Medicine and tropical forests
Medicine and the fate of tropical forests
J Caldecott - British Medical Journal 1987, 295:229-230 (register and download)
Reserpine, tubocurarine, and vincristine or vinblastine come to the doctor originally not from a laboratory but from the jungle. The pharmaceutical industry may have refined and adapted, but the inventions were made respectively by an Indian shrub (Rauwolfia serpentina), a Brazilian vine (Chondrodendron tomentosum), and a Madagascan herb (Catharanthus (Vinca) roseus) (1).
Many other drugs and drug families also come from the plants of the world's tropical forests-for example, the quinine related antimalarials from the Peruvian tree Cinchona ledgeriana; the amoebicidal emetine from a Brazilian shrub Cephaelis ipecacuanha; and the cocaine related local anaesthetics from the Andean shrub Erythroxylum coca. Almost imperceptibly medicines from tropical plants have come to dominate much of the pharmaceutical market, where already one product in four is derived from wild plants (2). As well as being therapeutically useful these medicines are worth collectively at least £35 billion annually (3). Thus we must consider the nature and future of the forests from which such useful chemicals have originated and from which many more may be expected to emerge.
The identification, analysis, and evaluation of forest medicines represent a continuing and possibly accelerating process. Many data have been collected on how forest dwelling people use tropical plants-for example, in Amazonia by Schultes (4), in the West Indies and West Africa by Ayensu (5, 6) and in East and South East Asia by Perry (7). This approach (known as ethnobotany) uses indigenous experience based on generations of trial and error to draw attention to those plants most likely to yield useful compounds (8). Ethnobotanical surveys in tropical rain forests may be especially productive. One conducted recently in south-eastern Peru compiled basic data on the human use of 175 species of medicinal plants at one community (M Alexiades and D Lacaze, unpublished observations). Several have been investigated in the laboratory: a local anaesthetic was found in a novel piperaceous shrub and a sedative in a bignoniaceous climber. Meanwhile, the latex of a fig from the same area, Ficus glabrata, has been evaluated clinically and shown to be a potent and well tolerated antihelminthic agent (9).
Yet only about one species in 100 has yet been screened for potential value to medicine or any other discipline. Much remains to be done, and since four fifths of all plant species live only within tropical forests it will be done only if these forests can be conserved.
The extreme chemical richness of tropical forests matches their taxonomic diversity: a greater proportion of species contain alkaloids (the class of compounds to which quinine, vinblastine, and reserpine belong) and other "secondary compounds" than in any other kind of vegetation-and the average tissue concentration is also much higher (2). These secondary compounds include a wide range of chemicals derived from plant metabolism - for instance, cardiac and cyanogenic glycosides, terpenoids, glucosinolates, saponins, phytohaemagglutinins, proteinase inhibitors, flavonoids, and tannins (10, 11). Most such chemicals seem to be there to defend the plant against attack by animals (especially insects) and micro-organisms. Their sites of action are various, but their net effect is to discourage predation and premature decay; for example, they may make leaves inedible to grazers or preserve fruits until they can be eaten by dispersing animals.
There are good evolutionary reasons why tropical moist forests are so rich in chemicals. Firstly, since there are so many species each is specialised - and therefore efficient in its own way - at retrieving nutrients from its environment in competition with others. Thus once a plant has succeeded in accumulating enough scarce raw materials to be able to grow a leaf it must be able to protect that leaf - it is just too valuable to lose. Although filling a leaf with toxic chemicals costs the plant further energy, the hostile rain forest environment makes this worthwhile. Secondly, the absence of winter in the rain forest means that the threat of predation is continuous. In temperate lands plants have an annual holiday from predation and can do most of their growing in spring before their enemies' numbers build up. In a rain forest, however, the threat of predation is not only continuous but also intense - so plants have been under great pressure to evolve defensive chemicals. Since these will have been selected specifically to affect animal systems or to inhibit fungal and bacterial growth, it is logical that potential pharmaceuticals should be abundant among them.
With so much of benefit to medicine still to be found in the tropical forests of the world it is distressing that these forests are being destroyed at a high and accelerating rate - presently at least 100 acres a minute worldwide. Many species are becoming extinct each year. The erosion of mankind's culturally acquired knowledge of rain forest medicine is even more critical: almost all ethnolinguistic groups indigenous to such forests are rapidly "forgetting" this skill under the impact of dominant cultures and new economic patterns. The fate of tropical forests is linked to the future of medicine-both as a source of many new drugs and as a gene bank for "growing pharmacies" in tropical villages (where effective herbal remedies can be provided at low cost to poor people). The time has come when to be a conscientious doctor should surely demand an active commitment to protecting tropical forests.
JULIAN CALDECOTT
Programme Director for Southeast Asia,
Earthlife Foundation, London SWIX 8PH
Address for correspondence: 7 Jalan Bukitan, Kuching, Sarawak, Malaysia.
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9 Hansson A, Veliz G, Naquira C, Amren M, Arroyo M, Aravelo G. Preclinical and clinical studies with latex from Ficus glabrata Hbk, a traditional intestinal anthelminthic in the Amazonian area. Journal of Ethnopharmacology 1986, 17:105-38.
10 Janzen DH. Ecology of plants in the tropics. London: Arnold, 1975.
11 Rosenthal GA, Janzen DH, eds. Herbivores: their interaction with secondary plant metabolites. New York: Academic Press, 1979.
