After a decade of annual dose mass ivermectin treatment in Cameroon and Uganda, onchocerciasis transmission continues

Objective To evaluate the effectiveness of 10 years’ annual single dose ivermectin treatment on onchocerciasis transmission in hyperendemic areas of Cameroon and Uganda. Methods Baseline nodule and microfilaria (‘skin snip’) prevalence data were available from 10 hyperendemic sentinel communities in Cameroon (from 1996) and hyperendemic 20 sentinel communities in Uganda (from 1993). We returned to these villages in 2005, 10 months after the last annual ivermectin distribution, to repeat the cross‐sectional surveys. Each sentinel community reported a mean interval treatment coverage of eligible persons of >88% (range 37–100%). Data were analyzed for more than 6200 person examinations. In Cameroon, 719 people ≥10 years were examined at the baseline survey in 1996 and 838 at the follow‐up survey in 2005. In Uganda, 1590 people ≥10 years were examined at the baseline survey in 1993 and 2122 people at the follow‐up survey in 2005. We also examined children under 10 in Cameroon (1996, n = 185; 2005, n = 448) and Uganda (1993, n = 177; 2005, n = 130). In Uganda, the vitality of worms was judged using standard histological criteria in 80 nodules excised in 2005. Results The prevalence of microfilaria carriers among older children and adults (≥10 years) in Cameroon sentinel communities dropped from 70.1% to 7.04% (P < 0.0001) over the 10‐year treatment period; that of nodule carriers from 58% to 9.55% (P < 0.0001). Similarly, in Uganda, the prevalence of microfilaria carriers fell from 71.9% to 7.49% (P < 0.0001) over the 13‐year treatment period, and that of nodule carriers from 53.21% to 9.66% (P < 0.0001). The number of microfilaria carriers among children <10 years in Cameroon decreased from 29.73% to 3.8% (P < 0.0001), and in Uganda from 33.89% to 3.1% (P < 0.0001). In 2005, worms excised from nodules in Uganda, 81.4% of males remained alive, and 64% of females, with 24% of them inseminated. Conclusion A decade or more of annual single dose ivermectin treatment in hyperendemic areas has reduced onchocerciasis to ‘hypoendemicity’, but onchocerciasis transmission persists. For now, annual treatment with ivermectin should be continued in formerly mesoendemic and hyperendemic zones.     

The Simulium damnosum complex in western Uganda and its role as a vector of Onchocerca volvulus

The status of onchocerciasis vectors in the former Ruwenzori focus in western Uganda was re-examined some 15 years after control measures against Simulium damnosum s.l. were suspended. The four cytoforms S. kilibanum, 'Sebwe', 'Nkusi' and S. pandanophilum were found. While the nonanthropophilic 'Sebwe' was still widely distributed in rivers north, east and south of the Ruwenzori, the only anthropophilic species and vector, S. kilibanum, had disappeared from most of its former habitats and was now restricted to two limited foci, where high biting densities were encountered. It was still a vector south of the Ruwenzori (Kasese focus), where 15.4% of the parous flies were infected with larval stages of Onchocerca volvulus and 34 infective larvae were found in the heads of 1000 parous flies. In the second focus along the Mahoma and Nsonge rivers, a chromosomally highly polymorphic population of S. kilibanum had replaced the former vector S. neavei, but does not act as a vector. Only 2.3% of the parous females were infected and just 1 infective larva was found in the heads of 1000 parous flies.     

Community-directed treatment of onchocerciasis with ivermectin in Takum, Nigeria

A study to identify factors within the community that can ensure sustainable community-directed treatment (ComDT) with ivermectin compared the effectiveness of programme-designed (PD) and community-designed (CD) strategies in 37 villages in the Takum area of Nigeria. In a subset of PD villages, designated PD1, communities were asked to use the village heads as community-directed distributors (CDD), and the other communities (PD2) were asked to select female distributors, and both were instructed to use the house-to-house method of distribution. Community-designed communities, on the other hand, were asked to design their own approach. All study communities received health education, treatment guidelines, and training enabling them to determine appropriate dosage. A total of 1744 people were interviewed about their experiences after two treatment cycles. Communities preferred honest, reliable community members as CDDs, but few women were selected. The results show striking similarity between PD and CD villages in many respects. In the PD1 villages, where the programme designated the village head as CDD, the mode of distribution was changed from house-to-house to central point, and distribution took place in the compound of the village head. In the PD2 villages, where the programme specified distributors should be women, the women who were selected were replaced by their male children. These changes to the original design were consistent with the local cultural norms and made the arrangement for distribution more acceptable to the people. Programme-designed villages that used the village head as distributors performed better than those that used women, and the coverage in the former group compares well with that of CD villages. Only five villages achieved coverage > 60%, but dosage was correct in most cases (87.4%). Drug shortage was the most frequent reason for non-treatment. Communities devised means for ensuring equity and fairness in sharing their limited supply and freely altered the original designs to fit local norms and values. These changes to the original design were consistent with local norms and were acceptable to the people. The success of this strategy should be tested in other parts of Nigeria. Long-term success of ComDT, however, requires a reliable drug supply and inputs from professionals in the health system for minimal supervision. The core issues that determine sustainability of ComDT appear to be not so much in the structure, but in the process by which they are introduced. Communities will only sustain a programme where the process of implementation fits well with local norms and where communities are free to alter PD procedures that are inconsistent with local customs.     

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.     

Variation in incidence of serious adverse events after onchocerciasis treatment with ivermectin in areas of Cameroon co-endemic for loiasis

Summary Objective To determine the incidence of serious adverse events (SAEs) after mass treatment with ivermectin in areas co-endemic for loiasis and onchocerciasis, and to identify potential risk factors associated with the development of these SAEs, in particular encephalopathic SAEs. Methods We retrospectively analysed SAEs reported to have occurred between 1 December 1998 and 30 November 1999 in central-southern Cameroon by chart review, interview and examination of a subset of patients. Results The overall incidence of SAEs for the three provinces studied was 6 per 100 000. However, for Central Province alone the incidence of SAEs was 2.7 per 10 000 overall, and 1.9 per 10 000 for encephalopathic SAEs associated with Loa loa microfilaremia (PLERM). The corresponding rates for the most severely affected district within Central Province (Okola) were 10.5 per 10 000 and 9.2 per 10 000 respectively. Symptoms began within the first 24–48 h of ivermectin administration but there was a delay of approximately 48–84 h in seeking help after the onset of symptoms. First-time exposure to ivermectin was associated with development of PLERM. Conclusion In Cameroon, the incidence of SAEs following ivermectin administration in general, and PLERM cases in particular, varies substantially by district within the areas co-endemic for loiasis and onchocerciasis. More intense surveillance and monitoring in the first 2 days after mass distribution in ivermectin-naïve populations would assist in early recognition, referral and management of these cases. The increased reporting of SAEs from Okola is unexpected and warrants further investigation. Research is urgently needed to find a reliable screening tool to exclude individuals (rather than communities) at risk of PLERM from the mass treatment program.     

Integrated community-directed treatment for the control of onchocerciasis, schistosomiasis and intestinal helminths infections in Uganda: advantages and disadvantages

Onchocerciasis is co-endemic with schistosomiasis and intestinal helminths infections, which are all diseases of the rural and the poorest communities in Africa. Community-directed treatment (ComDT) for the control of onchocerciasis is the only functional health approach in most of these communities and the strategy has proven to be effective for onchocerciasis control. This study was conducted to assess the feasibility of integrating ComDT with ivermectin for the control of onchocerciasis, and with praziquantel (PZQ) and mebendazole (MBD) for the control of schistosomiasis and intestinal helminths infections in children aged 5-14 years, and to assess advantages and disadvantages of the integrated ComDT over the routine ComDT and the school-based treatment approach. Integrated ComDT achieved higher treatment coverage (85%) for PZQ and MBD than the school-based treatment approach (79%) among children aged 5-14 years (P = 0.03). There were more reported adverse reactions after treatment with a combination of PZQ and MBD in the school-based treatment approach (33%) than for the combination of ivermectin and MBD on day 1 and PZQ on day 2 in the integrated ComDT (18%). However, all adverse reactions were mild (headache, nausea/vomiting and abdominal pain). The integrated ComDT also achieved higher ivermectin treatment coverage for all ages (81.3%) than routine ComDT (77.2%) (P = 0.0003). To achieve even better coverage for PZQ and MBD among the targeted high risk groups, integrated ComDT should treat all age groups in areas where the prevalence of schistosomiasis and intestinal helminths infections is >50%. This would minimize the shortage of the drugs targeted to treat the high risk groups, as the non-targeted groups, will inevitably demand and receive the treatment from the distributors. The results of this study show that PZQ and MBD treatment for the control of schistosomiasis and intestinal helminths, respectively, can be integrated with ivermectin treatment for the control of onchocerciasis without negatively affecting ivermectin treatment coverage.     

Feasibility of measuring compliance to annual ivermectin treatment in the African Programme for Onchocerciasis Control

The African Programme for Onchocerciasis Control (APOC) sponsors annual distribution of ivermectin to control onchocerciasis. Ivermectin should be taken annually by 65% of community members for a number of years to eliminate the disease as a public health problem. While many community coverage surveys have been undertaken during project monitoring, individual compliance could not be studied until several annual rounds of distribution had occurred. This paper reports on the efforts to determine whether adequate records could be found to enable a compliance study. A step-down process from project to district to community level was used to identify project sites where continuous ivermectin distribution up through 2004 had occurred. The first step consisted of selecting 17 of 25 projects by APOC. The second step determined adequacy of districts where distribution had occurred on a regular annual basis. Among 121 districts 58.6% undertook distribution in all 7 years. A total 852 villages were visited and community level records were found in all but three. Records showed that distribution had occurred for a minimum of five consecutive times in 429 villages, and ultimately 10 projects. While the feasibility study found an adequate number of villages to study compliance, the large number of projects, districts and villages that did not qualify for the follow-on compliance study should lead National Onchocerciasis Control Programme managers to strengthen the overall coverage and consistency of their efforts.     

A community trial of ivermectin for onchocerciasis in Sierra Leone: compliance and parasitological profiles after three and a half years of intervention

We have studied the compliance patterns and the long‐term effects of repeated ivermectin at various dosing intervals in a randomized controlled trial. The setting for the trial was six neighbouring communities hyperendemic for onchocerciasis in southern Sierra Leone. A total of 335 subjects attended a survey 18 months after the fifth treatment round. Of those randomized to ivermectin, over 85% had received at least three doses. There was no evidence that women of childbearing age were consistently under‐treated, despite the criteria for exclusion from treatment. An intention‐to‐treat analysis showed that a 6‐monthly ivermectin treatment regime satisfactorily suppressed microfilarial loads. Microfilarial repopulation was significantly slower over an 18‐month period after multiple doses compared to a single dose. Further analysis of microfilarial repopulation suggests that there is a cumulative suppressive effect after at least the first three doses of ivermectin, and that an annual treatment interval is as effective for short‐term microfilarial suppression as a 6‐monthly interval.     

Cytogenetic and PCR-based identification of S. damnosum "Nkusi J" as the anthropophilic blackfly in the Uluguru onchocerciasis focus in Tanzania

A combined molecular and cytogenetic analysis was used to identify the local blackfly vectors of onchocerciasis in the Uluguru Mts. focus, Morogoro region, central Tanzania. Cytotaxonomic identification revealed the occurrence of three cytospecies of the Simulium damnosum complex, along altitudinal gradients. The cytoform "Nkusi" was found breeding at low to mid-altitudes (100-500 m), while S. kilibanum was found at mid- and high altitudes (300-1260 m), being sympatric with "Sebwe" only in the highlands. The analyses further revealed the cytoform "Nkusi" from the nearby Kilosa focus. Simulium kilibanum and "Nkusi" are potential vectors of onchocerciasis, while the "Sebwe" form is considered non-anthropophilic and therefore not a vector. By means of molecular typing of wild-caught females, "Nkusi" was incriminated as the man-biting fly at all catching sites, and consequently, is the assumed vector of onchocerciasis over the full altitude range. In the high altitude zones, however, S. neavei group species may act as secondary vectors at the edge of the forest reserve, if transmission actually occurs there at all. Our results also suggest that vector control could be feasible because of the vector species breeding in the foothills below 500 m in relatively accessible open country.     

The elimination of the onchocerciasis vector from the island of Bioko as a result of larviciding by the WHO African Programme for Onchocerciasis Control

The island of Bioko is part of the Republic of Equatorial Guinea and is the only island in the World to have endemic onchocerciasis. The disease is hyperendemic and shows a forest-type epidemiology with low levels of blindness and high levels of skin disease, and the whole population of 68,000 is estimated to be at risk. Control of onchocerciasis began in 1990 using ivermectin and this yielded significant clinical benefits but transmission was not interrupted. Feasibility and preparatory studies carried out between 1995 and 2002 confirmed the probable isolation of the vector on the island, the high vectorial efficiency of the Bioko form of Simulium yahense, the seasonality of river flow, blackfly breeding and biting densities, and the distribution of the vector breeding sites. It was proposed that larviciding should be carried out from January to April, when most of the island's rivers were dry or too low to support Simulium damnosum s.l., and that most rivers would not need to be treated above 500 m altitude because they were too small to support the breeding of S. damnosum s.l. Larviciding (with temephos) would need to be carried out by helicopter (because of problems of access by land), supplemented by ground-based delivery. Insecticide susceptibility trials showed that the Bioko form was highly susceptible to temephos, and insecticide carry was tested in the rivers by assessing the length of river in which S. damnosum s.l. larvae were killed below a temephos dosing point. Regular fly catching points were established in 1999 to provide pre-control biting densities, and to act as monitoring points for control efforts. An environmental impact assessment concluded that the proposed control programme could be expected to do little damage, and a large-scale larviciding trial using ground-based applications of temephos (Abate 20EC) throughout the northern (accessible) part of the island was carried out for five weeks from 12 February 2001. Following this, a first attempt to eliminate the vectors was conducted using helicopter and ground-based applications of temephos from February to May 2003, but this was not successful because some vector populations persisted and subsequently spread throughout the island. A second attempt from January to May 2005 aimed to treat all flowing watercourses and greatly increased the number of treatment points. This led to the successful elimination of the vector. The last biting S. damnosum s.l. was caught in March 2005 and none have been found since then for more than 3 years.     

Simultaneous administration of praziquantel, ivermectin and albendazole, in a community in rural northern Ghana endemic for schistosomiasis, onchocerciasis and lymphatic filariasis

Objectives To compare (i) side effects associated with the simultaneous adminstration of praziquantel, albendazole and ivermectin with side affects associated with albendazole and ivermectin only and (ii) coverage by volunteers distributing three or two drugs. Methods Two‐arm comparative study in northern Ghana integrated praziquantel distribution into an existing lymphatic filariasis and onchocerciasis control programme using Community Directed Distributors. The control arm continued to distribute only ivermectin and albendazole. Dosages of ivermectin and praziquantel were based on height. Treatment was directly observed, and all two/three drugs were co‐administered. Adverse effects were recorded based on passive surveillance. Parasitological, anthropometric and haematological data were collected at baseline. Results Prevalence of Schistosoma haematobium infection among 1001 (boys: 47.9% girls: 52.1%) school‐age children (6–15 years) [intervention: 30.0% (CI: 23.1–34.2); control: 23.0% (CI: 18.9–27.0)], mean haemoglobin, weight and age were similar among the intervention and control groups. While 1676 (99.1%) compounds in the control area were visited and 15 020 (96.58%) people were treated, only 1375 (88.5%) compounds in the intervention area were visited and 8454 (80.97%) people treated (P < 0.001). The numbers of adverse effects were similar (intervention: 50/6896; control: 130/15 020). The most reported adverse effects was headache (intervention: 14/50; control: 13/130), followed by body weakness, which was reported more from the intervention group (intervention: 13/50, 95% CI: 14.6–40.3; control: 6/130, 95% CI: 1.7–9.8]. Sixty‐six per cent (6896/10 441) of the eligible population received praziquantel. Conclusions Reported adverse events were mild and managed at the subdistrict level with no cases of hospitalization; intensive health education will, however, be required to improve coverage.     

A health club for a community school in south-eastern Nigeria: influence on adult perception of onchocerciasis and compliance with community-based ivermectin therapy

The impact of a school health club on adult perception of onchocerciasis and compliance with ivermectin was evaluated in an onchocerciasis-endemic community in southeastern Nigeria. Venous blood was collected from each of 26, 32 and 124 randomly selected subjects during ivermectin distribution programmes in 1995 1996 and 1997 respectively. Ivermectin concentrations were measured in the samples. Data was also collected from 334 and 319 randomly selected household heads or their representatives (aged 24 to 65 years) before and after health talks by schoolchildren, using interviewer-administered questionnaires. There was an increasing number of subjects who participated in control programmes (116 in 1995, 437 in 1996 and 2055 in 1997). Compliance with ivermectin treatment was low (53.9%) in 1995 but increased dramatically (90.1%) in 1997. A significant proportion (chi2 = 108.7, df = 1, P < 0.0001) of respondents knew about onchocerciasis after health education, predominantly from health workers (64.5%) before the tests and children (92.3%) after. Knowledge and beliefs about causative agents (chi2 = 266.4, df = 5, P < 0.0001), diagnostic method (chi2 = 207.4, df = 3, P < 0.0001), prevention (chi2 = 67.0, df = 4, P < 0.0001) of onchocerciasis and the effectiveness of ivermectin (chi2 = 40.4, df = 1, P < 0.0001) also differed significantly between the periods before and after tests. The school health club increased adult knowledge about onchocerciasis and its treatment. Schoolchildren could therefore supplement the information, education and communication (IEC) aspect of health care delivery in a community through such health clubs.     

Profile of eye lesions and vision loss: a cross-sectional study in Lusambo,a forest-savanna area hyperendemic for onchocerciasis in the Democratic Republic of Congo

OBJECTIVE: To determine baseline data regarding eye lesions and vision loss in five villages of Lusambo, an onchocerciasis-hyperendemic forest-savanna area in the Democratic Republic of Congo (DRC), in preparation of mass ivermectin distribution. METHODS: Five villages were selected by simple randomization. Through a cross-sectional design, 750 subjects were examined ophthalmologically. The eye examination included acuity visual measurement, slit-lamp examination, ophthalmoscopy, intraocular pressure measurement, and visual field assessment by the Wu-Jones test. RESULTS: There was a high prevalence of onchocerciasis-related eye lesions compared with non-onchocercal lesions. Chorioretinitis (20%) was the most frequent disease, others were punctate keratitis and microfilariae in the anterior chamber in equal frequency (13.8%), white intraretinal deposits (10.4%) and iridocyclitis (8%). Vision loss was discovered in 8.5% of the subjects, of whom 0.5% had bilateral blindness, 2.2% had monocular blindness and 5.7% had visual impairment. Vision loss was mostly caused by onchocerciasis-related diseases, especially those affecting the anterior segment of the eye. CONCLUSION: Features of ocular onchocerciasis usually described in forest and savanna areas were both found in this forest-savanna zone of the DRC.     

pOVEX vector: prokaryotic expression and purification of onchocerciasis vaccine candidate antigens as fusion proteins with the 24 kD Onchocerca volvulus glutathione S-transferase

Summary An expression vector, pOVEX, has been designed and constructed, combining the advantages of the expression vectors pGEX-3X and pJC20. The pOVEX vector produces a fusion protein with the 24kD Onchocerca volvulus glutathione S-transferase (OvGST2) which is easy to purify in one step from bacterial extracts under non-denaturing conditions using glutathione-sepharose chromatography. High yields of fusion protein were produced from this T7 RNA polymerase-dependent expression vector, which were then cleaved by digestion with the factor Xa protease to separate the OVGST2 polypeptide from the expressed protein of interest. This vector will be particularly useful to O. volvulus investigators for the production of O. volvulus antigens for the analyses of host humoural and cellular responses to these proteins and for immunization studies.     

Detection of exhaled CO as a simple non-invasive tool for monitoring acute exacerbations of asthma in the elderly

Exhaled carbon monoxide (CO) concentrations were measured on a CO monitor by vital capacity maneuvers in asthmatic patients either receiving or not receiving inhaled corticosteroids, and in nonsmoking healthy control subjects. CO was detectable and measured reproducibly in the exhaled air of all subjects. The exhaled CO concentrations were higher in asthmatic patients not receiving inhaled corticosteroids and similar in asthmatic patients receiving inhaled corticosteroids and nonsmoking healthy control subjects (Am J Respir Crit Care Med 1997: 156: 1140-1143). All patients with inhaled corticosteroid treatment had reductions in exhaled CO concentration and eosinophil cell counts in sputum that were accompanied by an amelioration of airway obstruction. These results showed that detection of exhaled CO could be a simple non-invasive tool for monitoring airway inflammation and acute exacerbation of asthma.     

Assessment of a national monitoring and evaluation system for rapid expansion of antiretroviral treatment in Malawi

OBJECTIVES: Monitoring and evaluation of national antiretroviral therapy (ART) programs is vital, but routine, standardized assessment of national ART patient monitoring systems has not been established. Malawi has undertaken an ambitious ART scale-up effort, with over 57,000 patients initiated on ART by June 2006. We assessed the national ART monitoring and evaluation system in Malawi to ensure that the response to the epidemic was being monitored efficiently and effectively, and that data collected were useful. METHODS: The evaluation, performed in August 2005, generally followed the Updated Guidelines for Evaluating Public Health Surveillance Systems (CDC) and Interim Patient Monitoring Guidelines for HIV Care and ART (WHO). Assessment was conducted with qualitative methods, including twelve ART site visits, with standardized key informant interviews with ART clinic coordinators, clinical staff, and data managers, at each site. Meetings were also held with key governmental stakeholders, including Ministry of Health and National AIDS Commission. RESULTS: The national monitoring and evaluation system devised by the Ministry of Health HIV/AIDS Unit is successful in achieving its objectives, and facilitates important aspects of the national response to HIV. Several basic changes in the data collection tools and system would facilitate more effective long-term assessment of the ART program and support improved patient care. As the number of ART sites and patients continues to expand, the current manual paper-based system may be overwhelmed. Identification and implementation of a feasible electronic data system that would maintain and improve data quality and the efficiency of data recording and reporting and enhance patient care is a priority. CONCLUSIONS: The assessment of ART monitoring and evaluation systems can optimize the effectiveness of national ART programs, and should be considered in other resource-constrained countries rapidly scaling up ART.     

The importance of utilizing 24-h Holter monitoring as a non-invasive method of predicting the mechanism of supraventricular tachycardia

Despite the emergence of advanced invasive technology in identifying the various types of arrhythmia mechanisms, 24-h ambulatory electrocardiogram monitoring as a non-invasive method remains an invaluable informative tool in delineating such mechanisms. Furthermore, one observational study has supported the utilization of 24-h Holter monitoring in exploring AV Node (AVN) characteristics sufficiently in correlation with invasive studies when limited to patients without Wolf Parkinson White syndrome showing a positive predictive value of 98% in their supraventricular tachycardia (SVT) assessment (Fukuda et al., 2005).We describe in this report suspected tachycardia initiation mechanism in three SVT cases based on 24-h Holter recordings. Premature atrial contraction with subsequent AVN fast pathway conduction block initiated the common type AVN re-entrant tachycardia (AVNRT). Dual AVN physiology was documented during the electrophysiological studies in all three cases and a definitive therapy was achieved by the AVN slow pathway modification.     

Vector competence of Simulium oyapockense s.l. and S. incrustatum for Onchocerca volvulus: Implications for ivermectin-based control in the Amazonian focus of human onchocerciasis, a multi-vector-host system

Although it is now well established that in the Amazonian onchocerciasis focus, straddling between Venezuela and Brazil, the main vectors in the highland (hyperendemic) and lowland (hypoendemic) areas, are respectively Simulium guianense sensu lato Wise and S. oyapockense s.l. Floch and Abonnenc, investigation of the vectorial role of a third anthropophagic species, Simulium incrustatum Lutz has remained inconclusive. Here we compare the vector competence of S. incrustatum with that of S. oyapockense s.l. by conducting, in the Venezuelan part of the focus, a series of feeding experiments designed to analyze their relative: (a) microfilarial intakes when fed upon the same skin load; (b) proportions of microfilariae (mf) surviving damage inflicted by the cibarial armature (present in both species); and (c) infective (L3) larval outputs. Although the ability of S. oyapockense s.l. to ingest mf, for a given microfilaridermia, was markedly higher than that of S. incrustatum, the (density-dependent) proportions of those ingested mf that were damaged by the armature were also consistently higher, with the resulting output of L3 larvae being significantly lower in S. oyapockense s.l. than in S. incrustatum. These results indicate that S. incrustatum plays a more important role in onchocerciasis transmission in the Amazonian focus than previously realized. We discuss the implications of our findings for the control and elimination of onchocerciasis with mass administration of ivermectin in this focus, where the three main anthropophagic species often co-occur.