Landscape Epidemiology of Human Onchocerciasis in Southern Venezuela

Botto, Carlos; Villamizar, N. J.; Jokic, Zˇ; Noya-Alarcón, Óscar; Cortez, José Paulo; Escalona, Marisela; Grillet, María-Eugenia; Basáñez, María‐Gloria

doi:10.1016/b978-0-444-52272-6.00693-0

Cited by 6 publications

(11 citation statements)

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“…However, this association may also depend on the type of river and its geomorphological substrate, determining vector breeding suitability (and in Venezuela also an association with vector species composition and the relative vector competence of the various simuliid species for O . volvulus ), and not merely on altitude [ 96 , 97 ]. In this study, altitude was not found to be a significant predictor of onchocerciasis; however, this could be partly explained because both rainfall and temperature variables were included in the model, which may remove some of the effect of altitude on prevalence, and the fact that > 94% of the total land area in the OCP is < 500 metres elevation.…”

Section: Discussionmentioning

confidence: 99%

Model-Based Geostatistical Mapping of the Prevalence of Onchocerca volvulus in West Africa

et al. 2016

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BackgroundThe initial endemicity (pre-control prevalence) of onchocerciasis has been shown to be an important determinant of the feasibility of elimination by mass ivermectin distribution. We present the first geostatistical map of microfilarial prevalence in the former Onchocerciasis Control Programme in West Africa (OCP) before commencement of antivectorial and antiparasitic interventions.Methods and FindingsPre-control microfilarial prevalence data from 737 villages across the 11 constituent countries in the OCP epidemiological database were used as ground-truth data. These 737 data points, plus a set of statistically selected environmental covariates, were used in a Bayesian model-based geostatistical (B-MBG) approach to generate a continuous surface (at pixel resolution of 5 km x 5km) of microfilarial prevalence in West Africa prior to the commencement of the OCP. Uncertainty in model predictions was measured using a suite of validation statistics, performed on bootstrap samples of held-out validation data. The mean Pearson’s correlation between observed and estimated prevalence at validation locations was 0.693; the mean prediction error (average difference between observed and estimated values) was 0.77%, and the mean absolute prediction error (average magnitude of difference between observed and estimated values) was 12.2%. Within OCP boundaries, 17.8 million people were deemed to have been at risk, 7.55 million to have been infected, and mean microfilarial prevalence to have been 45% (range: 2–90%) in 1975.Conclusions and SignificanceThis is the first map of initial onchocerciasis prevalence in West Africa using B-MBG. Important environmental predictors of infection prevalence were identified and used in a model out-performing those without spatial random effects or environmental covariates. Results may be compared with recent epidemiological mapping efforts to find areas of persisting transmission. These methods may be extended to areas where data are sparse, and may be used to help inform the feasibility of elimination with current and novel tools.

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Section: Discussionmentioning

confidence: 99%

Model-Based Geostatistical Mapping of the Prevalence of Onchocerca volvulus in West Africa

et al. 2016

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“…On the other hand, onchocerciasis models and theory predict that risk of recrudescence will depend on the pre-control endemicity level and the vector species efficiency as indicators of the local potential of parasite transmission [ 25 ]. The pre-control endemicity of both foci (the prevalence and intensity of infection in the human population) was relatively low compared to the southern focus of the country [ 9 ]. As an example, in the northern onchocerciasis foci, the community microfilarial load (CMFL) before mass drug administration never reached levels above 5 mf per mg of skin in contrast to the southern focus figures [ 9 ].…”

Section: Discussionmentioning

confidence: 99%

“…The pre-control endemicity of both foci (the prevalence and intensity of infection in the human population) was relatively low compared to the southern focus of the country [ 9 ]. As an example, in the northern onchocerciasis foci, the community microfilarial load (CMFL) before mass drug administration never reached levels above 5 mf per mg of skin in contrast to the southern focus figures [ 9 ]. In addition, S. metallicum has a relatively low vectorial competence [ 7 ] compared to other Venezuelan and American vector black fly species [ 12 , 31 ], regardless of its moderate to high biting density (mean monthly biting rate ranging from 14, 284 to 34, 428 bites per person per month; unpublished results).…”

Section: Discussionmentioning

confidence: 99%

“…The remaining transmission zone in Venezuela, the southern onchocerciasis Amazonian focus [ 9 ], is the only active focus in the Americas shared by Brazil [ 4 ]. This endemic Yanomami area extends through remote and densely forested zones and is populated by the Yanomami people, a migratory indigenous group that routinely moves across the border at will [ 4 , 9 ].…”

Section: Discussionmentioning

confidence: 99%

“…Here, about 108,968 persons from the rural population are at risk [ 4 ] and infection is transmitted by Simulium metallicum sensu lato Bellardis [ 5 - 7 ]. In contrast, the southern onchocerciasis Amazonian focus is confined to the rainforest of the Upper Orinoco River region, affecting about 10,390 people [ 4 ] from the indigenous Yanomami population [ 8 , 9 ]. In this second endemic area, S. guianense s.l.…”

Section: Introductionmentioning

confidence: 99%

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Interruption of Onchocerca volvulus transmission in Northern Venezuela

et al. 2013

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BackgroundOnchocerciasis is caused by Onchocerca volvulus and transmitted by Simulium species (black flies). In the Americas, the infection has been previously described in 13 discrete regional foci distributed among six countries (Brazil, Colombia, Ecuador, Guatemala, Mexico and Venezuela) where more than 370,000 people are currently considered at risk. Since 2001, disease control in Venezuela has relied on the mass drug administration to the at-risk communities. This report provides empirical evidence of interruption of Onchocerca volvulus transmission by Simulium metallicum in 510 endemic communities from two Northern foci of Venezuela, after 10–12 years of 6-monthly Mectizan® (ivermectin) treatment to all the eligible residents.MethodsIn-depth entomologic and epidemiologic surveys were serially conducted from 2001–2012 in selected (sentinel and extra-sentinel) communities from the North-central (NC) and North-east (NE) onchocerciasis foci of Venezuela in order to monitor the impact of ivermectin treatment.ResultsFrom 2007–2009, entomological indicators in both foci confirmed that 0 out of 112,637 S. metallicum females examined by PCR contained L3 infection in insect heads. The upper bound of the 95% confidence intervals of the infective rate of the vector reached values below 1% by 2009 (NC) and 2012 (NE). Additionally, after 14 (NC) and 22 (NE) rounds of treatment, the seasonal transmission potential (±UL CIs) of S. metallicum was under the critical threshold of 20 L3 per person per season. Serological analysis in school children < 15 years-old demonstrated that 0 out of 6,590 individuals were harboring antibodies to Ov-16. Finally, epidemiological surveys made during 2010 (NC) and 2012 (NE) showed no evidence of microfilariae in the skin and eyes of the population.ConclusionsThese results meet the WHO criteria for absence of parasite transmission and disease morbidity in these endemic areas which represent 91% of the population previously at-risk in the country. Consequently, the two Northern foci are currently under post-treatment onchocerciasis surveillance status in Venezuela.

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