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PubMed1988

Identification of species-specific DNA sequences in North American blackflies.

Blackflies are vectors for Onchocerca volvulus, a filarial parasite that infects tens of millions of people in Africa and Latin America. Different species of the insect vectors, varying greatly in their ability to transmit the infection, may populate overlapping geographical regions and be morphologically similar. A rapid and reliable method for distinguishing among blackfly species is presently not available. We have isolated cloned DNAs coding for repetitive sequences that can distinguish between two species of North American blackflies, Simulium pictipes and Simulium vittatum. These DNAs do not cross hybridize to three African blackfly species. The assay is very sensitive, less than 1/1000th of the amount of DNA in a single fly being sufficient for detection with a radioactive probe. Diagnosis with nonradioactive probes by a procedure that is amenable for use in the field was also demonstrated.

Animals↗
PubMed1987

Infection rates and parasitic loads of Onchocerca volvulus, and other filariae, in Simulium sanctipauli s.l. and S. yahense in a rain-forest area of Liberia.

Simulium sanctipauli s.l. and S. yahense are common and widespread in the rain-forest zone of Liberia, but differ with regard to their biting densities and contribution to the transmission of Onchocerca volvulus. Although, in a study area on the St. Pauli River, S. sanctipauli s.l. (presumably S. soubrense in the sense of Post) was the predominant ma-biting species (74.3% of 30,855 females examined), S. yahense was shown to be the important vector. While 1000 biting females of S. yahense carried 96 3rd stage larvae indistinguishable from O. volvulus, only 14 were found per 1000 females of S. sanctipauli s.l. Of the parous females (3135 S. sanctipauli s.l./1621 S. yahense) 23.8/39.9% harboured 1st and/or 2nd stage filarial larvae and 1.9/9.4% 3rd stage larvae of O. volvulus. Animal filariae of unknown origin, indicative of zoophily, were very common in S. sanctipauli s.l. (13.8%) but practically absent from S. yahense (0.5%). In spite of its poorer vectorial performance S. sanctipauli s.l. cannot be neglected as a vector because it may occur in high biting densities and contribute considerably to the transmission, in particular in the vicinity of the St. Paul River. The interplay of two vector species, which develop in different types of water-courses explains the overall high endemicity of onchocerciasis in the study area.

Animals↗
PubMed1987

Cytotaxonomy of the Simulium sanctipauli sub-complex in Liberia.

Larvae of the Simulium sanctipauli sub-complex from three of the main rivers of Liberia, the Lofa, the St. Paul and the Farmington were examined. All larvae were identified as S. sanctipauli (sensu Vajime and Dunbar, 1975) but as S. soubrense using the new definition of Post (1986). It therefore appears that S. soubrense (sensu Post) and not S. sanctipauli is the main man-biting species of the S. sanctipauli sub-complex in Liberia. Larval populations from the three rivers were genetically distinct. Those from the Lofa and St. Paul were most closely related, but the double inversion 1L-B.C. which was in Hardy-Weinberg equilibrium in the Lofa at a frequency of 0.578 was absent in the St. Paul. An abrupt genetical change was observed between the populations of the St. Paul and Farmington, which are only about 30 to 40 km away from each other. The inversion 2L-D, which was fixed and present in more than 90% of specimens from the Lofa and St. Paul respectively, was practically absent in the Farmington. Of further interest was the sex-linkage of inversion 3L-5 in the Lofa and the St. Paul. This inversion was autosomal and rare in the Farmington.

Animals↗
PubMed1986

Onchocerciasis in Sudan: the distribution of the disease and its vectors.

The distribution of onchocerciasis and its vectors in Sudan has been reviewed with special emphasis on the hyperendemic foci where there is serious visual handicap caused by the disease. These blinding foci lie primarily in the south-west of the country, along the rivers flowing north and east from the borders with Central African Republic and Zaire, though at least one such focus is known from the eastern part of the country close to the Ethiopian border. In the blinding foci, often small villages localized to short stretches of the rivers, ocular onchocerciasis is as severe as that found in any other African foci. Only the S. damnosum s.l. species complex has been implicated in disease transmission and only the two dangerous, morphologically similar, savanna cytospecies, S. damnosum s.s. and S. sirbanum, have been identified from breeding sites close to known onchocerciasis foci. Near to the Uganda and Zaire borders it is very likely that other, less anthropophilic, cytospecies occur. Different Onchocerca-Simulium complexes (different strains of parasite with different pathogenicities transmitted by different vector species) may be responsible for the different severities of disease found in the 3 main areas of onchocerciasis in North, East and South-West Sudan. The localized, focal distribution of the communities seriously blinded by onchocerciasis, suggests that a strategy of tackling the disease on a focus basis may prove optimal. A control scheme, planned to treat all the vector breeding sites with insecticide, as in West Africa, would receive extensive invasion from the Zaire/Congo River Basin and the headwaters of the White and Blue Niles in neighbouring countries. While some foci are situated beside major river rapids, with vector breeding only controllable by regular insecticide treatments, several foci have been identified as lying close to removable man-made objects which provide excellent breeding sites at certain water levels e.g. causeways. The destruction of such breeding sites should be considered since localized vector control may produce substantial reductions in onchocerciasis transmission.

Animals↗
PubMed1987

Failure of Onchocerca gutturosa to develop in Simulium soubrense and Simulium yahense from Liberia.

Laboratory reared female S. soubrense and S. yahense from Liberia were intrathoracically inoculated with bovine skin dwelling microfilariae of Onchocerca gutturosa. Of a total of 174 flies, each injected with 50 fresh microfilariae, the maximum development was to late first-stage larvae. One S. soubrense also contained two second-stage larvae and a maldeveloped third-stage larvae. No development to normal third-stage larvae was observed in any fly despite maintenance at 28 degrees C for up to 10 days following infection. This contrasts with complete development of O. volvulus in both blackfly species under similar conditions.

Animals↗
PubMed1987

[Role of children in the evaluation of the Onchocerciasis Control Program in West Africa].

Incidence of onchocercal infections is studied in children borne since the start of the larvicidal treatments of the Onchocerciasis Control Programme in West Africa (OCP). A total of 8088 children, originating from 155 villages widely distributed throughout the initial area of the OCP were examined for microfilariae of O. volvulus. 37 were found infected. If there had not been control measures, 652 children would normally have become infected in such a sample. It is considered that in the centre of the OCP area which constitutes approximately 90% of the total, transmission has been interrupted (1 child infected among 5,886 examined in the centre). The remaining 36 cases are spread over two quite distinct marginal zones of the OCP where transmission persisted because of reinvasion by blackflies coming from untreated areas, or because of insecticide vector resistance, or because of occasional treatment failures. The entomological indices confirm perfectly the epidemiological findings in the central region as well as in the marginal zones.

Africa, Western↗
PubMed1986

Geographical distribution of the prevalence of nodules of Onchocerca volvulus in Guatemala over the last four decades.

In Guatemala a national nodulectomy campaign has been carried out since 1935 in three operational areas, namely the Eastern, Central and Western endemic zones. The prevalence of nodules was calculated in each 1 km grid square in these zones in each decade and plotted on mesh maps. A total of nine foci of high endemicity, three in each operational area, was found in the decade 1940-49. Nodule prevalence was concentric in each endemic focus; high in the centre and gradually falling towards periphery. The foci were found mainly on Tertiary or Quaternary-Tertiary volcanic rocks in the Eastern and Central zones, or on non-carbonate sedimentary rocks in the Western zone. Nodule carriers were found at altitudes from 220 to 2200 m above sea level. The altitude of the foci of high endemicity varied, being low in the Eastern zone, medium in the Central zone and high in the Western zone. The nodule prevalence declined from the decade 1940-49 to the decade 1970-79 in most of the grid squares. The Santa Rosa focus in the Eastern zone virtually disappeared before 1970. The extent of endemic area having a nodule prevalence of 1% or higher decreased from 2384 km2 in the decade 1940-49 to 1424 km2 in the decade 1970-79. The area recommended for vector control operation against Simulium ochraceum was estimated at 803 km2.

Altitude↗
PubMed1986

Development of a method for the cytological identification of man-biting sibling species within the Simulium damnosum complex.

A method is described for the preparation of adult Malpighian tubule polytene chromosomes from members of the Simulium damnosum complex. This method was used to show that the banding pattern is conserved between adult and larval tissues, and consequently it has been possible to demonstrate that within the S. sanctipauli subcomplex both S. soubrense 'B' and S. sanctipauli bite humans. The degree tubule development is correlated with the gonotrophic cycle and appears to be dependent on food source. This suggests that there is a physiological stage which is optimum for obtaining maximum polyteny for cytotaxonomy.

Animals↗
PubMed1986

Localized onchocerciasis vector control in the Bahr el Ghazal Region of South-Western Sudan. I. Precontrol.

Over a two year period, onchocerciasis vector biting and transmission rates were recorded from 11 sites within a 40 km radius of Wau, in the Bahr El Ghazal Region of South-Western Sudan. These data provide the baseline on which the efficacy of localized vector control in the third year of the study, has been assessed (Baker and Abdelnur 1986). During 1018 collection days, 59,660 Simulium damnosum s.l. vectors were attracted to man, 41,548 of which were dissected. 2.8% of the dissected flies were infective with head L3 Onchocerca with a mean of 2.4 head L3s per fly and a maximum of 19. Larval collections revealed that only Simulium sirbanum and S. damnosum s.str. were breeding in the study area. The results were analyzed to show the hourly, seasonal and annual patterns of biting and transmission. These patterns were found to be remarkably similar in both years even though vector biting and transmission was 24-28% lower in the second year. The sites could be divided into two groups. Those with Annual Transmission Potentials (ATPs) over 1000 lay beside fast flowing, well wooded stretches of river and those with ATPs below 500 lay near rivers flowing slowly in a wide grassy plain. Blinding onchocerciasis had been recorded only from the first group.

Animals↗
PubMed1986

The effect of 7-8 years of vector control on the evolution of ocular onchocerciasis in West African savanna.

The evolution of ocular onchocerciasis was studied in a cohort of 1170 persons over 5 years of age who were examined before the start of and after 7-8 years of effective vector control in 12 originally hyperendemic villages in the central part of the OCP area. The proportion of the cohort which at the outset of vector control was free from ocular onchocerciasis or had an early or recent infection in the form of punctate keratitis only, remained largely free of or lost their signs of ocular infection respectively and only a very insignificant proportion acquired microfilariae in the eyes or developed a severe onchocercal eye lesion at the initial stage. The proportion of the cohort with a heavy ocular microfilarial load had a reduced risk of developing severe eye lesions and no risk of going blind. The proportion of the cohort with already existing severe eye lesions at the advanced stage remained largely unchanged and some lesions at the initial stage disappeared. Blindness occurred only in those who had severe eye lesions at the outset and was comparatively less than in areas of on-going transmission.

Adolescent↗