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This study was stimulated by a publication of Mr Mike Bosch (Bosch 2007) that Cereus jamacaru DC (Cactaceae) used by him on his farm is effective for gastrointestinal nematode control in livestock. This plant, widely known as Queen of the night, is a serious invasive weed. We evaluated this claim in in vitro assays and in vivo experiments in sheep experimentally infected with Haemonchus contortus and Trichostrongylus colubriformis. This is the first report of such an anthelmintic trial in livestock involving C. jamacaru. The first study was to repeat the farmer’s work under strictly controlled conditions. In vivo studies were conducted to determine the possible direct anthelmintic effects of C. Jamacaru on ovine gastrointestinal nematodes. Eighteen sheep were allocated to three groups and were infected with 4000 H. Contortus and 6000 T. Colubriformis infective larvae given in 3 divided doses over a period of three days. From day 0 until day 49 of the experiment, sheep were drenched once a week with fresh blended C. jamacaru plants with the core removed at the same (32.33 g/sheep) or double the dose (64.66 g/sheep) used by Mr Bosch. No negative effects of the double dose were observed during the period of the experiment. Faeces were collected twice a week for faecal egg count. Based on the in vivo experiments, C. Jamacaru was not effective in reducing H. Contortus and T. Colubriformis in sheep to the 70% reduction levels of the control treatments considered to be a useful reduction in FEC. Nevertheless, its in vivo activity was substantial at the higher dose and reduced the FECs by 65%. To investigate the matter further some additional experiments were carried out. Specimens of C. Jamacaru were collected and dried in the shade. For the phytochemical analysis and in vitro experiments, the dried material was milled to a fine powder, it was extracted with acetone and five fractions (hexane, butanol, water, chloroform and 35% water in methanol) were obtained by solvent-solvent fractionation. The chemical composition of the fractions and crude extract was analysed by thin layer chromatography using three solvent systems of varying polarity and pH. To detect the separated compounds, vanillin-sulphuric acid-methanol was sprayed on the chromatograms and heated at 110°C for optimal colour development. The antioxidant activity in plant extracts may influence the immune systems of animals and have an indirect effect. The antioxidant activity was therefore determined. For qualitative analysis of antioxidant activity, the 2,2, diphenyl-1-picrylhydrazyl (DPPH) assay on TLC plates was used as a test for the radical scavenging ability of the compounds present in the different extracts. In the DPPH qualitative analysis of antioxidant activity there were a number of antioxidant compounds present in some of the extracts and fractions but the activity appeared to be low. This observation was confirmed in the TEAC quantitative analysis of antioxidant activity. Even the fraction with the highest activity was about 8 times less active than trolox or ascorbic acid. It therefore appears that stimulation of the immune system by antioxidant activity does not explain the results found on the farm. The antibacterial and antifungal activities were determined against 4 bacteria (Staphylococcus aureus, Enterococcus faecalis, Escherichia coli and Pseudomonas aeruginosa) and 3 fungi (Aspergillus fumigatus, Candida albicans and Cryptococcus neoformans). The minimum inhibitory concentration (MIC) values for plant extracts varied from 0.04 to 2.5 mg/ml. E. coli was the most sensitive and the fungi were in general resistant to the extract and fractions. In general the activity was not very good, especially if the dosage in an aqueous system on the farm is considered. Antimicrobial activity also does not explain the results obtained. The nematocidal activity was evaluated by an egg hatch assay and larval development viability assay (LDVA) in vitro. The acetone extract inhibited egg hatchability and killed infective larvae of H. Contortus nematodes of sheep in a concentration-dependent manner. The percentage inhibition of egg hatching of the acetone extract and the butanol, chloroform, 35% water in methanol, hexane and water solvent-solvent fractions were 100%, 100%, 94%, 91%, 9% and 16%, respectively. The percentage inhibition of larval development of the acetone extract and chloroform, 35% water in methanol, hexane and water fractions were 93%, 84%, 49%, 85%, 31%, respectively. The chloroform fraction was significantly more active than all the other fractions (p<0.05). The extracts had in vitro activity against the nematodes. The activity was however much lower than the positive control, albendazole. It is disappointing that our results do not provide an explanation for the success obtained by Mr Bosch on his farm. Even if we do not understand how it works it may be useful to prepare suitable dosages (using low level technologies adaptable to rural conditions) for use by resource-poor rural communities where C. jamacaru occurs as an invasive weed. Copyright / Dissertation (MSc)--University of Pretoria, 2010. / Paraclinical Sciences / unrestricted