Transmission Of Human Onchocerciasis Research Articles

Factors affecting the transmission of human onchocerciasis by <i>Simulium damnosum</i> s l. in a fringe savanna village in Cross River State, Nigeria

Factors affecting the transmission of human onchocerciasis by Simulium damnosum sensu lato were the focus of a study carried out in River Lewa, Etikpe, a fringe savanna village in Ikom LGA in Cross River State, Nigeria. The duration of the study was six months (August 1999 to January 2000). The thrust of the study was on ecological parameters that affected the transmission of this human scourge. Some entomological indices/parameters that are useful criteria in planning and evaluating the progress of onchocerciasis control programme were also considered. Trained flycatches working alternate hours (0700hr.-1800hr.) caught black flies used for the study. The black flies caught were either dissected fresh in the field to determine parity (physiological age) or processed in the laboratory to determine the infection/infective rates due to Onchocerca volvulus. Out of 844 black flies collected in the 30 days catching, 425 (50.4%) flies were parous. The monthly ecological parameters (that is, water pH and temperatures, dissolved oxygen, iron content, atmospheric temperatures, rainfall, relative humidity and conductivity) measured and recorded in the study area/site had no significant differences in their values (P>0.05). These observed parameters however, had fluctuations, changes and variations in values. The findings in this study, if corroborated with those of related studies based on parasitologic, ophthalmologic and socioeconomic monitoring will be useful for planning of effective treatments with ivermectin. Keywords : transmission, human onchocerciasis, savannah, Cross River State Mary Slessor Journal of Medicine Vol. 5(2) 2005: 59-64

Combined benefits of annual mass treatment with ivermectin and cattle zooprophylaxis on the severity of human onchocerciasis in northern Cameroon.

We studied the effect of 10 annual distributions of ivermectin for control of human onchocerciasis on the reduction of Onchocerca volvulus transmission by Simulium vector flies. Prevalence and infection load in the human population decreased, whilst the annual biting rates (ABR) of the vector remained unchanged. The annual transmission potential of infective larvae of O. volvulus fell to 40% of the pre-intervention level, but was still sufficient to maintain endemicity. However, recent immigration of herds of nomadic cattle into the study area has diverted the flies from man to cattle, creating an additional zooprophylactic effect. The predominant transmission of the bovine filaria O. ochengi to man also provides some concomitant cross-protective immunity against further infestations with O. volvulus. The effects of ivermectin on O. volvulus, combined with the zooprophylactic effects of the increased cattle population, have produced a complex beneficial influence on the transmission of human onchocerciasis.

Density-dependent processes in the transmission of human onchocerciasis: relationship between microfilarial intake and mortality of the simuliid vector.

In order to construct an analytical model of onchocerciasis transmission, it is necessary to elucidate the functional relationships of the various population rate processes taking place within the human and vector hosts. Two previous papers have explored the evidence for density-dependent regulation in relation to microfilarial intake by, and larval development within, the Simulium host. This paper investigates the survivorship of wild-caught blackfly samples fed on subjects with different intensities of Onchocerca volvulus microfilarial infection. Analyses were based on data for Guatemalan S. ochraceum s.l. (possessing a well-developed cibarial armature), West African S. damnosum s.l. (forest species), and South Venezuelan S. guianense (the latter two lacking a toothed cibarium). The mean survival times of samples of the 3 species, kept under laboratory conditions, decreased as parasite intake increased, the rate of mortality being dependent on the fly's age (measured as time post-feeding) and on the worm load acquired. An empirical, time-dependent hazard function was fitted to observed death rates/fly/day which rose very shortly after engorgement, declined subsequently, and rose again throughout the extrinsic incubation period of the parasite. The parameters of this hazard model were all positively correlated with the density of microfilariae in the bloodmeal. Expressions of survivorship and life-expectancy as explicit functions of time post-feeding and mean parasite intake were derived. The average expectation of life at engorgement for uninfected flies in the laboratory was estimated to be around 1 week for both, armed and unarmed blackflies. Residual life-expectancy decreased with time post-feeding and microfilarial load in both categories of vectors. This decline (resulting from age- and parasite-dependent mortality rates) was much more pronounced in those species lacking a toothed fore-gut. Whilst a fraction of heavily infected S. ochraceum was able to survive the latent period of the parasite, being therefore potentially capable of transmitting the infection, equivalent worm loads in S. guianense resulted in a drastic reduction of the expectation of infective life. These results provide additional evidence to support the hypothesis that, in the case of intrinsically susceptible vectors, unarmed simuliids are more efficient at low microfilarial loads, when the transmission rate from human to vector host is higher, and parasite-induced fly mortality is negligible. The opposite takes place in armed flies, which perform poorly at low parasite burdens and better at heavier loads, with little parasite-induced vector death.

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.

Density-dependent processes in the transmission of human onchocerciasis: intensity of microfilariae in the skin and their uptake by the simuliid host.

The transmission success of Onchocerca volvulus is thought to be influenced by a variety of regulatory or density-dependent processes that act at various points in the two-host life-cycle. This paper examines one component of the life-cycle, namely, the ingestion of microfilariae by the simuliid vector, to assess the relationship between intake of larvae and the density of parasites in the skin of the human host. Analysis is based on data from three areas in which onchocerciasis is endemic and includes published information as well as new data collected in field studies. The three areas are: Guatemala (Simulium ochraceum s.l.), West and Central Africa (savanna members of the S. damnosum complex), and South Venezuela (S. guianense). The data record experimental studies of parasite uptake by flies captured in the field and fed to repletion on locally infected subjects who harboured varying intensities of dermal microfilarial infection. Regression analyses of log transformed counts of parasite burdens ingested by the flies plotted against log transformed counts of microfilariae per mg of skin revealed little evidence for saturation in parasite uptake by the flies as the intensity in the human host increased. There was a positive and highly significant rank correlation between both variables for the three blackfly species. In an alternative analysis a model was fitted to data on prevalence of flies with ingested microfilariae (mff) versus dermal mean intensities. The model assumed an overdispersed distribution of the number of mff/fly and a given functional relationship between intake and skin load. The results of both approaches were consistent. It is concluded that parasite ingestion by the vector host is not strongly density dependent in the three geographical areas and ranges of dermal loads examined. It therefore appears that this transmission process is of reduced importance as a regulatory mechanism in the dynamics of parasite population growth.

Knowledge, Attitudes and Perceptions (KAP) of onchocerciasis: A survey among residents in an endemic area in Guatemala targeted for mass chemotherapy with ivermectin

If ivermectin distribution programs are to have maximum impact on the morbidity and transmission of human onchocerciasis there must be broad and sustained acceptance within the endemic communities. Educational activities, developed with careful consideration of community attitudes, should promote positive treatment seeking behavior while simultaneously addressing local reservations about the control effort. To better understand the ambient knowledge, attitudes, and practices concerning onchocerciasis in the context of ivermectin use in Guatemala, we conducted a survey among 145 heads of households in five endemic communities. Given the country's long-standing nodulectomy program, it was not surprising that 100% of persons interviewed had heard of the disease ‘ la filaria’, which they defined as a skin nodule that could cause blindness. Ninety-five percent of respondents identified surgery as the only cure for the condition. Relatively few (39%) knew that la filaria was caused by a worm, although slightly more (50%) knew that the condition was acquired by the bite of an insect. The term microfilaria was not broadly recognized. We also determined that onchocerciasis was not perceived as a serious health problem: few persons (12%) mentioned la filaria when requested to provide a complete list of illnesses that occurred in the community, and the gravity of infection (based on rank ordering of common illnesses) was similar to that of a bad cold. Recommendations were made which might assist long-term acceptance of a national chemotherapy initiative against onchocerciasis in Guatemala.

Identification of vector species (Diptera:Simuliidae) of human onchocerciasis in the amazonia focus of Brazil and Venezuela.

The taxonomic status of three Amazonian simuliid species, Simulium guianese Wise, S. oyapockense Floch & Abonnenc and S. yarzabali Ramirez Perez is reviewed. Simulium cuasisanguineum Ramirez Perez, Yarzabal & Peterson is synonymized with S. oyapockense, and S. yarzabali is revalidated from its synonymy with S. incrustatum Lutz. The role of these three species in the transmission of human onchocerciasis and mansonelliasis in Amazonia is reviewed.

Two New Species of Anthropophilic Culicoides from Guatemala (Diptera: Heleidae)

The purpose of this short paper is to make available names for two new species of Culicoides which have been submitted by Dr. Colvin L. Gibson, of the Laboratory of Tropical Diseases, National Institutes of Health, while assigned to the Pan American Sanitary Bureau in Guatemala. These species, along with C. guttatus (Coquillett) and C. paraensis (Goeldi), were collected in the area around San Pedro Yepocapa, Department of Chimaltenango, Guatemala, while biting man, and are the subject of a separate report by Dr. Gibson regarding their possible role in the transmission of human onchocerciasis in Guatemala.