Modelling the impact of ivermectin on River Blindness and its burden of morbidity and mortality in African Savannah: EpiOncho projections

Abstract

BackgroundThe African Programme for Onchocerciasis Control (APOC) has refocused its goals on the elimination of infection where possible, seemingly achievable by 15–17 years of annual mass distribution of ivermectin in some African foci. Previously, APOC had focused on the elimination of onchocerciasis as a public health problem. Timeframes have been set by the World Health Organization, the London Declaration on Neglected Tropical Diseases and the World Bank to achieve these goals by 2020–2025.MethodsA novel mathematical model of the dynamics of onchocercal disease is presented which links documented associations between Onchocerca volvulus infection and the prevalence and incidence of morbidity and mortality to model outputs from our host age- and sex-structured onchocerciasis transmission framework (EpiOncho). The model is calibrated for African savannah settings, and used to assess the impact of long-term annual mass administration of ivermectin on infection and ocular and skin disease and to explore how this depends on epidemiological and programmatic variables.ResultsCurrent onchocerciasis disease projections, which do not account for excess mortality of sighted individuals with heavy microfilarial loads, underestimate disease burden. Long-term annual ivermectin treatment is highly effective at reducing both the morbidity and mortality associated with onchocerciasis, and this result is not greatly influenced by treatment coverage and compliance. By contrast, impact on microfilarial prevalence and intensity is highly dependent on baseline endemicity, treatment coverage and systematic non-compliance.ConclusionsThe goals of eliminating morbidity and infection with ivermectin alone are distinctly influenced by epidemiological and programmatic factors. Whilst the former goal is most certainly achievable, reaching the latter will strongly depend on initial endemicity (the higher the endemicity, the greater the magnitude of inter-treatment transmission), advising caution when generalising the applicability of successful elimination outcomes to other areas. The proportion of systematic non-compliers will become far more influential in terms of overall success in achieving elimination goals.

Detection of Onchocerca volvulus in Skin Snips by Microscopy and Real-Time Polymerase Chain Reaction: Implications for Monitoring and Evaluation Activities

“…However, long-lived (10-15 years) reproductive adult filariae necessitate sustained long-term programs of annual or twice-per-year treatment for complete elimination of transmission risk. 3,4 The availability of effective tools to determine infection status will be of critical importance to elimination programs, both for monitoring and evaluation (M and E) of program success and for determining when to stop treatment.…”

Onchocerciasis transmission in Ghana: biting and parous rates of host-seeking sibling species of the Simulium damnosum complex

“…Models of O. volvulus transmission dynamics that investigate intervention outcomes are mostly parameterised using S. damnosum s.s./S. sirbanum [18][19][20][21][22][23][24][25][26], with the exception of the model by Davies (1993) [27], based on transmission of forest onchocerciasis by S. soubrense B sensu Post, some quantitative analyses on other S. damnosum complex species including S. leonense and S. squamosum B [28,29], and a recent modelling study of the effect of climate change on O. volvulus transmission in Ghana and Liberia, including S. soubrense [30]. This research gap will need addressing in areas where different species compositions exist in the human-biting blackfly population.…”

Reaching the London Declaration on Neglected Tropical Diseases Goals for Onchocerciasis: An Economic Evaluation of Increasing the Frequency of Ivermectin Treatment in Africa

“…This is because, in the absence of vector control, there is substantial transmission between consecutive annual treatments under scenarios of perennial transmission [18] (Figure 2). Although under these conditions, biannual ivermectin treatment would only have a small additional health impact—and would be deemed less cost effective than annual treatment in terms of the additional cost per additional DALY averted—it would lead to reduced program duration.…”

“…The DALY burden was estimated using a disease model that links the dynamic transmission model–derived prevalence and intensity of O. volvulus infection with the burden of onchocercal disease [18]. …”