Difference between revisions of "Dichapetalum cymosum"

Latest revision as of 11:27, 7 July 2010

Dichapetalum cymosum (common name gifblaar or occasionally the English translation, poison leaf) is a small shrub occurring in the northern parts of Southern Africa. It is notable as a common cause of lethal cattle poisoning in these areas and is considered one of the 'big 6' toxic plants of cattle in South Africa. A 1996 estimate of plant poisonings in South Africa^[1] attributes 8% of cattle mortality caused by poisonous plants to gifblaar. The majority (70%) of fatal cases is in Limpopo province, with 10% each in North West, Mpumalanga, and Gauteng. The chemical monofluoroacetate occurs in all parts of the plant and is responsible for the toxic effects shown^[2].

Description

Above ground the plant is seen as a clump of small, woody shrubs about 15cm high. Such a clump is typically 1 plant as gifblaar has a huge underground root system - likened to an underground tree - and sends numerous shoots above ground in favourable conditions. The most obvious above ground parts are the leaves - simple, alternate with initially fine hairs later becoming glabrous. The leaves are bright green in colour on both sides. The secondary veins form loops and do not reach the margin. Flowers are small & white and occur as dense clumps in the early spring. Fruit formation is rare; the fruit are orange & leathery - and are known to be consumed by the Bushmen.

Identification of gifblaar in the field is important in prevention of toxicity and also in assigning gifblaar as the cause of toxicity in an outbreak. It is a small, low-growing, non-descript shrub and thus easily confused with other species. There are four principal "confusers" in its habitat. These are Ochna pulchra (lekkerbreek) saplings, Parinari capensis (grysappel), Pygmaeothamnus spp. (goorappels) and the various gousiektebossies (various genera and species of the family Rubiaceae.) The first 3 of these are non-toxic, but gousiektebossies are also toxic and another of the 'big 6' cattle poisons.

Of the differentials, gousiektebossies & goorappel have opposite not alternate leaves. Goorappel leaves also have a characteristic bulge terminally, though only when mature. Grysappel & Ochna pulchra have alternate leaves, but grysappel has pale grey undersides to its leaves (its name means grey apple.) Ochna pulchra leaves have secondary veins that are not looped and reach the margin, and the margin itself is dentate not smooth.

Habitat and distribution

Gifblaar occurs in dry, sandy areas in acidic soils,as well as the northern slopes of rocky hills in the southern parts of the African savannah biome. In South Africa, the distribution is within the so-called 'gifblaar triangle' the points of which are Mmabatho; Middelburg, Mpumalanga; and Musina. The traditional southern border of distribution is the Magaliesberg mountains. It also occurs in an isolation region of the far north of Kwazulu-Natal. Gifblaar is also found in Namibia, Zimbabwe, Botswana as well as southern Angola. Within its habitat certain indicator species are used to identify veld which potentially harbours gifblaar - this veld is called 'gifveld' by farmers of the region. These are the trees Burkea africana, Terminalia sericea, and Ochna pulchra, and also the shrub Parinari capensis. It should be noted that the 2 above can easily be mistaken for gifblaar.

Toxicity & biochemistry

The toxic compound isolated as the cause of gifblaar poisoning is monofluoroacetate. It was first isolated by Marais^[3][4]. The LD₅₀ of this compound is 0,5mg\kg which translates to about 200g of dry plant material to kill a 500kg cow. The compound in itself is not toxic. However it undergoes lethal synthesis in the body reacting with Coenzyme-A to make Fluoroacetyl-Coenzyme A. This compound reacts with oxaloacetate to form fluorocitrate, which is toxic, being an alternate substrate for aconitase (normal substrate citrate.) It binds to the aconitase but cannot be released, irreversibly binding the aconitase. This causes the Krebs cycle to shut down, leading to massive energy shortages. Furthermore, fluorocitrate stops citrate from crossing from the cytoplasm into the mitochondrion, where it is needed. In the cytoplasm it gets degraded.

Pathology

In cattle, acute death by cardiac arrest is seen following drinking or some kind of exertion. Affected animals will show dyspnoea and arrhythmias prior to this. There may occasionally be nervous signs such as trembling, twitching and convulsions. Death occurs 4-24h after ingestion. In rare cases, an animal will survive the initial period only to drop dead months later of a heart failure - so-called chronic gifblaar poisoning. On post-mortem, leaves may be found in the rumen, cyanosis may be seen, as well as signs of heart failure - congestion, haemorrhage, and myocardial necrosis (on histopathology.) Diagnosis is based on these as well as the presence of gifblaar in the camp, particularly if signs of consumption are seen. Tests can be done for monofluoroacetate in rumen fluid, kidneys and liver.

Treatment

Ensure animals remain calm & rested. Remove the animals from infected camp, but without exciting them. It is thought withholding water for 48h can help too.

Pattern of toxicity

Cattle are mostly affected; with sheep, goats and game rarely being poisoned. The compound is equally poisonous to these species. An explanation is that the bulk grazing style of cattle, which is by nature less selective lends itself to the ingestion of the plant. Young sprouts have more monofluoroacetate, but all parts are lethal. The plants sprouts in late winter, before the spring rains, the cue for most plants - including grasses - to shoot. This makes it the predominant greenery during that period. Cases of poisoning are most frequent at this time. Later in the season, gifblaar poisoning is far less common, presumably enough other grazing occurs that gifblaar is not eaten. Autumn (late season) poisonings also occur. This is associated with heavy grazing, leading to denudation of preferred species, and gifblaar is again the predominant herbage within the camp. Poisoning of carnivores, including dogs, has been reported after consumption of ruminal contents of poisoned animals.

Management

From the above it is clear that gifblaar-infested camps are not ungrazeable per se. Nevertheless caution should be taken and animals should only be grazed later in the season, and the camps should not be overutilised.

References

Further reading

• Gifplante van Suider-Afrika wat veeverliese veroorsaak - J. Vahrmeijer
• Plant poisonings and mycotoxicoses of Livestock in South Africa - Kellerman, Coetzer, Naudé, and Botha
• Poisonous Plants of South Africa - van Wyk, van Heerden, and van Oudtshoorn