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PubMed1992

Distribution of Simulium yahense after larviciding activities in the southern extension of the Onchocerciasis Control Programme.

The distribution range of Simulium yahense in the Eastern Operational Area of the Onchocerciasis Control Programme increased after larviciding treatments were started in 1988, to control important Simulium damnosum s. 1. vectors of onchocerciasis. A distribution map indicating the extended range is drawn and the importance of this expansion of S. yahense to disease transmission discussed.

Africa, Western↗
PubMed2005

Use of Geographic Information Systems in the development of prediction models for onchocerciasis control in Ethiopia.

A risk assessment model was developed for onchocerciasis distribution and its control in Ethiopia using Geographic Information System (GIS) methods. GIS data analysis was done to generate 3 separate risk models using selected environmental features of (1) earth observing satellite data on Normalized Difference Vegetation Index (NDVI) and midday Land Surface Temperature (LST) prepared from AVHRR sensor data of the Global land 1-km project for the years 1992 and 1995, (2) FAO agroclimatic databases from the Crop Production System Zone (CPSZ) of the Intergovernmental Authority on Development (IGAD) sub-region of East Africa, and (3) a climate-based forecast index based on the growing degree days (GDD) and water budget concepts. Parasitological data used for the analysis were published and unpublished reports of onchocerciasis surveillance made between 1969 and 2000 in various parts of the country. Analysis of queries based on 1992 and 1995 annual wet and dry season data of the Global land 1-km project resulted in a predictive value of 95.1%, 94.0% and 96.3%, respectively, using data values extracted from buffers centered on sites above 5% prevalence. The model based on CPSZ data predicted an endemic area that best fit the distribution of sites over 5% prevalence; the query was based on CPSZ values of average altitude (442-2134 m), annual mean temperature (18-28 degrees C), annual rainfall (822-1980 mm), annual potential evapotranspiration (1264-1938 mm), rain minus potential evapotranspiration (-124 - 792 mm), average NDVI x 100 (2000-5000) and average terrain percent slope (9-34). The climate-based model based on GDD and water-budget predicted high risk to severe risk areas in the western and southwestern parts of the country. All three of the models predicted suitable areas for the transmission of onchocerciasis outside known endemic areas, suggesting the need for ground-based validation and potential application to current community-directed treatment programs with ivermectin (CDTI) for control of onchocerciasis in Ethiopia.

Agriculture↗
PubMed2005

Geographical patterns of onchocerciasis in southern Venezuela: relationships between environment and infection prevalence.

Onchocerciasis is a chronic filarial infection transmitted by Simulium flies that has a focal geographical distribution in Latin America. The southern Venezuelan focus has a gradient of endemicity that includes the largest number of hyperendemic communities in the continent, many of them in remote forest and mountainous areas, where it is an important public health problem among the Yanomami indigenous population. The recent introduction of Geographical Information Systems (GIS) tools and a landscape epidemiology approach for study of vector borne diseases is helping to understand relationships between environment and transmission dynamics of onchocerciasis. Striking differences in the transmission dynamics of onchocerciasis between different river courses were detected. A significant relationship between onchocerciasis and temperature was also demonstrated. The geologic substrate, kind of landscape and vegetation seemed also to influence the transmission of onchocerciasis. In the Venezuelan Amazon, different kinds of landscapes associated with distinctive vector species, show different intensities of transmission of onchocerciasis. In this sense, landscape analysis aided by GIS, may prove to be a useful tool for better identification of the spatial distribution of onchocerciasis risk in the Orinoco basin.

Altitude↗
PubMed2005

Physico-chemical indices of breeding sites of Simulium damnosum in the lower Cross River Basin, Nigeria.

To determine the factor influencing the abundance development and distribution of Simulium damnosum (S.I) immature stages in the lower Cross River Basin, Nigeria, the physico-chemical characteristics of water at breeding sites were analysed longitudinally over a 12 month period. Analysis of the water samples showed that water current correlated positively and significantly (p < 0.05), with dissolved oxygen (r = 0.95 vs 0.10); conductivity (r = 0.95 vs 0.81); hydrogen ion concentration (r = 0.03 vs 0.46); and biochemical oxygen demand (r = 0.80 vs 0.64); from Agbokim Waterfalls and Afi River respectively. The result indicates that these parameters and low ionic concentration are the most essential requirement for the development of S. damnosum pre-imaginal stages. Amplitudes of annual variability as measured by co-efficient of variation varied between the parameters. From the trends in hydrological variables it was deduced that the integrated impact of precipitation, input of surface runoff and municipal effluents played an overriding role in determining the absolute levels and temporal pattern in the water quality attributes. Of the 3578 pre-imaginal stages of S. damnosum collected, 52.52% and 47.4% were collected from Agbokim Waterfall and Afi River respectively. There was a significant difference (p < 0.05) in the number of pre-imaginal stages collected from both sites. The highest number of pre-imaginal stages of S. damnosum collected were at the peak of rain (July-September) indicating that S. damnosum in the River Basin is a wet season breeder. The need for long term laboratory colonization of blackfly immature stages becomes imperative especially how these parameters could be exploited in control programme through the testing of larvicides without adverse ecological damage to the aquatic habitat.

Animals↗
PubMed2005

Status of forest onchocerciasis in the Lower Cross River basin, Nigeria: entomologic profile after five years of ivermectin intervention.

In the Lower Cross River basin in Nigeria, no pre-control entomologic profile of Onchocerca volvulus infection in the local Simulium damnosum population was available prior to the initiation of an ivermectin control program in 1995. A longitudinal entomologic study was therefore carried out over a 12-month period (January-December 2001) at the Agbokim waterfalls and Afi River, which are breeding sites of S. damnosum in the river basin. A total of 9,287 adult S. damnosum were caught on human bait; 9,048 (97.43%) were dissected, of which 313 (3.46%) were infected. Annual biting rates (ABRs) of 42,419 and 28,346 bites per persons per year were recorded at the Agbokim Waterfalls and Afi River, respectively. The annual transmission potential (ATP) was 419 infective larvae per person per year at the Agbokim Waterfalls and 427 at the Afi River. Monthly biting rate and monthly transmission potential varied significantly (P < 0.05) at the two sites. Transmission was highly seasonal from April to September, corresponding to the peak biting period of the vector. The high ATP and ABR values are a measure of the mesoendemicity of onchocerciasis in the river basin. There was a significant F(0).05 (1, 10) (P < 0.05) variation in the relative fly abundance from both sites. It was observed that human activities such as farming, fishing, timber cutting, and hunting are done in the early morning and late afternoon, which corresponds to the peak diurnal biting period of the vector. Changes in these practices and attitudes may markedly affect the disease intensity and transmission.

Animals↗
PubMed1991

Importance of migrants infected with Onchocerca volvulus in west African river valleys protected by 14 to 15 years of Simulium control.

A study was done to determine the importance of human migration from non-controlled endemic onchocerciasis foci to the river valleys that have been protected for the past 14 to 15 years by the vector control operations of the Onchocerciasis Control Programme in West Africa. The aim of the study was to assess the contribution of migrants to the prevalence and intensity of infection in villages from 5 major river valleys and their potential role in causing relapse of transmission once the vector is allowed to return. In Burkina Faso the migrant population varied from 0.0% to 18.1% of the village population, and averaged 4.9%. Migrants accounted only for 0.6% of the population in Ghanaian and Ivorian villages along the Black Volta river. The prevalence of infection was significantly higher in migrants (8.2%) than in non migrants (1.1%) in the surveyed villages in Burkina Faso, and 1.5% of migrants had infections with more than 16 microfilariae per snip as against 0.2% of non migrants. Nearly all infected migrants came from the south of the Côte d'Ivoire. The study shows that human migration has caused the importation of Onchocerca volvulus from non-controlled areas. However, the epidemiological importance of this phenomenon is limited because of the very small number of infected migrants per village while two-third of the infected migrants are believed to be infected with the less pathogenic forest strain of the parasite. Because migration patterns changes geographically and over time similar studies will be continued on a regular basis.

Animals↗
PubMed2006

Consequences of vector behavior in epidemiology of onchocerciasis on the Firestone Rubber Plantation in Liberia.

In the current study, we have assessed the distribution of black fly vectors and the occurrence of onchocerciasis among residents of the Firestone Rubber Plantation (FRP) in Liberia. The degree of endemicity of onchocerciasis on the FRP is correlated with the breeding of Simulium yahense on the Du River and Simulium soubrense on the Farmington River, as well as with the feeding behavior of these black fly vectors. S. yahense is highly anthropophilic and highly susceptible to Onchocerca volvulus, thus, it is apparently the primary vector of O. volvulus on the plantation. S. soubrense is predominantly zoophilic; consequently, it has a low capacity for transmitting of O. volvulus to humans. The prevalence of the disease is hyperendemic in the divisional camps near the Du River, affecting 77.6% to 94.4% of males and 64.9% to 88.9% of females. In the camps along the Farmington River, the disease is hypoendemic, affecting 26.1% to 49.2% of males and 25.6% to 35.1% of females. The mean intensity of infection in the camps near the Du River was 14.1 microfilariae (mf)/mg of skin in males and 13.7 mf/mg of skin in females. The density of microfilariae in people in the camps along the Farmington River was 4.8 mf/mg of skin in males and 1.9 mf/mg of skin in females. The age-specific infection in camps along both rivers substantially increased after the ages 21-30, reaching a maximum at ages 41-50. The mean density of mf/mg of skin in three anatomic locations (calf, hip, and shoulder) was 28.0, 24.5, and 14.1 mf/mg of skin, respectively, for those living on the Du River, and 10.2, 7.6, and 5.8 mf/mg of skin, respectively, for those on the Farmington River.

Adolescent↗
PubMed2006

Large-scale entomologic assessment of Onchocerca volvulus transmission by poolscreen PCR in Mexico.

To study the impact of mass Mectizan treatment on Onchocerca volvulus transmission in Mexico, entomological surveys were carried out in the endemic foci of Oaxaca, Southern Chiapas, and Northern Chiapas. Collected flies were screened by polymerase chain reaction (PCR) for O. volvulus parasites. The prevalence of infected and infective flies was estimated using the PoolScreen algorithm and with a novel probability-based method. O. volvulus infective larvae were not detected in flies from 6/13 communities. In 7/13 communities, infective flies were detected, with prevalences ranging from 1.6/10,000 to 29.0/10,000 and seasonal transmission potentials ranging from 0.4 to 3.3. Infected and infective flies were found in a community in Northern Chiapas, suggesting that, according to World Health Organization criteria, autochthonous transmission exists in this focus. These data suggest that O. volvulus transmission in Mexico has been suppressed or brought to a level that may be insufficient to sustain the parasite population.

Algorithms↗
PubMed2003

[Transgenic bioinsecticides inimical to parasites, but imical to environment].

Identification of Bacillus thuringiensis (Bt) parasporal crystalline inclusions composed of Cry proteins (=delta-endotoxins) resulted in introduction of microbial pesticides for biological control of some parasites. Delta-endotoxins are encoded by cry genes and are active against pest and nuisance insects (mostly mosquitoes and black flies--vectors of still important infectious diseases). The recent significant progress in DNA recombination technique may overcome limitations (a short residual persistence and a narrow spectrum of activity) associated with application of Bt conventional products. An introduction of cry genes from mosquitocidal subspecies B. th. israelensis (Bti) to the aquatic microorganisms inhabiting the same water bodies as mosquito and fly larvae (Diptera), has considerably improved the toxin delivery system to target insects. However, in the first experiments, in which Bti genes were cloned in cyanobacteria (Agmenellum quadruplicatum, Synechocystis PCC6803), a low gene expression was observed. Thus, it was necessary to integrate cry genes with strong promoters or to increase the number of vector-introduced copies. To overcome the obstacles of low gene expression and regulatory restriction for recombinant organisms, Bti spore/crystal formulations were encapsulated in the aquatic protozoan, Tetrahymena pyriformis. Large numbers of crystals (180 to 240/cell) were accumulated in its food vacuoles. This system resulted also in an increase in toxin persistence from 24 to 71 h. Cloning Bti genes in B. sphaericus (which also produces mosquitocidal proteins) was another way of an increasing Bt crystal residual activity. In this case, the crystals were additionally protected by B. sphaericus exosporium. These transgenic bacteria produced large amounts of delta-endotoxins that remained under water surface longer than the wild B. sphaericus strains. Moreover, they had a broader spectrum of insecticidal activity, because B. sphaericus is toxic mostly to Culex and Anopheles, and Bti--mostly to Culex, Aedes and some Simmulidae. Gram-negative bacteria (Asticcacaulis excentricus, Caulobacter crescentus and Ancylobacter aquaticus) turned out also to be effective delta-endotoxin producers. They grow on simple media and do not contain proteases which could degrade Cry proteins. In some cases, 100% mosquito larvae mortality was observed as a result of an exposure to transgenic microorganisms containing Bti genes. However, transgenic techniques are still not very popular in the world, despite their efficacy in biological control of insects. The transgenic organism construction is expensive and time-consuming. Genetic engineering is still raising a lot of anxieties and doubts concerning inappropriate use of modified organisms. On the other hand, this technology could solve many problems associated with vectors of important diseases, which are still unapproachable to contemporary medicine.

Animals↗
PubMed1990

[Campaign against the vectors of onchocerciasis in the surroundings of a refugee camp located in a savanna zone of Cameroon].

In view of the risks of onchocerciasis facing Chadian refugees who have settled in a camp in a savanna hyperendemic area of northern Cameroon, the authors concluded that a vector control directed against the black fly larvae located on small seasonal tributaries of the main water course of this area should be carried out during the four to five months of the rainy season, i.e., during the whole period of transmission of this disease. The operations were undertaken during two consecutive rainy seasons. The first year consisted in developing ground-based control techniques and evaluating the larvicidal efficacy at the larval level. The second year was, particularly, devoted to the study of the impact of the vector control on the biting rate and on the transmission of onchocerciasis. Every week, three of the tributaries close to the camp were treated with an Emulsifiable Concentrate of temephos. Throughout the rainy season, the larvicide eliminated almost all the pre-adult black fly stages on these water courses. At the point in a high onchocercal endemicity area, it also reduced the biting rate by 60% and the Annual Transmission Potential by 72%, i.e., to values corresponding to the lower meso-endemicity threshold. Since these results were obtained at a very unfavourable point from the standpoint of black fly density and onchocerciasis transmission, the authors consider that vector control carried out under these conditions protects the local populations and the Chadian refugees effectively from black fly bites and a risk of severe onchocerciasis.

Animals↗
PubMedDate not supplied

[Molecular approaches to the control of onchocerciasis in Mexico].

Onchocerciasis is one of the major causes of blindness in the World, with about 17.7 million infected, particularly in West Africa. In Mexico, onchocerciasis is also present and has been subjected to control since 1923. The standard diagnosis of onchocerciasis is by the detection of microfilariae by skin biopsy and transmission is evaluated by detection of Onchocerca volvulus larvae in the vector. Classically, this was carried out by manual dissection of Simuliumn ochraceun s.l. However, with the use of ivermectin, a drug that kills microfilariae but not the adult worms, the skin biopsy is becoming no longer useful for detecting microfilariae levels and due to the reduced transmission, fly dissection is no longer viable. The subject of this paper is to present the immunological and molecular techniques developed to supersede the skin biopsy and fly dissection, and their diagnostic ability to assess the impact of multiple bi-annual mass ivermectin treatments on O. volvulus transmission in Mexico.

Africa, Western↗
PubMed2006

[Onchocerciasis in the area of Lastourville, Gabon. Clinical and entomological aspects].

The African Programme for Onchocerciasis Control (APOC) has implemented a series of surveys aimed at evaluating the long-term impact of its activities. The region of Lastourville (Gabon) is one of the selected sites for this study. A total of 886 persons was examined for skin lesions, and 459 out of them participated in detailed ocular examinations. Blackflies were collected during one year and dissected. Although the focus was found to be hypoendemic (prevalence of nodules: 7.7%), the frequency of onchodermatitis was relatively high. The lesions of the anterior segment of the eye were rare, but the prevalence of optic nerve disease, and of choroido-retinal lesions reached 5.2 and 2.7%, respectively. The annual transmission potential (2,171 infective larvae per man) was high, when compared with the results recorded in the human population. This may be due to the presence of Onchocerca spp. of animal origin in the blackflies. These results indicate that in the area of Lastourville, though regarded as the main focus of onchocerciasis in Gabon, the disease is relatively mild.

Adolescent↗
PubMed2006

[African Programme for Onchocerdasis Control (APOC): Onchocerca Simulium squamosum in two regions in the Republic of Cameroon].

In 1998, the African Programme for Onchocerciasis Control (APOC) decided to launch a long-term impact assessment of its operations. This paper reports the baseline entomological data collected throughout a whole year in two sites of Cameroon (Kahn and Bolo). The Simulium populations of the two study sites were characterized by parous rates of 7.2% and 33.5% respectively and infectivity by O. volvulus of 31 and 190 infective larvae per 1000 parous flies respectively The Annual Transmission Potentials (ATP) were respectively 523 and 9972 infective larvae per man and per year in Kahn and in Bolo. The Simulium populations studied in both sites, even though the ATP in Kahn is 19 times lower than that of Bolo, showed a pattern of an onchocerciasis hyperendemic zone in terms of vector capacities and entomological indices.

Animals↗
PubMedDate not supplied

[Vector capacities of Similium damsnosum s.l and risk for Onchocerca volvulus transmission in Inga (Democratic Republic of Congo)].

This article analyzes the results of a one-year entomological survey conducted near the Yalala rapids in the Inga region, Democratic Republic of Congo, prior to the implementation of Community-Directed Treatment with Ivermectin (CDTI). A vector control program had been undertaken from October 1969 to December 1980, 20 kilometers upstream from the study site. The results of the present study show that during 122 collection days, 5,824 females were captured, of which 3,899 were dissected; 2,001 (51.3%) were parous while 44 (2.2%) were infected while 4 (0.2%) were infective. The crude Annual Transmission Potential (ATP) was 65 infective larvae per man and per year at the river as against 12 in Yalala village located two kilometers from the rapids. The average biting rate was 5 flies/man/day during the dry season and 90 during the rainy season. The vector capacity of similium shows that the risk of transmission of Onchocerca volvulus to the human population was not only reduced in time (2-3 months), but was also negligible all year round.

Animals↗
PubMed1991

Vector immunity to filariasis: new potential for disease control?

These experiments describe investigations into the mode of microfilarial killing in the haemocoel of immune Aedes aegypti and Simulium ornatum. Antibody blocking experiments indicate that sarcotoxin IIA-like and sarcolectin-like molecules maybe involved. The use of carbohydrate in the infecting blood meal indicates that gut lectins may be involved in preventing migration of Brugia microfilariae into the haemocoel. This appears to be a strain factor linked to innate susceptibility. These physiological mechanisms may develop into useful handles in the control of vectors by genetic manipulation, or in developing new chemotherapeutic agents.

Aedes↗
PubMed1991

Transmission of Onchocerca dukei by Simulium bovis in North-Cameroon.

"Wild" Simulium bovis females were collected after they had taken blood on a bait oxen with skin microfilariae (mf) of Onchocerca dukei and Onchocerca ochengi. Flies ingested O. dukei mf in high numbers (13.1 mf per blood fed fly) but rarely mf of O. ochengi (3 mf in 60 flies). In 32 flies dissected 14 to 17 hours after the blood meal, a total of 130 O. dukei mf but no of O. ochengi had migrated to the thorax. Six to 9 days after the blood meal, a total of 203 third stage larvae (L3) were found in 61 surviving flies. Invasive larvae from the head measured 738.0 x 17.8 microns and resembled those of O. dukei obtained by intrathoracic injection of mf in a previous study, but were shorter and thinner than O. ochengi L3. Thus, S. bovis can be seen as an efficient natural vector of O. dukei. Its role in the transmission of bovine and human onchocerciasis in North Cameroon is discussed.

Animals↗
PubMed1991

A temporary focus of savanna species of the Simulium damnosum complex in the forest zone of Liberia.

Savanna species of the Simulium damnosum complex, the major vectors of the blinding savanna type of onchocerciasis, were considered to be rare in Liberia, until the dry season of 1988. In 1988 they became a serious nuisance, biting people at the Bong iron ore mine within the rain forest zone. S. damnosum s.str. and S. sirbanum were found breeding, in association with S. adersi a non man-biting savanna species, in a stream emerging from the mine's tailings pond. The local forest species were extremely rare in this stream. The water of the stream was characterized by an increased hardness and higher temperatures in comparison with those of natural watercourses in the area. The mass occurrence of flies was probably related to expansions of the tailings ponds, when rich nutrition was provided for the blackfly larvae by the decaying forests submerged by the rising water levels. The phenomenon did not recur in 1989 when only a few savanna flies were caught, and none were seen in the dry season of 1990. Although it is not known why the artificial environment of the tailings ponds was so attractive for the savanna species the events clearly demonstrated that savanna flies seasonally invading the area, possibly aided by the northeasterly harmattan winds, can become established in the rain forest zone if suitable conditions are met. No infections with Onchocerca volvulus were found in more than 1000 flies caught by vector collectors but, after experimental infection with the local forest strain, a few parasites developed to the infective stage. As yet there is no evidence that the occurrence of savanna flies in the rain forest zone of Liberia was of epidemiological significance.

Animals↗