Page 82 - VHSA - Onderstepoort 100 Years - Part 3

OND
ERSTEPOORT 100
Disheartened by the lack of cross-immunity between the various cardiac glycosides – a major stumbling block in the development of a vaccine – we began to investigate other means of controlling plant poisonings. First, we tried to exploit the well documented, inherent ability of stock to learn to avoid certain harmful plants. This naturally-developed aversion, for instance to tulp, is so strong that animals reared on infested veld seldom eat the plant and, as a result, poisoning usually occurs only in stock newly introduced from non-infested areas. One of the big factors that drew us to CFA (conditioned feed aversion), of course, was that, being one of nature’s ways of protecting animals against plant poisonings, it was completely environmentally friendly. This new approach to the control of plant poisoning by CFA, interestingly, is founded also on folk medicine. Traditional and commercial farmers in the eastern parts of the country believe that stock can be taught to avoid yellow tulp (Moraea pallida) by predosing them respectively with suspensions of charred leaves, subtoxic quantities of chopped leaves or infusions of the plant. To isolate the unknown aversion (CFA) factor from yellow tulp, Kellerman, Snyman and Schultz designed a rather unique trial. In essence, this amounted to feeding a small (non-toxic) quantity of milled M. pallida, mixed in a small amount
Artificially averted animals grazing with impunity on a tulp-infested pasture during a conditioned feed aversion trial
with epoxyscillarosidin during the induction of aversion. Some evidence was also found that the epoxiscillirosidin used in the induction of aversion could contribute towards poisoning of animals during the aversion process on tulp-infested veld. This new setback was addressed by partially replacing
of maize meal, to sheep until they were averted. The averted sheep were then used to ‘sniff out’ the CFA factor in chemical fractions of yellow tulp obtained by column and thin layer chromatography.
Employing this system, a pure aversion factor was isolated from the plant which could induce conditioned aversion to M. pallida in naive sheep. Research has its
“Isolation by means of the senses of taste and smell of a sheep of a single CFA factor from the myriad of organic compounds contained by a plant must surely be one of the more unique experiments conducted at Onderstepoort! It was now up to the ingenious Snyman in collaboration with Schultz to find a practical application for this laboratory finding.”
epoxyscillarosidin with lithium chloride, a relatively non-toxic aversive agent that does not affect the heart. Averting cattle with a combination of epoxyscillarosidin and lithium chloride, together with a tulp-hexane extract acting as an identification factor for tulp, effectively restricted severe poisoning of cattle on tulp-infested pastures.
The second most important plant poisoning listed in the 1996 study is geel- dikkop. In a nutshell, geeldikkop can be described as a hepatogenous photosensi- tivity disease of sheep and goats grazing on Tribulus terrestris (dubbeltjies) in the Karoo. Paradoxically, T. terrestris normally is a highly nutritious shrub, providing valuable summer grazing for sheep throughout the Karoo. We do not yet know why in some years it causes massive outbreaks of geeldikkop, while in others it does not. In the past, outbreaks of geeldikkop involving half a million or more small stock were not uncommon, though in recent times the incidence of outbreaks seems to have
172 memorable moments: one of which was
when Snyman showed us the, as yet, un-
identified CFA factor, as a single spot on
a thin layer plate! This aversion factor was
subsequently – to our slight disappointment
– identified as epoxyscillarosidin, the active
principle of M. pallida. Isolation by means
of the senses of taste and smell of a sheep
of a single CFA factor from the myriad of
organic compounds contained by a plant
must surely be one of the more unique
experiments conducted at Onderstepoort!
It was now up to the ingenious Snyman in
collaboration with Schultz to find a practical
application for this laboratory finding. It must be pointed out that field trails are notoriously difficult to carry out because the exact circumstances under which poisonings occur in the field are still imperfectly understood; for instance, cattle averted with epoxyscillirosidin unexpectedly had difficulty in identifying the compound in green M. pallida growing on naturally infested kikuyu grass pastures. This problem of non-identification was circumvented by dosing an epoxyscillirosidin-free hexane tulp extract (serving as an identification factor for tulp) together
decreased somewhat. Despite this tendency, the 1996 study attributed 28% of all small stock deaths from plant poison- ings in South Africa to geeldikkop. Today, after more than 90 years of more or less constant research and despite the attentions of notable researchers such as Theiler and Claude Rimington, the final resolution of its aetiology still eludes us. Geeldikkop arguably is one of the most intractable problems ever investigated at Onderstepoort. Unravelling its aetiology has therefore become an almost irresistible challenge for
PART 3
History of Individual Disciplines
1908-2008
Years