Page 112 - VHSA - Onderstepoort 100 Years - Part 3

OND
ERSTEPOORT 100
passage of lung material in horses resulted in the peracute pulmonary form whereas serial passage of spleen material resulted in a lengthening of the period of illness and even- tually failure to kill. Serial passage of AHS virus in cell cultures (e.g. BHK21 cells) resulted in apparent attenuation, but when large numbers of horses were vaccinated,
immunogenicity of the serotype 4 vaccine strain (Vryheid), as the vast majority of field isolates proved to be serotype 4, including those from Onderstepoort and the Institute’s neighbouring farm, Kaalplaas where horses were meticulous- ly vaccinated every year. During 1963 Erasmus proved that the
some succumbed to fatal AHS, indicating the presence of a low percentage of fully virulent particles amongst avirulent viruses.
The results of the preceding experi- ments, coupled to the hypothesis that “attenuation” of virus in a foreign host or cell type is due to selection of existing avirulent virus particles rather than modi- fication of the virus, strongly indicated plaque selection as a means to obtain a uniform virus population with regard to any given property.
“The overwintering of AHS viruses from one year to the next was another vexing problem for many decades. This riddle was eventually solved by Barnard in the 1990s when he demonstrated low level virus activity in young zebra foals in the Kruger National Park during the winter months.”
inherent immunogenicity of the serotype 4 vaccine strain was virtually non-existent. This serotype was duly omitted from the vaccine in 1964 until 1974 when it was replaced with the plaque-selected vaccine strain. In subsequent years all the neurotropic AHS vaccine strains were replaced by large plaque-selected vaccine strains, a process that was completed in 1989.
Another fact that emerged over many years was that a polyvalent AHS vaccine would not necessarily induce polyvalent immunity in all vaccinated horses, even though subsequent annual boosters ten- ded to broaden the protection. The best scientific approach to the problem of com- petition between different serotypes of live attenuated viruses would be the serial administration of single vaccine strains at
The selection of genetically stable large
plaques and small plaques on Vero cell
cultures indicated that the latter regularly
produced the peracute pulmonary form of
AHS whilst the former produced immunity
in the absence of disease. This observation
was exploited and led to the development of a Type 4 vaccine strain, urgently needed to control the occurrence of the disease in vaccinated horses in 1973. During the 1950s, and more particularly in 1958, many vaccinated horses contracted AHS and several died. Everything pointed to poor
intervals of 3 weeks. This approach, though scientifically sound, was not acceptable to horse owners. The eventual compromise was two quadrivalent vaccines to be administered at an interval of at least 3 weeks followed by annual boosters. Although these attenuated strains were safe and highly
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Zebras can be a source of African horsesickness virus but show no clinical signs of the disease
PART 3
History of Individual Disciplines
1908-2008
Years