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PubMed2000

Knowledge and beliefs about causes, transmission, treatment and control of human onchocerciasis in rural communities in south western Nigeria.

Descriptive and quantitative information on onchocerciasis transmission and control were collected using focus group discussions and structured questionnaire to determine what changes if introduced, could upset the established human-vector-parasite relationship in limiting transmission. People's knowledge of cause of infection and transmission was very poor. Of the 1012 people interviewed, only 3% related the clinical manifestations of onchocerciasis to Simulium bites. Thirty six percent had no idea of the cause of infection while the rest attributed the clinical symptoms of the disease to many other causes. People's knowledge of the current treatment and control measures was also poor. However an impressive knowledge of the daily and seasonal distribution of Simulium flies was observed. The study identifies the need for health education campaigns aimed at relating the clinical manifestations of onchocerciasis to Simulium bites. This could help people in taking personal protective measures and seeking appropriate treatment.

Animals↗
PubMedDate not supplied

Deforestation and the spatio-temporal distribution of savannah and forest members of the Simulium damnosum complex in southern Ghana and south-western Togo.

Spatio-temporal data on cytotaxonomic identifications of larvae of different members of the Simulium damnosum complex collected from rivers in southern Ghana and south-western Togo from 1975 until 1997 were analysed. When the data were combined, the percentages of savannah blackflies (S. damnosum sensu stricto and S. sirbanum) in the samples were shown to have been progressively increasing since 1975. The increases were statistically significant (P < 0.001), but the rates of increase were not linear. Further analyses were conducted according to the collection seasons and locations of the samples, to account for possible biases such as savannah flies occurring further south in the dry season or a preponderance of later samples from northern rivers having more savannah flies. These analyses showed that the increasing trend was statistically significant (P < 0.0001) only during the periods April to June and October to December. The presence of adult savannah flies carrying infective larvae (L3) indistinguishable from those of Onchocerca volvulus in the study zone was confirmed by examinations of captured flies. The percentages of savannah flies amongst the human-biting populations and the percentages with L3s in the head were higher during dry seasons than wet seasons and the savannah species were found furthest south (5 degrees 25'N) in the dry season. Comparisons of satellite images taken in 1973 and 1990 over a study area in south-western Ghana encompassing stretches of the Tano and Bia rivers demonstrated that there have been substantial increases in urban and savannah areas, at the expense of forest. This was so not only for the whole images but also for subsamples of the images taken at 1, 2, 4, 8 and 16 km distant from sites alongside the River Tano. At every distance from the river, the percentages of pixels classified as urban or savannah have increased in 1990 compared with 1973, while those classified as degraded or dense forest have decreased. The possibility that the proportionate increases in savannah forms of the vectors of onchocerciasis, and hence in the likelihood of the transmission of savannah strains of the disease in formerly forested areas, were related to the decreases in forest cover is discussed.

Animals↗
PubMedDate not supplied

Onchocerciasis in the Amazonian focus of southern Venezuela: altitude and blackfly species composition as predictors of endemicity to select communities for ivermectin control programmes.

In preparation for an ivermectin distribution programme, the prevalence and intensity of infection due to Onchocerca volvulus as well as the species composition and abundance of Simulium vectors were investigated in 22 Yanomami communities situated along 2 altitudinal transects in the southern Venezuelan onchocerciasis focus. These transects corresponded to the Ocamo-Putaco and Orinoco-Orinoquito river systems, covering a range of elevation between 50 m and 740 m above sea level (asl). A total of 831 people underwent parasitological examination in this survey and an additional 196 patients from a previous study, at an altitude of 950 m, were included in the analysis. A total of 92,659 man-biting blackflies were collected and identified to morphospecies. S. oyapockense s.l. was the predominant simuliid up to 150 m asl, whereas S. guianense s.l. and S. incrustatum s.l. prevailed above 150 m. Communities located below 150 m were found to range from hypo- to mesoendemic; all villages above 150 m proved to be hyperendemic (> 60% microfilarial prevalence) and mass ivermectin treatment should be implemented. Age above 10-14 years, altitude of the village and biting rate of S. guianense s.l. up to 200 m asl were found to be statistically significant independent predictors of infection by multivariate logistic regression using a spline model. There were no differences in infection status according to sex. Above 200 m, microfilarial rate and density remained approximately constant, prevalence averaging 79% regardless of blackfly abundance. For the implementation of ivermectin-based onchocerciasis control programmes in the Amazonian focus, altitude and species composition of the blackfly population might be adopted as useful indicators aiding selection of the most affected communities. However, below 200 m additional parasitological indicators may also be necessary. As a direct result of this study, regular mass-ivermectin delivery to meso- and hyperendemic communities is now in progress.

Adolescent↗
PubMedDate not supplied

Similar blackfly attraction by onchocerciasis patients and individuals putatively immune to Onchocerca volvulus.

In areas endemic for onchocerciasis, a small number of individuals show no detectable infection with Onchocerca volvulus despite an apparently similar exposure to the transmitting blackflies. Such individuals have been termed putatively immune. Since several studies have indicated marked host differences in attractiveness for blood-seeking insects, putative immunity in O. volvulus infection may result, at least in part, from low vector attractiveness of the respective individuals. In an area hyperendemic for onchocerciasis (Guinea), where Simulium yahense is the predominant vector, we organized fly catches by putatively immune individuals and individuals with moderate-to-high worm counts. No differences were found between the 2 groups with respect to (i) the attraction of blackflies, (ii) the attraction of blackflies infected with O. volvulus, or (iii) the numbers of O. volvulus larvae carried by the attracted blackflies.

Adolescent↗
PubMed2001

Risk assessment of etofenprox (Vectron) on non-target aquatic fauna compared with other pesticides used as Simulium larvicide in a tropical environment.

Within the rotational scheme developed by the Programme to fight the resistance of Simulium damnosum to chemical larvicides, there was an operational gap at discharges between 5 and 70 m3 s(-1) for the treatment of rivers where resistance to organophosphates was present. The use of permethrin and carbosulfan was precluded because of risk of environmental impact and, Bacillus thuringiensis ser. H-14 treatments were not envisageable due to cost and logistics constraints. Among the possible complementary groups of larvicides tested, the pseudo-pyrethroids, held promise, because of a mode of action similar to that of pyrethroids, but along with a usually lower toxicity for fish. Etofenprox, one of the pseudo-pyrethroids tested, shows a global detachment of non-target insects in 24 h close to that of pyraclofos, an organo-phosphorus compound (27 against 23%). In laboratory conditions, six times the operational dose which is 0.03 mg l(-1) 10 min, is needed to cause 50% mortality of Caridina sp. (a small shrimps species) and 30 times this same dose for 95% mortality. For fish species, a safety margin of 400-800 times the operational dose is observed for Oreochromis niloticus and 200-400 times for Tilapia zillii.

Animals↗
PubMed1998

Long-term quantitative ecological assessment of insecticides treatments in four African rivers: a methodological approach.

In West Africa different insecticides had been applied in selected river areas for the reduction of the blackfly populations vectors of Onchocerca volvulus, a parasite causing blindness. To evaluate the possible long term effects of the larvicides on the non target fauna an aquatic monitoring programme has been up from the initial phase of the project. Addressing the attention to the invertebrates data collected in four countries during a maximum period ranging from 1977 to 1996, this paper shows and discusses the data analysis strategy for the measure and interpretation of the biological variation. In particular the application of quantitative ecological analysis methods: Principal Component Analysis, rank abundance models and the community diversity indexes, is critically discussed and comments are given to the ecological interpretation of the results.

Africa↗
PubMed2006

Control of onchocerciasis.

Onchocerciasis is a filarial infection which causes blindness and debilitating skin lesions. The disease occurs in 37 countries, of which 30 are found in Africa (the most affected in terms of the distribution and the severity of the clinical manifestations of the disease), six in the Americas and one in the Arabian Peninsula. The latest WHO Expert Committee on Onchocerciasis estimated that in 1995 around 17.7 million persons were infected, about 270,000 of whom were blind and another 500,000 severely visually impaired. The disease is responsible for 1 million DALYs. Eye disease from onchocerciasis accounts for 40% of DALYs annually although severe skin disease is also recognized as of public health significance. Great progress has been made in the last thirty years in the control of onchocerciasis, both in Africa and the Americas, and this progress has been due largely to international public-private partnerships, sustained funding regional programmes, and new tools and technology. Landmarks in the global control of river blindness include the significant success of the Onchocerciasis Control Programme of West Africa (1975-2002), and the donation of ivermectin (Mectizan) by Merck & Co. Inc., in 1988, a medicine that is distributed to millions free of charge each year. Future major technical challenges of onchocerciasis control include ivermectin mass administration in areas co-endemic for the parasite Loa loa in the light of possible severe adverse reactions, ivermectin treatment in hypoendemic areas hitherto excluded from African control programmes, sustainability of ivermectin distribution, post-control surveillance for recrudescence detection, surveillance for emergence of resistance, and decisions of when to stop mass ivermectin treatments. There is the need to develop the appropriate information systems and diagnostic tools to help in accomplishing many of these tasks. A search for a second-line treatment or as an additional drug to ivermectin as well as a search for a macrofilaricide are issues that need to be addressed in the future.

Africa↗
PubMed1997

Vector-parasite transmission complexes for onchocerciasis in West Africa.

BACKGROUND: In West Africa, there are two strains of the filarial parasite Onchocerca volvulus, which differ in their ability to induce ocular disease. Transmission studies have suggested that six sibling species of the parasite vector, the black fly Simulium damnosum sensu lato, allow development of the two strains of O volvulus with varying efficiency. We aimed to test the hypothesis of parasite-vector complexes, whereby the two parasite strains, known as forest and savanna, are preferentially transmitted by distinct groups of the species of S damnosum S l. METHODS: During 1993 and 1994, wild black flies were collected from 11 river basins within the area covered by the Onchocerciasis Control Programme (OCP). The flies were dissected and filarial larvae, ovaries, and malpighian tubules removed. Genomic DNA was extracted from larvae, and PCR amplification was used to classify O volvulus parasites as forest or savanna strains. PCR-amplified DNA from ovaries and malpighian tubules was used to distinguish sibling species of S damnosum s l. S yahense and S squamosum were distinguished by body colour. FINDINGS: 214 of 105105 flies dissected were infected with filarial larvae; 84 of these were infected with mature O volvulus parasites. Of the 35 savanna-dwelling infected flies. 17 carried forest-strain parasites and 18 savanna-strain parasites. Of the 45 infected flies identified as the forest dwelling sibling species. 20 carried savanna-strain parasites and 25 forest-strain parasites. No significant differences were found in the numbers of mature larvae of each strain carried by the forest-dwelling species of fly or in the number of forest and savanna larvae in savanna-dwelling vector species. INTERPRETATION: Vector-parasite transmission complexes do not currently play a part in the biology of O volvulus transmission in the area of the OCP in West Africa. This finding has important strategic implications for the future of efforts to control onchocerciasis in West Africa.

Africa, Western↗
PubMed1996

Immunity to eukaryotic parasites in vector insects.

Mosquitoes and blackflies have been the focus of recent efforts to elucidate factors influencing the susceptibility of vector insects to metazoan and protozoan parasites of medical significance. Vector species exhibit variation in cellular and humoral immune responses, as highlighted by studies of melanotic encapsulation and components of the phenoloxidase system. Significant progress has been made in the development of genetic maps based upon molecular markers, leading to the genetic analysis of loci influencing susceptibility. The identification of specific inducible antibacterial peptides, and the cloning of genes encoding immune effector proteins as well as potential regulatory factors, open the path to fruitful studies of vector insect innate immunity and its relationship to insect-parasite interactions.

Animals↗
PubMed1997

Rapid induction by a blood meal of a carboxypeptidase gene in the gut of the mosquito Anopheles gambiae.

A search for genes induced rapidly (< 3 h) after a blood meal in the gut of the human malaria vector Anopheles gambiae led to the identification of a carboxypeptidase gene (AgCP). We report the sequence of the 1302 nt AgCP transcribed sequence, 710 nt of upstream and 585 nt of downstream DNA. The AgCP open reading frame is 60.4% identical at the nucleotide level to a blackfly, Simulium vittatum, carboxypeptidase gene. The transcriptional start site of AgCP was determined by primer extension. Expression of AgCP mRNA is detectable in the guts of pupae and sugar-fed adult female mosquitoes and is induced (approximately 10-fold) within 3 h of a blood meal. By 24 h after a blood meal, mRNA abundance returns to a level close to that present before a blood meal. Whole-mount in situ hybridization shows that AgCP mRNA expression is restricted to most or all cells of the posterior midgut. Expression of the AgCP and trypsin genes were compared and shown to differ in two fundamental ways: (1) the peak of AgCP expression after a blood meal occurs approximately 20 h before that of trypsin; and (2) induction of the AgCP gene is independent of the composition of the ingested meal whereas trypsin induction requires the presence of protein. The potential use of the AgCP promoter for driving the expression of genes that hinder the development of parasites in the mosquito gut is discussed.

Amino Acid Sequence↗
PubMed1991

Onchocerca lienalis: comparison of techniques for the cryopreservation of microfilariae within skin-snips or free of host tissues.

Onchocera lienalis microfilariae (mf) were cryopreserved in liquid nitrogen within skin-snips using methanol as a cryoprotectant and their viability evaluated and compared to mf cryopreserved free of host tissues using ethanediol as a cryoprotectant. Despite an initial delay in emergence, the methanol technique did not significantly affect the total numbers of mf emerging from skin-snips of various sizes (3.3-59.81 mg) compared to untreated controls over a 6 h period. Following thawing, the initial motility index (MI) scores of mf cryopreserved by either method were not significantly different from untreated controls; however, over a period of 15 days in culture the MI scores of both cryopreserved groups showed a small but significant overall decline, with the methanol technique producing the lowest scores. These changes in motility levels correlated with the numbers of mf which developed to the infective stage following intrathoracic injection into Simulium ornatum, although this ability to develop was a much more sensitive measure of parasite viability; compared to untreated control recoveries of 3rd-stage larvae, 63.9-71.7% (ethanediol technique) and 34.2-36.9% (methanol technique) of this number were recovered from cryopreserved groups. There were no significant differences in the lengths of infective larvae recovered from the insect heads from each treatment group, nevertheless there were higher numbers of 2nd-stage larvae recovered from the cryopreserved groups compared to the untreated controls. The methanol technique has the advantage of being easier to carry out under field conditions, while parasite viability is significantly better using the ethanediol technique.

Animals↗
PubMed1988

Surface carbohydrate changes on Onchocerca lienalis larvae as they develop from microfilariae to the infective third-stage in Simulium ornatum.

Use was made of seven FITC labelled lectins as tools to investigate the surface of Onchocerca lienalis larvae as they develop through to the infective third-stage in a natural vector, Simulium ornatum. The lectins were derived from Canavalia ensiformis (Con A), Lens culinaris (lentil), Triticum vulgaris (wheat germ), Arachis hypogaea (peanut), Helix pomatia, Phaseolus vulgaris (kidney bean) and Tetragonolobus purpureus (asparagus pea). Between 70 and 100 living parasites were examined for each developmental stage; i.e. skin microfilariae, late first-stages, second-stages, preinfective third-stages and infective third-stages isolated from the mouth parts of the flies. None of the lectins used bound to the surface of the microfilariae. However, progressive binding to the cuticle of the first- and second-stages was observed using Con. A, lentil lectin and wheat germ agglutinin (WGA). Following moulting to the third-stage, binding of these three lectins declined. Furthermore, as these lectins decreased, peanut and Helix pomatia lectins progressively increased in their binding, despite the fact that they showed little or no binding to the first- and second-stages; stages at which Con A, lentil and WGA were at their maximum. Asparagus pea and kidney bean lectins failed completely to bind to any of the larvae examined. Carbohydrate inhibition tests showed that the lectin was indeed binding specifically to glycoconjugates on the parasite surface. WGA binding was not inhibited by prior incubation with N-acetyl-D-glucosamine, even at high concentrations, but neuraminic acid did completely inhibit its binding. Judging from the patterns of binding on the nematodes themselves, the carbohydrates may not be vector in origin, but derive from the worms. The lectin specificities indicate that initially mannose/glucose type derivatives are present on the surface. Following moulting to the third-stage these are progressively replaced, or overlaid with galactosamine type derivatives, also present on the infective third-stage as it enters the bovine host. The availability of these surface glycoconjugates to attack mediated by natural insect lectins may be of importance in the parasite regulatory mechanisms of the blackfly. Variability in these surface carbohydrates, and in the response to them could well be a contributing factor in the cytospecific variation in S. damnosum susceptibility to geographical variants of O. volvulus.

Animals↗
PubMed1990

Avian trypanosomes in Simulium and sparrowhawks (Accipiter nisus).

Isolates of avian trypanosomes from nestling sparrowhawks (Accipiter nisus) and from Simulium latipes were compared by isoenzyme electrophoresis with the previously described avian trypanosomes Trypanosoma corvi and T. everetti. Simulium isolates developed into trypomastigotes in semi-defined medium at 37 degrees C confirming that they belong to the genus Trypanosoma. Cellulose acetate electrophoresis (CAE) and polyacrylamide gel electrophoresis (PAGE) methods were employed. A total of 11 enzymes was examined, of which 8 gave satisfactory results for all lysates (MDH, ICD, PGM, 6PGD, SOD, PEP I, PEP II and PEP D). The zymograms of 3 isolates from Simulium and 4 from A. nisus were similar indicating that they were the same organism and the isoenzyme patterns coincided with those of T. corvi. It is concluded that A. nisus and S. latipes are infected with T. corvi. Flagellate infections were found in the midgut, hindgut and rectum of Simulium where large numbers of epimastigotes were found in the lumen and attached to the cuticular intima of the rectal ampullae by hemidesmosomes at the distal end of an expanded flagellum. Out of 285 nestling sparrowhawks 13 (5%) were infected with trypanosomes. In view of their age and inability to preen themselves it is suggested they became infected via infected prey.

Animals↗
PubMed1995

Density-dependent processes in the transmission of human onchocerciasis: relationship between the numbers of microfilariae ingested and successful larval development in the simuliid vector.

A previous paper reported that the intake of Onchocerca volvulus microfilariae (mff) by different species of Simulium is essentially proportional to the parasite load in the skin of infected carriers. This paper examines the fate of the ingested mff in susceptible vectors to assess the relationship between parasite intake and infective larval output in blackfly species with and without well-developed cibarial armatures. Analysis is based on data from 3 onchocerciasis endemic areas: Guatemala (S. ochraceum s.l.), West Africa (S. damnosum s.l./S. sirbanum) and the Amazonian focus between South Venezuela and Northern Brazil (S. guianense and S. oyapockense s.l.). The data, which include published and unedited information collected in the field, record experimental studies of parasite uptake by wild flies maintained in captivity until the completion of the extrinsic incubation period. The relationship between L3 output (measured as the mean number of successful larvae/fly or, as the proportion of flies with infective larvae) and average microfilarial intake, was strongly non-linear. This non-linearity was best represented by a sigmoid function in case of armed simuliids (S. ochraceum s.l., S. oyapockense s.l.), or by a hyperbolic expression in that of unarmed flies (S. damnosum s.l., S. guianense). These results are compatible, respectively, with the patterns of 'initial facilitation' and 'limitation' described in culicid vectors of lymphatic filariases. A maximum mean number of 1-3 L3/fly was observed in all 4 vectors. It is concluded that O. volvulus larval development to the infective stage is regulated by density-dependent mechanisms acting at the early phase of microfilarial migration out of the blackfly's bloodmeal. Damage by the bucco-pharyngeal armature may also be density dependent. A hypothesis, based on this density dependence is forwarded to explain initial facilitation, so far only recorded in vectors with well-developed cibarial teeth. Our results provide quantitative support for the conjecture that chemotherapy alone is likely to have a greater impact on reducing onchocerciasis transmission in endemic areas where the main vector has a toothed fore-gut than in foci where the vectors have unarmed cibaria.

Africa, Western↗