Acta Leidensia最新文献

It is a little over 60 years since Blacklock, in an elegant study, demonstrated that in Sierra Leone human onchocerciasis was transmitted by a Simulium fly. Amazingly within 6 years of Blacklock's discovery an attempt was made, mainly using environmental management, to control Simulium in the Chiapas focus of Mexico, though this was not successful. Later Buckley was successful in eradicating Simulium neavei from the small Riana focus in Kenya by clearing riverine forest. However, it was not until DDT became widely available that Simulium control entered its heroic phase with campaigns in Kenya, Uganda and Zaire, some of which were highly successful. This led to an interest in controlling the vector in West Africa. The very different pattern of disease with very large foci virtually contiguous across tens of thousands of square kilometers suggested a difficult proposition and early attempts were not very successful. There is much of interest in the East and Central African schemes and in the West African forerunners of the OCP. In this talk an attempt will be made to capture something of the flavour of these pioneering efforts, to give credit to the men involved, and to highlight the successes against both the S. neavei and S. damnosum complex, and the advances which contributed to the successful planning of the OCP.

A traditional herbal drug, called "Ganna Ganna", is used in rural areas of Liberia to treat onchocerciasis. It is prepared from the bark of the "Ganna Ganna" tree, which was identified as Cassia aubrevillei. An aqueous extract was prepared from the bark according to informations from local people and its effects on microfilaria (mf) density and pruritus were studied in eleven patients with onchocerciasis. In four patients a 1.8-4.8 fold increase in mf density was observed after three weeks and in two patients mf counts dropped to almost zero. Histology and embryogram of adult worms, isolated after treatment, revealed no significant changes. All patients reported a rapid decline of pruritus and this effect lasted for up to three months. The in vitro effect of different extracts on mf viability was tested. The alcoholic extract gave the best results, killing mf within 24 h at concentrations of 50 micrograms/ml. Chrysophanic acid, an anthraquinone identified in the drug, also showed good in vitro microfilaricidal activity. It is concluded, that "Ganna Ganna" could be a useful adjuvant in the therapy of onchocerciasis and that further evaluation of the drug seems promising.

Human onchocerciasis is reported to occur in all States of the Federal Republic of Nigeria with serious medical, social and economic impact on affected communities. Attempted control schemes have failed because of reinvasion caused by limited area coverage, short duration of the scheme and non-involvement of the affected communities. As the Nigeria National Onchocerciasis Control Programme (NOCP) takes off with baseline data collection, it is essential to point out the need for community participation in the programme. Information obtained from group interviews in 8 villages in Akamkpa Local Government Area of Cross River State, Nigeria, reveal that ignorance of the cause of onchocerciasis. Location of villages and farms near fast flowing waters and outdoor working habits, have resulted in continued endemicity of the disease in the affected poor farming communities. For successful control of the disease, the communities should be involved in all stages of the control programme. During the preparatory stage, control officials should carry out intensive health education; encourage, and assist in, formation of village health committees (VHC) and selection of village based workers (VBW); and work with the VHC and VBW in developing skeletal control plans and deciding on input from community. During screening for prevalence, communities will, if adequately mobilized, accommodate field workers, act as field guides and interpreters and cooperate despite conflicting religions and traditional beliefs and practices. Before chemical treatment of breeding sites of Simulium, permission will be required from the communities, members of which will, if trained, take part in the exercise.(ABSTRACT TRUNCATED AT 250 WORDS)

A survey to establish the extent and possible impact of deforestation and increased farming activities on the prevalence of onchocerciasis was carried out in all the 17 villages in the eastern Usambara mountains. Results revealed that the disease is widespread. Out of 2,968 inhabitants of all ages skin-snipped in 1984 in the area, 22.7% were positive for Onchocerca volvulus microfilariae. The infection rates in the under nineteen and under ten age groups were 10.3% and 6.6% respectively. The overall prevalence rose steadily with age, with the mean microfilarial density reaching a plateau at age 24. Of the population studied 35.4% were found to present with at least one clinical manifestation commonly associated with onchocerciasis. Visual impairment (9.4%) and blindness (0.1%) rates suggest that ocular onchocerciasis is not a health problem in the area at present. Comparison with results obtained in the area during the last survey show that during the past 25 years there has been a significant decline in prevalence of infection. This drop is thought to be due to the effects of extensive deforestation on the vector population. Control of transmission, which is likely to rise as a result of a re-afforestation programme in the area is discussed.

Microorganisms isolated from soil at the Kitasato Institute in Japan were tested in a variety of biological assays in Merck laboratories. One of the cultures was found to be active against a nematode parasite. It yielded a series of novel macrocyclic lactones, named avermectins, which proved active against a variety of nematode and arthropod parasites. An extensive programme of chemical modification resulted in the selection of ivermectin (22,23-dihydro-avermectin B1) for commercial development. The producing organism is a new actinomycete species, Streptomyces avermitilis. Strain selection and fermentation improvement were necessary to scale-up from laboratory flasks to 50,000 L fermentors. Using tritium-labelled drug it was shown that ivermectin is absorbed rapidly after oral or parenteral dosing and is excreted almost entirely in the faeces. Pharmacokinetic behaviour depends upon formulation and route of administration. Residues are highest in liver and fat and lowest in brain. Ivermectin has a high therapeutic index in target animals. Idiosyncratic toxicity has been observed in certain strains of dog at more than 8-fold the recommended dose of 6 ug/kg. Ivermectin causes paralysis in susceptible parasites. It is believed that this is the result of lowered cell membrane resistance produced by increased chloride ion influx.

Maintenance of the control of O. volvulus in Africa would be assisted by the development of assays to identify the origin of outbreaks of transmission within the controlled regions. The isolation of pFS-1, a DNA sequence specific for forest form O. volvulus, has suggested that it might be possible to develop such an assay based on form specific DNA probes. The isolation and characterization of pSS-1, a clone which hybridized preferentially to savannah form O. volvulus is described here. DNA sequence analysis demonstrated that pSS-1 was a representative of a highly repeated sequence family of O. volvulus. Comparison of the sequence of pSS-1 to other known examples of this family has identified a region of pSS-1, designated pSS-1BT, which appeared to be specific for savannah form O. volvulus. Tests of isolates from different areas demonstrated that the specificity of pFS-1 and pSS-BT were not confined to a small geographic region. However, the close relationship of pFS-1 and pSS-BT to less specific members of the repeat family means that their specificity and sensitivity are affected by small changes in the conditions under which they are used.

Disease due to virtually all of the human helminth parasites can be prevented simultaneously by a simple system based on the following premises: 1) helminths do not multiply within their definitive hosts, and their distribution in host populations is overdispersed; 2) disease manifestations occur largely in the small proportion of hosts, often school children, with heavy worm burdens; and 3) a few single-dose, broad-spectrum anthelmintics given in low doses at prolonged intervals can maintain worm burdens below pathogenic levels for almost all of the major human helminth parasites. The three drugs might be albendazole or one of the other benzimidazoles for hookworm, ascaris and trichuris; ivermectin for the filaria, including onchocerca and many other nematodes such as strongyloides; and praziquantel for virtually all trematodes and cestodes. The target group would be school children treated at intervals set to maintain worm burdens below the disease-inducing threshold. It is possible that all three drugs could be administered in low doses simultaneously at yearly intervals. Studies will be necessary to examine drug interactions, development of drug resistance, optimal dosages and timing, and effects on morbidity.