Onchocerciasis Focus Research Articles

Detailed epidemiological mapping of three onchocerciasis foci in West Africa

Detailed epidemiological mapping of three isolated foci of hyperendemic blinding onchocerciasis was undertaken in three West African countries as part of community trials of ivermectin, a new microfilaricide for the treatment of human onchocerciasis. The results show that the geographical distribution of the prevalence and intensity of onchocerciasis infection in the community can be very different from what was expected on the basis of demographic and entomological information. The technique of detailed epidemiological mapping is an important tool for the identification of target populations for large scale ivermectin treatment of onchocerciasis. It is being used extensively in the Onchocerciasis Control Programme in West Africa.

Changes in ocular onchocerciasis after two rounds of community-based ivermectin treatment in a holo-endemic onchocerciasis focus

In a longitudinal study to determine the effect of annual community-based treatment of ocular onchocerciasis with ivermectin, the population living in the 3 most affected villages in the holo-endemic onchocerciasis focus of Asubende in Ghana were re-examined 16 and 24 months after initiating treatment. Ocular microfilarial loads had decreased to very low levels in nearly all of the 334 examined persons who were treated twice. Only very few subjects had ocular loads of 32 microfilariae or more in the anterior chamber of the eye, but this was not associated with deterioration of ocular lesions. Important regression of both early and advanced lesions of the anterior segment of the eye was observed, which was highly statistically significant with respect to iridocyclitis. Lesions of the posterior segment of the eye remained stable. Though no systematic change in the visual acuity of the population was observed, 3 new cases of blindness occurred in persons who already had eye lesions at such an advanced stage that ivermectin treatment could no longer affect the outcome. The results suggest that annual ivermectin treatment is adequate to control onchocercal ocular disease even in populations with very high endemicity levels.

Differences in the larval head patterns and body coloration of members of the Simulium metallicum species complex (Diptera: Simuliidae) from Mexico

AbstractSimulium metallicum Bellardi larvae were collected from a variety of sites in the three onchocerciasis foci in Mexico. Their head patterns and body coloration were scored before the larvae were cytologically analysed. Cytotypes A, B, H and I were collected and in the majority of cases they could be distinguished from each other by larval morphology. An undescribed cytotype (termed cytotype X) was found in Oaxaca which, on the basis of larval morphology, was indistinguishable from cytotype B.

Changes in ocular onchocerciasis four and twelve months after community-based treatment with ivermectin in a holoendemic onchocerciasis focus

The impact of ivermectin mass treatment on ocular onchocerciasis was studied in a holoendemic focus of blinding onchocerciasis in Ghana. A cohort of 417 persons, 369 of whom were treated, was followed up at 4 and 12 months after treatment. The mean ocular microfilarial load in the anterior chamber of the eye and in the cornea of treated persons was reduced to less than 20% and 10% of the pretreatment levels respectively at the 4 months follow-up but had increased significantly by 12 months. Lesions of the eye at the advanced stage of development remained stable. There was significant regression of early lesions of the anterior segment of the eye, particularly iridocyclitis, after ivermectin treatment. In view of the substantial increase of ocular microfilarial loads after 12 months, 6-monthly treatment may be indicated in such highly endemic foci. However, longterm observation is needed to give a correct estimate of the full benefit to be derived from mass treatment with ivermectin.

The onchocerciasis focus at Kinsuka/Kinshasa (Republic of Zaire) in 1985. II. Parasitological and clinical aspects

Since the antivectorial control in 1948, the parasitological and clinical aspects of the onchocerciasis focus at Kinsuka/Kinshasa have not been studied further until now. The population examined was selected for its daily contact with the vector. It constituted 143 adults, including fishermen, stonebreakers and women cultivators, and 26 children aged from 10 to 16 years. People were subjected to an ophthalmological examination as well as to an investigation for Onchocerca volvulus microfilariae. Adults were examined for the presence of nodules as well as for dermal and lymphatic manifestations of onchocerciasis. The microfilaria carrier rate (MfCR) was 59.2%. The mean microfilaria density (MfD) was 9.7 mf mg-1. Only 20% of the adults presented with small onchocercal nodules. Cutaneous lesions were mild. Lymphatic complications such as adenolymphocele, hydrocele and elephantiasis of the lower limbs were observed in 18% of the adults. No one was blind from onchocerciasis, although nine patients of 60 years or over had lost the use of one eye. Punctate keratitis occurred commonly in young people. Ocular lesions, except for punctate keratitis, developed progressively after 55 years and they became worse with the degree of infection, age and length of stay. Hyperendemic and insufferable in 1940, onchocerciasis has become, in 1985, hypoendemic and no longer a public health problem. One could deduce that a situation similar to that in Kinsuka may occur within 30 years in other foci which are today hyperendemic and where prophylaxis has been started.

The onchocerciasis focus at Kinsuka/Kinshasa (Republic of Zaire) in 1985

The entomological aspect of the onchocerciasis focus at Kinsuka/Kinshasa has not been studied since vector control was carried out in 1948. As in 1940, larvae of Simulium damnosum s.l. are located in the arm of the Zaire River flowing between Mimosa Island and the Zairian bank. They are, however, scarce. Cytotaxonomic studies showed that the S. squamosum group was the only member of the S. damnosum complex present in the region of Kinshasa. Physico-chemical analysis of the water at breeding sites determined that S. squamosum larvae develop in neutral or acid water with a conductivity of less than 50 micronhos cm-1 and a low concentration of dissolved salts. Longitudinal study showed that the 'annual biting rate' (ABR) and the 'annual transmission potential' (ATP) fluctuate from 2406-5999 bites man-1 year-1 and from 120-236 infective larvae man-1 year-1 respectively. The low transmission rate at Kinsuka is mainly due to the low level of the biting population occurring during the main river rising. The increase in Kinsuka's population has reinforced this situation by 'diluting' the parasite in the human reservoir.

Biosystematics and distribution of simuliid vectors of human onchocerciasis in South America.

A brief review is given of the taxonomic status, biology and medical importance, and distribution of the vectors of human onchocerciasis in Latin America. Key reference works are cited and distribution maps of each vector species in relation to the known onchocerciasis foci are given.

The status of Simulium oyapockense and S.limbatum as vectors of human onchocerciasis in Brazilian Amazonia

In an attempt to explain the current distribution of onchocerciasis in the forests of northern Brazil (Moraes et al., 1979, 1986), and its potential for dispersal to other areas, this study compares the vector status of Simulium oyapockense Floch and Abonnenc, 1946 in both a hypoendemic and an onchocerciasis free area with that of S. limbatum Knab, 1915 in the latter area. Both species allowed the full development of Onchocerca volvulus (Leuckart) to the infective L3 stage after experimental infection with microfilariae. Their vector competence was significantly lower than for other efficient vector species in South America and Africa because of the lethal effect of the cibarial armature on ingested microfilariae. The low vector capacity of S. oyapockense, together with the low prevalence and intensity of infection of O. volvulus, probably explains why onchocerciasis has not significantly increased in intensity over the last 10 years in the hypoendemic part of the Amazonian focus. Omnipresence of both vector species in the adjacent savanna region, however, could facilitate the spread of onchocerciasis if human population movements continue from the hyperendemic part of the onchocerciasis focus.

Studies on the biology of Simuliidae (Diptera) at the Santiago onchocerciasis focus in Ecuador, with special reference to the vectors and disease transmission

A survey showed the presence of seven simuliid species in the onchocerciasis focus in Ecuador Simulium exiguum and S. quadrivittatum were the two most common anthropophilic species and were shown to be both experimental and natural vectors of Onchocerca volvulus. S. antillarum only occasionally bit man. Observations were made on the biology of the two vector species and it was evident that S. exiguum was the primary vector of onchocerciasis in the rainy season. The relevance of these findings to the epidemiology and control of onchocerciasis in Ecuador are discussed.

Additional data on an inland focus of Venezuelan onchocerciasis.

"Additional data on an inland focus of Venezuelan onchocerciasis." Annals of Tropical Medicine & Parasitology, 76(2), p. 233

Report on a focus of onchocerciasis in Esmeraldas province of Ecuador.

A case suggesting the occurrence of onchocerciasis in Ecuador was first reported in 1952. The appearance of another case in 1977 provided the stimulus to do an epidemiologic survey on an isolated 20-km section of the Cayapa River in Esmeraldas Province. The Mazzotti test, skin biopsy, and nodulectomy were performed on 300 inhabitants of the area. Of the 300 who were tested, 210 (70%) reacted positively to the Mazzotti test, only 20 of whom had a negative skin biopsy. An additional 10 patients with a negative Mazzotti test had a positive skin biopsy. Combining the results of the Mazzotti test and skin biopsy show a 73% prevalence of onchocerciasis. Sixty-four patients (21%) had nodules, and evidence suggests that ocular pathology may be present.

Données sur le cycle d’Onchocerca ochengi chez Simulium damnosum s.l. au Togo

In the focus of human onchocerciasis in Togo (Mono River), the authors collected 250 Simulium damnosum s.l. engorged on a cow parasitized by O. dukei, O. ochengi, O. gutturosa and O. armillata. On 39 Simulium damnosum dissected 24 h after the blood meal, 5 had ingested O. gutturosa, 1 O. armillata, 18 O. dukei and 6 O. ochengi; only for O. ochengi, the microfilariae have reached the hemocoel (3. S. damnosum on 6). From 48 h to 5 days, 6 on 132 S. damnosum contained developing larvae (2 Simulium with young first stage larvae at 48th hour; 1 with young first stage at 3rd day; 1 with first molting at 4th day; 2 with second molting larvae at 5th day). From 6th to 9th days, 2 Simulium on 89 have infective stages. All these larvae cannot be distinguished from those of O. volvulus. A good presumption does exist that O. ochengi can develop in S. damnosum: crossing of the stomach wall only by these microfilariae, presence of larvae of which the development-stage corresponds to the moment of the blood-meal, observation of the second molting five days later. It is difficult to ascertain that the three infective stages observed from 6th to 9th day pertain to O. ochengi, because the presence of a natural (human or animal) infestation; but it appears quite impossible that larvae which can develop to the second molting cannot be transformed in an infective stage.

Onchocerciasis in Colombia

Ophthalmic examinations were made on 292 inhabitants of an area in Colombia where a small focus of onchocerciasis was discovered. Ocular lesions were found in 15 (34.1%) of 44 persons with proven onchocerciasis; 1 had superficial punctate stromal keratitis, 4 had sclerosing keratitis, and 2 had active anterior uveitis, all of which were probably caused by the filarial infection. Five of these persons were blind and in three the blindness was apparently due to inflammatory lesions produced by the parasite. Microfilariae were observed in the anterior chamber of the eye in 5 persons. Of the 248 persons without onchocerciasis, 51 (20.6%) had ocular lesions and 15 (6.0%) were blind, trauma being the chief cause of blindness. On the basis of the small number of ocular lesions attributable to the parasite and the absence of severe onchocercal skin lesions, the infections in this Colombian focus seem relatively mild. This is apparently due to the low intensity of the infections as indicated by the small number of microfilariae in the skin and the small number of onchocercomata.

Onchocerciasis in Colombia

Abstract Ophthalmic examinations were made on 292 inhabitants of an area in Colombia where a small focus of onchocerciasis was discovered. Ocular lesions were found in 15 (34.1%) of 44 persons with proven onchocerciasis; 1 had superficial punctate stromal keratitis, 4 had sclerosing keratitis, and 2 had active anterior uveitis, all of which were probably caused by the filarial infection. Five of these persons were blind and in three the blindness was apparently due to inflammatory lesions produced by the parasite. Microfilariae were observed in the anterior chamber of the eye in 5 persons. Of the 248 persons without onchocerciasis, 51 (20.6%) had ocular lesions and 15 (6.0%) were blind, trauma being the chief cause of blindness. On the basis of the small number of ocular lesions attributable to the parasite and the absence of severe onchocercal skin lesions, the infections in this Colombian focus seem relatively mild. This is apparently due to the low intensity of the infections as indicated by the small number of microfilariae in the skin and the small number of onchocercomata.

The localization of onchocercomata

On the basis of a detailed investigation of a focus of neavei-transmitted onchocerciasis in the Belgian Congo, together with some considerations derived from veterinary onchocerciasis, it is concluded that the localization of onchocercomata is determined by the site of inoculation of the larval filariae and by the minute anatomy of the dermis. The migrating filariae are directed along dermal planes in flowing tissue fluid towards bony prominences, where they are arrested by intermittent or long-continued pressure.

Observations on Simulium neavei Roubaud, with special reference to a focus of onchocerciasis in the Belgian Congo

Parts of a district situated in the tropical rain-forest of the Belgian Congo in which onchocerciasis was generally endemic, were investigated to identify the vector and to ascertain its phoretic host, breeding places, habits, etc.The vector found biting human bait (usually less than 1 metre from the ground) was Simulium neavei Eoub. Dissection of adult females revealed developmental forms of Onchocerca volvulus.Larvae and pupae were found on crabs of the genus Potamon in fast-flowing stretches of the numerous small streams flowing into the Eiver Congo. The infested streams in the main focus of onchocerciasis were treated by DDT emulsion at a concentration of one part per 500,000 for 30 minutes, every ten days for ten applications.The good results observed initially were partly offset by the reappearance six months later of small numbers of Simuliids apparently developing from eggs laid by females entering the disinfested area from without.

Studies on Human Onchocerciasis and Simulium in Nyanza Province, Kenya

1. An experiment in bush-clearing was carried out at a small endemic focus of onchocerciasis in South Kavirondo district with a view to observing its effect on the adult population of S. neavei.2. The clearing consisted of the complete removal of undergrowth and partial removal of trees, from the banks of fly-infested portions of two confluent rivers, the Riana and the Yabe.3. In the first year of the experiment the clearing was carried out on the infested section of the Riana River, after which there was a decrease in the incidence of S. neavei on this section compared with that on the uncleared Yabe River.4. In the second year a similar clearing was effected on the infested section of the Yabe River, after which the S. neavei incidence on both sections came together again and during the third year showed a steady increase.

A new focus of onchocerciasis occurring in Kenya colony

1. 1. A new focus of onchocerciasis is described around Kakamega, in the western part of Kenya Colony. Among fifty-three natives, taken at random, the infestation was found to occur in twenty, i.e., 38 per cent. 2. 2. Eye-lesions appeared to be common in the district, although it is not yet determined how far these are due to the onchocerciasis. 3. 3. Subcutaneous nodules were relatively uncommon. 4. 4. Eosinophilia was more marked in cases with onchocerciasis (average percentage of eosinophils, 15·2) than in those uninfested (average, 9·7); but the association was not sufficiently regular to have much diagnostic significance.