A study of onchocerciasis with severe skin and eye lesions in a hyperendemic zone in the forest of southwestern Cameroon: clinical, parasitologic, and entomologic findings

Why onchocerciasis transmission persists after 15 annual ivermectin mass drug administrations in South-West Cameroon

Introduction

Onchocerciasis is targeted for elimination mainly with annual community-directed treatment with ivermectin (CDTI). High infection levels have been reported in South-West Cameroon, despite ≥15 years of CDTI. The aim of this study was to assess factors associated with continued onchocerciasis transmission and skin disease.

Methods

A large-scale cross-sectional study was conducted in 2017 in 20 communities in a loiasis-risk area in South-West Cameroon. A mixed-methods approach was used. Associations between infection levels, skin disease and adherence to CDTI were assessed using mixed regression modelling. Different community members’ perception and acceptability of the CDTI strategy was explored using semi-structured interviews.

Results

Onchocerciasis prevalence was 44.4% among 9456 participants. 17.5% of adults were systematic non-adherers and 5.9% participated in ≥75% of CDTI rounds. Skin disease affected 1/10 participants, including children. Increasing self-reported adherence to CDTI was associated with lower infection levels in participants aged ≥15 years but not in children. Adherence to CDTI was positively influenced by perceived health benefits, and negatively influenced by fear of adverse events linked with economic loss. Concern of lethal adverse events was a common reason for systematic non-adherence.

Conclusion

CDTI alone is unlikely to achieve elimination in those high transmission areas where low participation is commonly associated with the fear of adverse events, despite the current quasi absence of high-risk levels of loiasis. Such persisting historical memories and fear of ivermectin might impact adherence to CDTI also in areas with historical presence but current absence of loiasis. Because such issues are unlikely to be tackled by CDTI adaptive measures, alternative strategies are needed for onchocerciasis elimination where negative perception of ivermectin is an entrenched barrier to community participation in programmes.

Comparison of Diagnostic Tests for Onchocerca volvulus in the Democratic Republic of Congo

Onchocerciasis is diagnosed by detecting microfilariae in skin snips or by detecting OV16 IgG4 antibodies in blood by either enzyme linked immunosorbent assay (ELISA) or a rapid diagnostic test (RDT). Here, we compare the sensitivity and specificity of these three tests in persons with epilepsy living in an onchocerciasis endemic region in the Democratic Republic of Congo. Skin snips and blood samples were collected from 285 individuals for onchocerciasis diagnosis. Three tests were performed: the OV16 RDT (SD Bioline) and the OV16 ELISA both on serum samples, and microscopic detection of microfilariae in skin snips. The sensitivity and specificity of each test was calculated with the combined other tests as a reference. Microfilariae were present in 105 (36.8%) individuals, with a median of 18.5 (6.5–72.0) microfilariae/skin snip. The OV16 RDT and OV16 ELISA were positive in, respectively, 112 (39.3%) and 143 (50.2%) individuals. The OV16 ELISA had the highest sensitivity among the three tests (83%), followed by the OV16 RDT (74.8%) and the skin snip (71.4%). The OV16 RDT had a higher specificity (98.6%) compared to the OV16 ELISA (84.8%). Our study confirms the need to develop more sensitive tests to ensure the accurate detection of ongoing transmission before stopping elimination efforts.

Relationship between oral declaration on adherence to ivermectin treatment and parasitological indicators of onchocerciasis in an area of persistent transmission despite a decade of mass drug administration in Cameroon

Background

Onchocerciasis control for years has been based on mass drug administration (MDA) with ivermectin (IVM). Adherence to IVM repeated treatment has recently been shown to be a confounding factor for onchocerciasis elimination precisely in rain forest areas where transmission continues and Loa loa co-exists with Onchocerca volvulus. In this study, participants’ oral declarations were used as proxy to determine the relationship between adherence to IVM treatment and parasitological indicators of onchocerciasis in the rain forest area of Cameroon with more than a decade of MDA.

Methods

Participants were recruited based on their IVM intake profile with the aid of a semi-structured questionnaire. Parasitological examinations (skin sniping and nodule palpation) were done on eligible candidates. Parasitological indicators were calculated and correlated to IVM intake profile.

Results

Of 2,364 people examined, 15.5 % had never taken IVM. The majority (40.4 %) had taken the drug 1–3 times while only 18 % had taken ≥ 7 times. Mf and nodule prevalence rates were still high at 47 %, 95 % CI [44.9–49.0 %] and 36.4 %, 95 % CI [34.4–38.3 %] respectively. There was a treatment-dependent reduction in microfilaria prevalence (r_s =−0.986, P = 0.01) and intensity (r_s =−0.96, P = 0.01). The highest mf prevalence (59.7 %) was found in the zero treatment group and the lowest (33.9 %) in the ≥ 7 times treatment group (OR = 2.8; 95 % CI [2.09–3.74]; P < 0.001). Adults with ≥ 7 times IVM intake were 2.99 times more likely to have individuals with no microfilaria compared to the zero treatment group (OR = 2.99; 95 % CI [2.19–4.08], P < 0.0001). There was no clear correlation between treatment and nodule prevalence and intensity.

Conclusion

Adherence to ivermectin treatment is not adequate in this rain forest area where L. loa co-exists with O. volvulus. The prevalence and intensity of onchocerciasis remained high in individuals with zero IVM intake after more than a decade of MDA. Our findings show that using parasitological indicators, reduction in prevalence is IVM intake-dependent and that participants’ oral declaration of treatment adherence could be relied upon for impact studies. The findings are discussed in the context of challenges for the elimination of onchocerciasis in this rain forest area.

Situation analysis of parasitological and entomological indices of onchocerciasis transmission in three drainage basins of the rain forest of South West Cameroon after a decade of ivermectin treatment

BACKGROUND

Community-Directed Treatment with Ivermectin (CDTI) is the main strategy adopted by the African Programme for Onchocerciasis control (APOC). Recent reports from onchocerciasis endemic areas of savannah zones have demonstrated the feasibility of disease elimination through CDTI. Such information is lacking in rain forest zones. In this study, we investigated the parasitological and entomological indices of onchocerciasis transmission in three drainage basins in the rain forest area of Cameroon [after over a decade of CDTI]. River basins differed in terms of river number and their flow rates; and were characterized by high pre-control prevalence rates (60-98%).

METHODS

Nodule palpation and skin snipping were carried out in the study communities to determine the nodule rates, microfilarial prevalences and intensity. Simulium flies were caught at capture points and dissected to determine the biting, parous, infection and infective rates and the transmission potential.

RESULTS

The highest mean microfilaria (mf) prevalence was recorded in the Meme (52.7%), followed by Mungo (41.0%) and Manyu drainage basin (33.0%). The same trend was seen with nodule prevalence between the drainage basins. Twenty-three (23/39) communities (among which 13 in the Meme) still had mf prevalence above 40%. All the communities surveyed had community microfilarial loads (CMFL) below 10 mf/skin snip (ss). The infection was more intense in the Mungo and Meme. The intensity of infection was still high in younger individuals and children less than 10 years of age. Transmission potentials as high as 1211.7 infective larvae/person/month were found in some of the study communities. Entomological indices followed the same trend as the parasitological indices in the three river basins with the Meme having the highest values.

CONCLUSION

When compared with pre-control data, results of the present study show that after over a decade of CDTI, the burden of onchocerciasis has reduced. However, transmission is still going on in this study site where loiasis and onchocerciasis are co-endemic and where ecological factors strongly favour the onchocerciasis transmission. The possible reasons for this persistent and differential transmission despite over a decade of control efforts using ivermectin are discussed.

Experimental studies on the transmission of Onchocerca volvulus by its vector in the Sanaga valley (Cameroon): Simulium squamosum B. Intake of microfilariae and their migration to the haemocoel of the vector

As large-scale ivermectin distribution is becoming the mainstay of onchocerciasis control in Africa, the issue of its impact on local transmission is increasing in importance. The vector competence of Simulium squamosum B in the severe focus of the Sanaga valley, Cameroon, was therefore investigated, by feeding 1320 flies on 14 carriers of Onchocerca volvulus microfilariae (mff). The results enabled the relationships between skin microfilarial load, microfilarial intake by the flies, the proportion and mean number of ingested mff that succeed in reaching the fly's haemocoel, and the frequency distribution of the ingested mff to be described, as functions of time post-engorgement (p.e.) and parasite density (while taking account of possible measurement error in the predictor variable). The proportion of flies with haemocoelic mff and the mean number of mff/fly increased up to 3 h p.e. The proportion of flies with ingested mff was non-linearly related to mean intake, via the negative-binomial distribution, with the overdispersion parameter k best described as an increasing (power) function of the mean. Approximately one in every three ingested mff escaped imprisonment by the peritrophic matrix, irrespective of the skin microfilarial load or the intake of mff. The relationship between successful and input mff is nearly linear (indicating proportionality) in S. squamosum B. These results are compared with those from O. volvulus-S. damnosum s.l. combinations in other West African foci.

Onchocerciasis hyperendemic in the Unturán Mountains: the value of recombinant antigens in describing a new transmission area in southern Venezuela

A recently described hyperendemic onchocerciasis area, located in the Unturán Mountains (between the Siapa and Orinoco basins) of southern Venezuela was studied using a cocktail of 3 low molecular weight onchocercal recombinant antigens (OvMBP/10, OvMBP/11, and OvMBP/29). The resulting seroepidemiological data were compared with those from a hypoendemic community (Altamira) situated in the northern coastal mountain range. Parasitological (skin biopsy) and serological (enzyme-linked immunosorbent assay, ELISA) methods for the specific diagnosis of Onchocerca volvulus in these 2 very different endemic areas were, respectively, 88% and 96% sensitive in Unturán, and 57% and 91% sensitive in Altamira. The mean microfilarial load, the mean optical density (OD), and the seropositivity rates all increased significantly with age in both communities. The serological variables (mean OD and prevalence of anti-O. volvulus antibodies) were both significantly higher in Unturán than in Altamira for children and young adults (aged < 25 years), although above this age no differences between communities were detected. Seroprevalence had already reached 50% in the under 15 year-olds examined at Unturán but was just 5% at Altamira for the same age-class. The prevalence of specific antibodies (mainly a marker of exposure to risk of infection) exceeded 85% in the remaining age-categories at the hyperendemic area. This is in agreement with the high community microfilarial load recorded in Unturán (> 20 mf/mg) and the presence of sclerosing keratitis and hanging groin, suggesting that onchocerciasis is a public health problem in this community. The ELISA test used here, based on a cocktail of 3 low molecular weight onchocercal recombinant antigens, appears, therefore, to constitute a practical tool for the description of endemicity levels in remote areas, particularly given the fact that finger-prick blood samples are routinely taken from children in the Upper Orinoco region for surveys of malaria incidence. Such studies could aid in defining the true extent of the Amazon focus (still unknown) and providing priority indicators for the selection of communities where onchocerciasis control programmes should be implemented.

Onchocerciasis hyperendemic in the Unturán mountains: an extension of the endemic region in southern Venezuela

A new region with human onchocerciasis is reported in the Unturán mountains, South Venezuela, affecting Yanomamö populations not surveyed in previous studies conducted in the Venezuelan-Brazilian border area. Its distribution probably extends towards the Upper Toototobi endemic area in Brazil. The age-standardized prevalence of Onchocerca volvulus microfilariae (mf) (67%), the prevalence of infection in those aged > or = 20 years (86%), and the community microfilarial load (CMFL) (24 mf/mg), are consistent with hyperendemic transmission. Both prevalence and mean intensity increased monotonically with age without reaching a plateau, the highest values being recorded in the > or = 45 years age class (respectively, 95% and 42 [geometric mean of Williams] or 172 [arithmetic mean] mf/mg). The degree of parasite overdispersion (measured by the variance/mean ratio) also increased with host age. The CMFL value, the presence of sclerosing keratitis, hanging groin, and severe skin lesions, indicated that the infection poses an important public health problem in the region.

Forest onchocerciasis in Cameroon: its distribution and implications for selection of communities for control programmes

The prevalence of infection and disease due to Onchocerca volvulus in regions at different altitudes and distances from the nearest breeding site for the Simulium vectors were studied in a forested area of Cameroon. A total of 1,785 subjects in 14 villages underwent clinical and parasitological evaluation. According to WHO endemicity criteria, villages 267 and 720 m above the sea level (asl) were hyperendemic, whereas lower villages, at 55 n asl, were meso-endemic. Altitude was positively correlated and distance from vector breeding site negatively correlated with several clinical indicators of onchocerciasis: prevalence of infection, microfilarial density, onchocercomata, and blindness. Location and altitude of his or her village and the subject's duration of residence in the village, age and sex were all found to be independent predictors of infection by logistic regression. The observed pattern of forest onchocerciasis reflects an interplay of multiple factors rather than a simple relationship between endemicity and distance from the nearest river. In order to select communities most affected by onchocerciasis for control programmes, two indicators, the prevalence of blindness and the prevalence of onchocercomata, might be used.

Molecular cloning of a gene expressed during early embryonic development in Onchocerca volvulus

Little attention has been paid to the reproductive biology of filarial nematode parasites as a possible target for immunological or chemotherapeutic intervention. An interruption of the reproductive process would, in addition to breaking the cycle of transmission, reduce the morbidity associated with certain filarial infections. As part of our efforts to define molecules that have important functions during filarial embryogenesis, antibodies against embryo-associated proteins were used to identify a 6308-bp cDNA sequence (ovt1) from an Onchocerca volvulus cDNA expression library. The ovt1 cDNA contained an open reading frame that coded for 2022 amino acids. The deduced amino acid sequence was highly hydrophilic, alpha-helical in nature and included two leucine zipper domains. OVT1 also contained a single Arg-Gly-Asp (RGD) site. The results of Southern blot analyses demonstrated that an ovt1-like gene occurs in a number of different species of filarial nematodes. In situ hybridization experiments to identify tissues that contain ovt1 transcripts showed that ovt1 was transcribed at high levels in the late morula/early blastocyst stage of embryonic development. Transcripts for ovt1 were also detected in O. volvulus larvae and in the hypodermal cells of adult parasites. Two fragments of ovt1 were expressed as fusion proteins and the fusion proteins were used to produce antibodies in rabbits. Both antibodies recognized a native protein with an apparent molecular mass of 230 kDa in extracts from gravid female O. volvulus. In addition, the antibodies reacted with a restricted number of lower-molecular mass bands which may represent the products of post-transcriptional or post-translational processing. The predicted coiled-coil structure and the sites of transcription suggest that OVT1 may be a component of the extracellular matrix.