Tannin content of leaf extracts of 53 trees used traditionally to treat diarrhoea is an important criterion in selecting species for further work

Abstract

AIM OF THE STUDY : In southern Africa many plant species are used to treat different ailments and diseases related to infections. Diarrhoea is not only an important disease of humans but also causes large losses in the animal production industry. The antimicrobial activity of many of these species may however, be based on their tannin content. Even though high tannin content may be therapeutically effective against pathogens causing diarrhoea, nutritional side effects limit its prophylactic use in animal production. The aims of the study were two-fold. In the first place it was to compile a list of species used traditionally to treat diarrhoea. The second aim was to identify and remove those species where a potential antidiarrhoeal activity may be due to tannins, because it would be counterproductive to use in production animal systems. MATERIALS AND METHODS : After a literature study, 53 tree species used to treat diarrhoea or dysentery in humans or animals in southern Africa were identified. Plant material was collected and dried powdered leaves were extracted with acetone. To select plants with potential use as prophylactics against diarrhoea, the tannin content was determined by a radial diffusion method of precipitation of bovine serum albumin in agar and expressed as gallic acid equivalents. RESULTS : Based on our literature research plant species from at least 37 tree families are used traditionally to treat diarrhoea in southern Africa. Most of the species were from the Fabaceae (9), Euphorbiaceae (6), Anacardiaceae (3) and Combretaceae (3). The highest tannin content of 11.3 mg/ml gallic acid equivalents was detected in leaf extracts of Combretummolle and Sclerocarya birrea. About 40% of the species tested contained no tannin based on the assay used. CONCLUSIONS : As other authors have found, the tannin content within a family varied strongly. Therefore it is dangerous to make any conclusions on the tannin content of the same plant family. About 42% of the species had no detectable tannin content and another 25% contained 0–2 mg/ml gallic acid equivalent tannins. In general it appears that inmost cases tannins do not play a major role in treating diarrhoea in southern African ethnomedicine. If aqueous extracts were used, the situation could have been different.