Page 83 - VHSA - Onderstepoort 100 Years - Part 3

toxicologists. Though at times it may seem to despondent investigators that geeldikkop is a mystical ‘act of God’, a logical explana- tion for it can surely be found. Finding this explanation has nevertheless proved more easily said than done.
The generally held belief by farmers
that geeldikkop was caused by T. terrestris
was confirmed by the inimitable Theiler in
1918 by feeding the plant to sheep. Since
then the condition has been repeatedly
reproduced – though with difficulty – in
a similar fashion by several investigators.
Amongst the questions that still remained
unanswered, though, was how did T.
terrestris sporadically become toxic and how
could two disparate plants such as the grass,
Panicum coloratum (a monocotyledon) and
T. terrestris (a dicotyledon) induce indistinguishable diseases, viz. dikoor (Panicum photosensitivity) and geeldikkop? In an effort to answer these questions, J.M.M.
Brown and L.P. Neethling in the early 1970s
pursued the possibility that geeldikkop was
an autoimmune disease caused by aberrant
selenoprotein haptens in T. terrestris.
At about the same time, Kellerman and Marasas, drawing on their experiences with lupinosis and facial eczema, proposed that geeldikkop was a mycotoxicosis. This was not the first time, however, that such a possibility had been mooted: Theiler in 1918 had speculated that a fungus species growing parasitically on dubbeltjies was involved in the disease, and Steyn in 1928
ONDERSTEPOORT 100
suggested that smutted Panicum colora- tum grass was responsible for dikoor. The hypothesis that geeldikkop and Panicum photosensitivity was a mycotoxicosis had its merits, as it would amongst others explain both the sporadic nature of the outbreaks and the close relationship between the two diseases. Disappointingly, an exhaustive mycotoxicological survey by Marasas and Kellerman of pastures during outbreaks of geeldikkop and dikoor to identify this hypothetical fungus, provi- ded no evidence that the condition was a mycotoxicosis. The only fungus capable of causing hepatogenous photosensiti- vity to be isolated, Pithomyces chartarum, clearly was not the primary cause of either disease. Later trials revealed that at most,
sporidesmin, the toxin produced by P. chartarum, might act as a predisposing factor for geeldikkop.
Since all the previous hypotheses had
led to nought, we decided to re-examine
the possible role of saponins in the aetio-
logy of the disease. The potential im- portance of the saponins in dubbeltjies
was first recognized by Marguerite Henrici
(1952), a determined plant physiologist appointed by Theiler who, according to Anderson, long before it was fashionable
for ladies to do so, bestrode the southern
Free State in khaki trousers. She suggested
that saponins acting in concert with un-
known factors, might be involved in the 173 aetiology of geeldikkop. Regrettably, as
it transpired, when 20g of crude saponins isolated from T. terrestris by W.T. Kock and P.R. Enslin of the CSIR (1958) failed to induce geeldikkop in a sheep, interest in these compounds waned. Our new investigation into the role of saponins in the aetiology of geeldikkop was part- ly prompted by a never re- peated report from Scandinavia (1984) that a similar ovine pho- tosensitization, alveld, caused by a lily, Narthecium ossifragum, could be reproduced by dosing crude saponins from the plant to sheep.
Tribulus terrestris or dubbeltjie is the plant responsible for geeldikkop in sheep
“Though at times it may seem to despondent investigators that geeldik- kop is a mystical ‘act of God’, a logical explanation for it can surely be found. Finding this explanation has nevertheless proved more easily said than done.”
Toxicology
1908-2008
Years