Abstract
Onchocerciasis (river-blindness) in Africa is targeted for elimination through mass drug administration (MDA) with ivermectin. Onchocerciasis may cause various types of skin and eye disease. Predicting the impact of MDA on onchocercal morbidity is useful for future policy development. Here, we introduce a new disease module within the established ONCHOSIM model to predict trends over time in prevalence of onchocercal morbidity. We developed novel generic model concepts for development of symptoms due to cumulative exposure to dead microfilariae, accommodating both reversible (acute) and irreversible (chronic) symptoms. The model was calibrated to reproduce pre-control age patterns and associations between prevalences of infection, eye disease, and various types of skin disease as observed in a large set of population-based studies. We then used the new disease module to predict the impact of MDA on morbidity prevalence over a 30-year time frame for various scenarios. ONCHOSIM reproduced observed age-patterns in disease and community-level associations between infection and disease reasonably well. For highly endemic settings with 30 years of annual MDA at 60% coverage, the model predicted a 70% to 89% reduction in prevalence of chronic morbidity. This relative decline was similar with higher MDA coverage and only somewhat higher for settings with lower pre-control endemicity. The decline in prevalence was lowest for mild depigmentation and visual impairment. The prevalence of acute clinical manifestations (severe itch, reactive skin disease) declined by 95% to 100% after 30 years of annual MDA, regardless of pre-control endemicity. We present generic model concepts for predicting trends in acute and chronic symptoms due to history of exposure to parasitic worm infections, and apply this to onchocerciasis. Our predictions suggest that onchocercal morbidity, in particular chronic manifestations, will remain a public health concern in many epidemiological settings in Africa, even after 30 years of MDA.
Highlights
Onchocerca volvulus is a parasitic filarial nematode transmitted through the bite of infected blackflies
For highly endemic settings with 30 years of annual mass drug administration (MDA) at 60% coverage, the model predicted a 70% to 89% reduction in prevalence of chronic morbidity. This relative decline was similar with higher MDA coverage and only somewhat higher for settings with lower pre-control endemicity
We present generic model concepts for predicting trends in acute and chronic symptoms due to history of exposure to parasitic worm infections, and apply this to onchocerciasis
Summary
Onchocerca volvulus is a parasitic filarial nematode transmitted through the bite of infected blackflies (genus Simulium). Clinical manifestations caused by inflammation are diverse, including onchocercal skin disease (OSD) and onchocercal eye disease (OED). Mf-induced damage to the eye can lead to visual impairment and eventually blindness. 218 million people in 30 countries worldwide (2018) are at risk of onchocerciasis; 99% of those people live in sub-Saharan Africa [12]. About 7.5 million people were infected with O. volvulus in West-Africa around 1974 (prior the implementation of the Onchocerciasis Control Programme [OCP]) [13]. Another study estimated that 36 million people would have been infected in the APOC countries by 2011 if there had been no ivermectin treatment [14]. Onchocerciasis (river-blindness) in Africa is targeted for elimination through mass drug administration (MDA) with ivermectin. Onchocerciasis may cause various types of skin and eye disease. We introduce a new disease module within the established ONCHOSIM model to predict trends over time in prevalence of onchocercal morbidity
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