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Differential length of Onchocerca volvulus infective larvae from the Cameroon rain forest and savanna.
Infective larvae from the savanna strain of Onchocerca volvulus are significantly longer (662.6 +/- 66.8 microns; n = 99) than those from the forest strain (624.2 +/- 62.0 microns; n = 236). The length of the infective larvae is not influenced by the size, species or age post infectionem (pi) of the Simulium damnosum s. l. vectors nor by their localization in the fly's head, thorax or abdomen, the worm load per fly or the thoracic volume per larva. However, the lengths of infective larvae within one individual fly have a conspiciously low variance.
[The fat body of prepupal and pupal blackflies and its utilization by the female during its life cycle].
The study performed revealed lipids, proteins and carbohydrates in the content of trophocytes which formed greater assemblies (lipid-protein-glycogen complexes, lipid-protein and protein-glycogen granules). The prepupal stage off stream Simulium noelleri was featured by neutral lipids and granules consisting of protein and glycogen. Mature pupae and emerged females had acid lipids and lipid-protein-glycogen complexes. Prepupal stages of the river S. morsitans, Schönbaueria pussilla sp. 1, Sch. pussilla demonstrated acid and neutral lipids that formed lipid-protein granules. The aforementioned granules were not infrequent in the river population of S. noelleri but in some of them protein-glycogen granules were revealed. The different correlations of lipid-protein and protein-glycogen granules were documented in the pupae of the studied species and emerged females. Gradual consumption of lipid accumulations was noted in the development of females ovaries in all species studied. Besides, the greater complexes were the first to be consumed, than followed proteins, carbohydrates and finally lipids.
OCP intercountry collaboration.
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Epidemiological impact of vector control. I. Incidence and changes in prevalence and intensity of Onchocerca volvulus infection.
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.
Epidemiological impact of vector control. II. Changes in ocular onchocerciasis.
The impact of 10-11 years of successful vector control on ocular onchocerciasis was evaluated in the population of 13 villages in the central part of the Onchocerciasis Control Programme area. The prevalence of ocular microfilariae was found to have reduced remarkably and loads over 20 microfilariae in the anterior chamber of the eye or the cornea which was rampant before the start of vector control, were rare. Whilst limited change in the age specific prevalence of lesions of the posterior segment of the eye was recorded, significant reduction in the prevalence of lesions of the anterior segment of the eye was found. The small decrease in the age specific prevalence of lesions of the posterior segment of the eye was partly explained by a reduced occurrence of the type of lesions of the anterior segment of the eye which previously obstructed the effective view of the fundus during examination. The prevalence of blindness was found to have reduced by 40% and onchocercal blindness was no longer found below the age of 20 years. It was concluded that 10-11 years of successful vector control has effectively reduced the incidence of onchocercal eye lesions, the deterioration of the existing eye lesions and the risk of developing an onchocercal eye lesion as well as going blind to virtually nil.
Resistance to reinfection of Simulium with onchocerca, and potential mechanisms for control.
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Population decline of Simulium woodi and reduced onchocerciasis transmission at Amani, Tanzania, in relation to deforestation.
To investigate an apparent decline of the onchocerciasis vector Simulium woodi, in the Simulium neavei group, weekly 12-hour biting catches on man were carried out for 13 months near Amani and compared with those obtained 22 years earlier. The results showed a marked fall in biting S. neavei s.l. females and supported earlier findings on the aquatic stages. The mean biting-catch in 1985/6 was only 13% of that in 1963/4 and comprised 91% S. woodi and 9% the more common, but largely zoophilic, Simulium nyasalandicum (Amani form). The infection rate of S. woodi with Onchocerca volvulus larvae had also fallen markedly from 17% to 3% of parous flies; no S. nyasalandicum females were infected. Recent onchocerciasis surveys indicate that reduced transmission has resulted in lower infection rates in man. Exposure by deforestation of the breeding sites of S. woodi in small shaded streams seems to have caused the simuliid's population decline. Reduced populations of S. neavei group vector species have been reported in other deforested parts of eastern Africa. Our long-term quantitative data on declining numbers of biting females without control measures, help to corroborate such reports. Control may no longer be necessary in some areas allowing a greater concentration of limited resources.
Computer simulation of the long term effects of vector control and drug treatment regimes on onchocerciasis in a village community in Sierra Leone.
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Increase with age and fly size of pteridine concentrations in different members of the Simulium damnosum species complex.
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Vectors of onchocerciasis in the Thyolo Highlands and other onchocerciasis foci in Malawi.
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Entomological results of vector control in the Onchocerciasis Control Programme.
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Maintenance of effective control of Simulium damnosum in the face of insecticide resistance.
The control of onchocerciasis through reduction of its vector Simulium damnosum depends on highly effective insecticide treatments directed against the larvae. As these treatments must be applied weekly over wide areas for many years, ideal conditions for the development of resistance have been created. Resistance to organophosphate compounds has occurred. To maintain effective vector control, an intense operational research effort has been necessary. Close and constant monitoring of susceptibility, combined with cytotaxonomic identification of the tested larvae, has revealed the pattern of spread of the resistance by migration and apparently by hybridization between populations. These studies have helped indicate when it might be possible to contain or eliminate a resistant population. Screening of all available appropriate compounds has led to the selection of replacement compounds in the microbial (Bacillus thuringiensis H14), pyrethroid (permethrin), and carbamate (carbosulfan) groups. Laboratory studies have revealed esterase and multi-function oxidase mechanisms for the existing organophosphate resistance. Cross-resistance tests have shown no cross-resistance to carbamate insecticides with these mechanisms, but negative correlations with most pyrethroids. These considerations, along with estimated risk of future resistance, effects on non-target organisms, and costs have led to a complex alternation pattern of insecticides, with Bacillus thuringiensis H14 being used in the dry season and a series of classical chemical compounds in the wet season. Through these measures, vector control remains a viable method of onchocerciasis control for the WHO Onchocerciasis Control Programme in West Africa.
Localized onchocerciasis vector control in the Bahr el Ghazal Region of South-Western Sudan. II. Control.
Onchocerciasis vector breeding along 41 km of the Bussere River in the Wau Area of South-Western Sudan was controlled with temephos (Abate) insecticide for one annual transmission period. Vector biting and transmission was compared to the two years of precontrol data. Up to 70% reductions in vector biting and 80% reductions in transmission were recorded at sites with the highest precontrol levels. The flies invading in the control year (principally those invading from the south-west in the main part of the rainy season) were found to be much less infective - 2390 fly bites would be needed to transmit 100 larvae in the control year, compared to 1490 in the precontrol years. It was concluded that localized vector control could have a major impact on onchocerciasis transmission and that in this, and many other foci in Sudan, permanent reductions could be achieved by causeway removal and selective vegetation clearance.
Identity of the OP-insecticide resistant species in the Simulium sanctipauli subcomplex.
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Onchocerciasis: a review of clinical, pathologic and chemotherapeutic aspects, and vector control program.
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Ocular onchocerciasis and intensity of infection in the community. II. West African rainforest foci of the vector Simulium yahense.
A novel method of analysis was used to describe community patterns of ocular onchocerciasis in relation to the intensity of infection in West African forest villages where S. yahense is the sole vector. The pattern is completely different from that found in the savanna, even after correction for the intensity of infection as measured by the Community Microfilarial Load (CMFL). Lesions of the anterior segment of the eye as well as onchocercal blindness either do not occur or occur only sporadically with increasing CMFL in the Yahense forest whilst a steep linear relation exists between the prevalence of these lesions and the CMFL in the savanna. Lesions of the posterior segment of the eye are also less common in the Yahense forest. For a given skin microfilarial load, the ocular microfilarial load is lower in the Yahense forest. For a given ocular microfilarial load, a lower prevalence of eye lesions is found in the Yahense forest compared to the savanna. It is concluded that microfilariae of Onchocerca volvulus in the Yahense forest are less eye invasive than microfilariae from the savanna. Furthermore, they appear to be also less pathogenic to the eye. These findings explain why ocular onchocerciasis is relatively mild in the Yahense forest, in spite of the high intensities of O. volvulus infection in the community.
[Blood-sucking flies and the protective measures against them].
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