"Piperine is more toxic than Pyrethrin to the housefly and 0.05% of piperine + 0.01% of Pyrethrin is more toxic than a 0.1% solution of Pyrethrin (Harvill and Hartzell in Kerharo and Adam, 1974, p. 637).

(b) Molluscicidal plants

As already mentioned, snails can play an important part in the transmission of diseases, especially those caused by Schistosoma spp. in their various forms. At present over 200 million people in about 70 tropical and subtropical countries are affected by endemic schistosomiasis (Hostettmann etal., 1982). The developmental cycle in the snail (mostly Bulinus spp.) takes about 6 weeks. In the infected snail the parasites are very prolific, a single miracidium can produce in optimal conditions 100 000-250000 cercaria, these emerging at the rate of 50-1000 daily (none emerging on dark days).

Active constituents

A number of plant extracts lethal to schistosome-transmitting snails were submitted by Mozley (1939, 1952) to comparative tests. The fruits of Balanites aegyptiaca Del.,Swartzia madagascariensis* Desv. andSapindussaponaria L. (grown

Fig. 4.11. Milletia aboensis (Hook.) Bak.

Fig. 4.11. Milletia aboensis (Hook.) Bak.

in West Africa, introduced from tropical America) were found to be amongst the most promising molluscicides. Later, Lemma (1970) reported from Ethiopia the strong molluscicidal properties of the dried berries of Phytolacca dodecandra L'Herit. *, which also occurs in West Africa and which is widely used in Ethiopia as a soap substitute. The active constituents in all these plants were found to be haemolytic saponosides. Another plant, Croton macrostachys Hochst. ex Del., with active molluscicidal properties, was reported from the Sudan (Amin et al., 1972). The active factors of this were also saponosides (El-Kheir and Salih, 1979).

In 1977 Dossaji et al., performed tests with Polygonum senegalense*, a powerful molluscicide in Kenya, where it is a common weed along rivers, lakes, etc. The active components are pseudo-cyanogenetic glycosides. In West Africa, the molluscicidal properties of Jatropha gossypiifolia* and J. curcas* fruits were examined by Adewunmi and Marquis (1980) and the methanol-related extract of the fruit of J. gossypiifolia was found to be particularly potent. Hydrocyanic acid and steroid saponins are present in the plant.

In Securidaca longepedunculata (Polygalaceae) the molluscicidal action of the roots is due to triterpenic saponins (see Table 4.6). In testing the molluscicidal activity of 24 saponins from various plants against Biomphalaria glabrata, Hostettmann et al. (1978, 1982) reported that at concentrations of 32 p.p.m. and even of 16 p.p.m. monodesmosidic triterpenoid saponins and saponins of the spirostanol series are potent molluscicides. Bidesmosidic saponins and the aglycones (e.g. hederagin and oleanolic acid) were found to be inactive. In Acacia nilotica galloyl tannins bind the protein of snails, inhibiting their enzymes (Hussein Ayoub, 1984).

Swartzia madagascariensis Desv. CAESALPINIACEAE

L The fruits are used in parts of West Africa, the Zambesi valley, Madagascar and Portuguese East Africa as a fish poison, (although fish poisons are forbiddeen in Nigeria). In Northern Nigeria the roots are occasionally added to the crushed fruits (Dalziel, 1937; Githens, 1949). In Senegal the roots are used in the treatment of leprosy and syphilis and the fruits as an abortifacient (Kerharo and Adam, 1974). The heartwood is termite and borer proof (Dalziel, 1937). In Bechuanaland the leaves are considered a valuable means of combating the schistosoma-carrying snail and are said to have insecticidal properties, especially against termites (Schultes, 1979).

C From the heartwood, pterocarpanoid constituents have been isolated (Harper et al., 1969). The fruit valves were found to contain a yellow flavonoid pigment which gave, on hydrolysis, swartziol (=kaempferol), rhamnose and glucose. In addition, triterpenoid saponosides and catechuic tannins have been reported. The seeds also contain saponosides and are strongly haemolytic (Beauquesne, 1947; Paris and Bezanger-Beauquesne, 1956). Harborne«aZ. (1975) confirm the presence of isoflavones.

P The fruits have been shown experimentally to possess ichthyotoxic properties and to kill fish in concentrations of 0.1% within 2 h, whereas they are toxic to guinea pigs only at higher concentrations (5 g/kg given perorally) (Gaudin and Vacherat, 1938; Beauquesne, 1947). They are also lethal to the snails transmitting bilharzia at higher concentrations (schistosomiasis).

Phytolacca dodecandra l'Herit. syn. (P. abyssinica Hoffm.) PHYTOLACCACEAE Endod, soapberry tree

L In some parts of West Africa the young shoots are used as a potherb and in the Ivory Coast as a substitute for spinach. On the other hand in East Africa and in Madagascar the berries and the juice of the fresh leaves are said to have caused death and the leaves are said to be the most dangerous of the Chagga abortifacients (a cheekful of leaves and young shoots is chewed and the juice swallowed, abortion commencing about 10 h later) (Watt and Breyer-Brandwijk, 1962). In Southern Nigeria the ashes of the burnt plant are used for making soap and in Somalia, Ethiopia and Uganda an infusion of the seeds or berries is used as a soap for washing clothes (Dalziel, 1937; Watt and Breyer-Brandwijk, 1962). A higher mortality of molluscs along the rivers in Ethiopia where the inhabitants washed their clothes with the berries induced Lemma to study the molluscicidal properties of the plant (Lemma, 1970).

C Investigations into the nature of the active principle showed that the active part was a saponoside, endod, the genin of which could be traced back to oleanolic acid by several authors. A freeze-dried aqueous extract was actively molluscicidal at concentrations of 1.25-2.5 p.p.m. (Jewers, Tropical Products Institute London, 1968 quoted in Lemma (1970)). A steroid saponin has been found in the roots (Ahmed et al., 1949).

P After 24 h of exposure endod killed all species of snails tested in comparative experiments with other molluscicides (N-tritylmorpholine, copper sulphate, ethanolamine of niclosamide). The test animals were killed in all cases by concentrations of less than 30 p.p.m. Whole and powdered berries kept at room temperature (22°C) for 4 years displayed no change in their molluscicidal potency during six-monthly tests, whilst the crushed fresh berries, and a solution prepared from them, lost their potency within a few days. The potency of the crude berries remained stable over a wide range of pH values and temperatures, under ultra-violet irradiation and in various concentrations of river-bed mud. The low solubility of the saponin from the whole berries also appears to be a notable feature. For mammals and birds, the LD50 is above 2 g/kg. The berries do not harm germination, growth rate or the morphology of vegetation. They have no insecticidal or larvicidal action and their toxicity to fish is similar to that of the above-mentioned molluscicides. In a concentration of 4 p.p.m., leeches are killed in 6 h. The Cercariae and miracidia of Schistosoma haematobium are killed in 10 min by 1000 p.p.m., in 1 h by 100 p.p.m. and in 2 h by 50 p.p.m. Thus P. dodecandra seems to be a cheap and effective means of eliminating Schistosoma in certain areas (Lemma, 1970). Only short staminate plants produce berries and the haemolytic activity of the samples runs parallel with the molluscicidal effect. Although in Ethiopia the unripe berries are always used for washing clothes (highest saponin level), sometimes the over-ripe berries have also shown a marked capacity to kill snails (Lugt, 1980). Endod has a weak ovicidal effect: it is not ovicidal at the concentration at which it kills adult snails. This difficulty can, however, be overcome by repeated treatment as has been demonstrated in the field. The molluscicidal potency (CL90) for eggs of Bulinus truncatus sericinus is of the order of 50-100 p. p.m. in6h(Lemma, 1970). The berries show approximately equal potency against different species of snail in a 24-h exposure at 30 p.p.m., whereas in a 6-h exposure, Physa acuta was the least and Biotnphalaria pfeifferi rupelli the most resistant species of snail examined (Lemma, 1970).

Croton macrostachyus Hochst. ex Del. syn. (C. guerzesiensis Beille)

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