringing to 16 the total number of BT virus serotypes known at the time. In 1973 Howell relocated to the Faculty of Veterinary Science as Professor and Head of the Department of Infectious Diseases which was subsequently incorporated into a new Department of Veterinary Tropical Diseases. Details of his and his colleagues’ continued involvement in orbivirus research in these departments can be found in Part 2.
Van Rooyen and his team immediately
set about to attenuate the newly identified
serotypes in embryonated eggs and Howell and his team tested the potential vaccine strains at various passage levels to determine safety and immunogenicity. The number of serotypes in the polyvalent vaccine increased rapidly and in 1968 it contained 14 serotypes. The production of BT vaccine in eggs was extremely labour intensive and the logical solution was found in the middle 1960s when vaccine production was achieved in lamb kidney cells in stationary cultures. A further major improvement was instituted in 1968 when production was switched to BHK21 cells in roller cultures.
Further investigations revealed that, despite all the apparent improvements, the polyvalent vaccine (containing 14 BT virus serotypes) was not yielding the desired results. The most important problem was the low immunogenicity of certain strains, which was further aggravated by the competition to replicate in a sheep which received so many other serotypes in a single inoculation. As an interim measure
“Further investigations revealed that, despite all the apparent improvements, the polyvalent vaccine (containing 14 BT virus serotypes) was not yielding the desired results. The most important problem was the low immunogenicity of certain strains, which was further aggravated by the competition to replicate in a sheep which received so many other serotypes in a single inoculation.”
Virology
1908-2008
Years