Page 74 - VHSA - Onderstepoort 100 Years - Part 3

OND
ERSTEPOORT 100
Anaplasma marginale, cause of gallsickness (anaplasmosis), in a bovine blood smear
in splenectomized cattle ever since. It was therefore surprising when Potgieter demonstrated that it was still infective to its tick vector R. simus 70 years later. This implies that its widespread use as vaccine must contribute to disease stability in many parts of South Africa. It has been found that it is possible to freeze-dry (lyophilize) both Anaplasma spp. ( ( (A . marginale and A. centrale). However, if stored above –20°C the parasites are unstable which makes the lyophilized A. centrale product impractical for use as a vaccine in the field.
D.T. (Theo) de Waal joined the Section in 1983. His interest in biometrics brought a new scientific dimension to the sec- tion when he obtained a Dip. Datametrics in 1988. Equine babesiosis became his main field of expertise and he was awarded a PhD degree by the Onderstepoort Veterinary Faculty, University of Pretoria in 1995 for his thesis on the distribution, transmission and serodiagnosis of Babesia equi and Babesia caballi of equines in South Africa. Some of his
important contributions to its scientific productivity. His main contribution was the introduction of the MASP (micro-aerophilus stationary phase) culture system for the in vitro culture of blood parasites. He had considerable success with the cultivation of B. bovis, to start off with. Parasitaemia levels of up to 15% were reached. The blood was frozen successfully and new cultures initiated from the frozen material. Attempts to establish B. bigeminum cutures were less successful. Mason and co-workers also showed that the attenuated, Onderstepoort
164 vaccine strain of B. bovis was no longer readily transmissible by ticks.
“Some of De Waal’s most
important findings were that the indirect fluorescent antibody test could be used to identify carriers of Babesia equi and Babesia caballi. De Waal and his co-workers also showed that the red tick, Rhipicephalus evertsi evertsi, and the bont-legged tick, H. truncatum, are vectors of B. caballi.”
most important findings were that the indirect fluorescent antibody test could be used to identify carriers of these two parasites. These serological studies were used to elucidate the epidemiology of equine babesioses in various parts of the country. On two farms in the Pretoria district antibodies to B. equi were present in 100% and 76% of the horses respective- ly whereas the incidence of B. caballi sero- positivity was only 33% and 28%.
De Waal and his co-workers showed that the red tick, Rhipicephalus evertsi evertsi, and the bont-legged tick, H. trunca- tum, are vectors of B. caballi. Edmour Blouin from the USA, a visitor to Onderstepoort from 1986-1989, and de Waal also studied the fine structure of the development of B. caballi in the bont-legged tick by electron microscopy.
J.S. Gray, a visiting scientist from Ireland
from January to September 1980, studied
a non-pathogenic Babesia sp. occurring
in cattle in South Africa. He found that
the parasite was transmitted by the tick
Hyalomma marginatum rufipes and proposed the name of Babesia occultans for it.
When dealing with outbreaks of redwater in the field and with the movement of cattle into endemic areas, it is some- times necessary to vaccinate large numbers of highly susceptible cattle. The vaccine strains may cause deaths in such animals. However, it was found by De Vos and Potgieter that a block- treatment with a third of the therapeutic dose of diminazene (Berenil) 7 days after vaccination will protect the animals from disease without sterilizing either B. bovis or B. bigemina infections, thereby ensuring the development of immunity. Block treatment with the recommended dose of imidocarb (imizol) to control outbreaks of B. bigemina in a herd will not only sterilize the infection, but also interfere with effective vaccination for 3 – 4 months due to its residual effect.
The A. centrale isolate used in the live anaplasmosis (gallsickness) vaccine since 1912, has been needle-passaged
D. T. de Waal (1988-2002) being congratulated by D. W. Verwoerd (Director) as researcher of the year
PART 3
History of Individual Disciplines
1908-2008
Years