Acta Leidensia最新文献

The avermectins are a family of macrocyclic lactones, produced by the soil organism Streptomyces avermitilis, which were discovered in the mid-1970's as a direct result of a screening effort for natural products with anthelmintic properties. Avermectin B1 (abamectin), the major component of the fermentation, also showed potent activity against arthropods in preliminary laboratory evaluations and was subsequently selected for development to control phytophagous mites and insect pests on a variety of agricultural and horticultural crops worldwide. Major applications for which abamectin is currently registered include uses on ornamental plants, citrus, cotton, pears and vegetable crops at rates in the range of 5 to 27 grams abamectin per hectare as a foliar spray. Abamectin has shown low toxicity to non-target beneficial arthropods which has accelerated its acceptance into Integrated Pest Management (IPM) programs. Extensive studies have been conducted to support the safety of agricultural uses of abamectin to man and the environment. Abamectin is highly unstable to light and has been shown to photodegrade rapidly on plant and soil surfaces and in water following agricultural applications. Abamectin was also found to be degraded readily by soil microorganisms. Abamectin residues in or on crops are very low, typically less than 0.025 ppm, resulting in minimal exposure to man from harvesting or consumption of treated crops. In addition, abamectin does not persist or accumulate in the environment. Its instability as well as its low water solubility and tight binding to soil, limit abamectin's bioavailability in non-target organisms and, furthermore, prevent it from leaching into groundwater or entering the aquatic environment.

Autoimmune mechanisms are thought to be involved in the pathogenesis of the chorioretinal changes in ocular onchocerciasis. The humoral autoimmune response was determined by measuring serum levels of autoantibodies, directed against human S-antigen and interphotoreceptor retinoid binding protein (IRBP) using an enzyme immunoassay. The cell-mediated immune response to these antigens and a crude retinal extract was investigated by means of a two-step migration inhibition factor assay. Patients with onchocerciasis (n = 50) were subdivided into three groups: 1. without ocular involvement (n = 10), 2. with ocular onchocerciasis limited to the anterior segment (n = 19), 3. with onchocercal chorioretinopathy (n = 21). A group of endemic controls from Sierra Leone, West Africa were also studied. The cellular immune response to Concanavalin A was measured to assess the general capacity of lymphocytes to respond to a mitogen. High levels of anti-human S-antigen and IRBP antibodies were detected in patients with onchocerciasis and endemic controls. The levels of both anti-human S-antigen and IRBP antibodies were significantly higher in onchocerciasis patients compared to endemic controls (Mann-Whitney ranksum test; p less than 0.001 respectively 0.002). No relationship could be demonstrated between the anti-retinal antibody level and the occurrence of chorioretinitis in ocular onchocerciasis. The occurrence of the anti-retinal antibodies as a result of crossreactivity of anti-retinal antibodies with parasitic antigens or of induction of polyclonal B-cell activation due to parasitic infection is discussed, since high antibody levels were also found in patients with Bancroftian filariasis from Papua New Guinea and Surinam. The migration inhibition factor assay, in which the cell-mediated immune response to human S-antigen, IRBP and retinal extract was tested, showed that four out of 50 (8%) patients with onchocerciasis and four out of 25 (16%) endemic controls reacted with at least one retinal antigen. From the patients with onchocercal chorioretinopathy two out of 21 (10%) showed a positive cellular response. The general mitogen response tested with Con A was positive in all these individuals. In conclusion, circulating antibodies against human S-antigen or human IRBP are thus nor specific for onchocerciasis and in themselves not sufficient to cause chorioretinopathy in onchocerciasis, although their pathogenic role in an ongoing chorioretinitis cannot be excluded. Furthermore a role for a cell-mediated anti-retinal autoimmune mechanism in the pathogenesis of chorioretinitis in onchocerciasis as studied with human S-antigen, IRBP or crude retinal extract could not be demonstrated.

Since 1974, the Onchocerciasis Control Programme (OCP) has been engaged in a large scale attempt to control the savanna species of the vector of onchocerciasis in seven West African countries. The effect of the vector control effort has been measured by epidemiological evaluation. For this purpose 474 villages have been examined by means of skin snip surveys between 1975 and 1983 and of these, 184 have been retained to-date for follow-up surveys which have documented over the years the reduction of the parasite population. The latest results of the epidemiological evaluation clearly demonstrate an outstanding success of the vector control campaign. The parasite has been or is close to being eliminated from the hyperendemic foci of the core area of the Programme. Major improvements have been registered in the reinvaded areas located at the Western and Eastern borders of the Programme. A major improvement has been found along the river Marahoué, the only focus of the intermediate area between the savanna and the forest where at the previous survey, the endemic situation was still similar to the pre-control situation. The exceptions to this gratifying picture are foci along the Dienkoa and Kulpawn rivers, both located in the core area, where transmission has relapsed and several more years of an effective vector control will be needed to eliminate the local parasite population.

This work collates previous and recent cytotaxonomically defined segregates of the Simulium damnosum complex from western and eastern Africa. Standard cytotaxonomic procedures were used on new samples from Nigeria, Cameroon and Malawi. The main onchocerciasis vectors comprising cytotypes, cytoforms and cytospecies are highlighted and indications of host preferences are given. Thyolo form, the vector implicated in the Thyolo Highlands of Malawi, is reported for the first time. Also reported are new foci of two genetically distinct savanna taxa: Volta form and S. damnosum s.s.. The associations between forest taxa and onchocerciasis in the Forest Zone of west Africa together with the interplay of vector distributions at the interface of this and the Savanna Zone in relation to the epidemiology of onchocerciasis are discussed. The role of individual members of the species complex in epidemiology is less understood in east Africa and the Yemen but is briefly discussed.

The general characteristics of Onchocerca volvulus infection and its transmission are outlined in this overview of human onchocerciasis. The pathogenic role of the microfilariae, producing lesions of the skin, lymphatic system, eye and deep organs, are described, along with the main clinical manifestations of the disease. The global prevalence and distribution of onchocerciasis are given. Best estimates in 1985 gave 86 million persons at risk, 17.8 million infected, 336,400 blind and a like number suffering from severe visual impairment. The vast majority was in Africa. The impact of onchocerciasis on communities in the Sudano-Guinean savanna zone of Africa is outlined, emphasizing the very high blindness rates and the increased mortality among the blind. Communities so affected cannot remain economically viable. They are forced to desert their villages and the fertile land near rivers. The background to the establishment of the Onchocerciasis Control Programme in West Africa (OCP) is given and the successful 10-year results of this campaign, which is based on prolonged, regular Simulium larviciding, are outlined. In the context of the future of the OCP and of the control of onchocerciasis elsewhere in the world, the need for improved chemotherapy is discussed. The prospects for large-scale suppressive therapy have greatly improved following the registration of ivermectin in 1988 for use in human onchocerciasis. The potential and possible uses of this drug, as a single-dose, non-toxic microfilaricide, which excites very little Mazzotti reaction and has a prolonged microfilarial suppressant action, are discussed. It is considered that an effective non-toxic macrofilaricide is still a prime need for onchocerciasis control.