Impact of human movement between hypo- and hyperendemic areas on sustainability of elimination ofOnchocerca volvulustransmission

Abstract

Background Onchocerciasis is a vector-borne disease caused by the filarial nematodeOnchocerca volvulus. Endemic countries target elimination of parasite transmission using primarily annual ivermectin mass administration. Elimination is particularly challenging in sub-Saharan Africa, where there are large contiguous areas with varying levels of endemicity and intervention history. We examined one challenge to elimination that has received little attention to date: movement of people between areas. Methodology/Principal Findings We extended one of the commonly used onchocerciasis transmission models, EPIONCHO, to allow modelling the effect of movement of people and/or flies between areas ("patches"). We explored the impact of humans travelling between a hypoendemic area (i.e., with low vector biting rates) with no history of interventions and a hyperendemic area (high vector biting rates) that stopped intervention (ivermectin mass administration) after infection prevalence decreased below 1.4%. Our results suggest that human travel in either direction will accelerate recrudescence in the hyperendemic area to pre-intervention levels, relative to recrudescence without travel, and can reduce the annual biting rate required for parasite transmission within hypoendemic areas. Conclusions/Significance Our results emphasize the importance of decisions on which hypoendemic areas to include in interventions and suggest that transmission mediated by human movement needs to be considered when planning (a) the geographic areas and sampling density for evaluations for decisions on when and where to stop interventions, (b) where, how often, and for how long to conduct post-intervention surveillance for verification of elimination of transmission and (c) where and how frequently to conduct post-elimination surveillance. Given the cost implications of stopping interventions too early or later than necessary, we encourage the development of models such as the one presented here for quantitating the impact of human and vector movement between areas on the risk and timeframe of recrudescence after interventions are stopped to inform economic analyses.

An Updated Economic Assessment of Moxidectin Treatment Strategies for Onchocerciasis Elimination

“…The cost reduction with moxidectin- relative to ivermectin-based strategies is particularly relevant for highly endemic areas for which EPIONCHO-IBM suggests that bCDTM is needed to reach EoT 90 within 20 years for both coverage scenarios ( Figure 1 and [ 19 ]). Such areas have the potential to be very costly to NTD programs in the long term, and continuing transmission in these areas could conceivably reintroduce infection to others [ 30 ].…”

“…To maximize usefulness for countries, other factors should be included, such as transmission seasonality, to guide optimal annual or biannual treatment timing [ 17 ]. Including spatial heterogeneity in endemicity (and vector biting rates) within transmission zones would help us to better understand the consequences of people and/or vector movement between communities on achieving and maintaining EoT [ 30 ], with potential repercussions on required treatment duration, number of treatments, and associated delivery costs to achieve EoT across the whole transmission zone. The fact that bCDTM is projected to reduce the difference in the number of treatments needed to achieve EoT across different endemicity settings (ie, less difference between number of treatment rounds needed to achieve EoT in areas ranging from lower to higher baseline prevalence compared with ivermectin-based strategies) could be an added advantage for country programs.…”

Synthesizing environmental, epidemiological and vector and parasite genetic data to assist decision making for disease elimination

“…Where there is high parasite mobility, but low blackfly mobility, we propose a risk of transmission mediated by human movement (Figure 6). Note that while high biting rates drive high endemicity of the disease, vector or human mobility might help maintain onchocerciasis endemicity despite MDAi, or increase the baseline prevalence in a community relative to that expected due to lower blackfly biting rates (McCulloch et al., 2023).…”

“…1. Resistance surfaces and connectivity maps could be used For example, recent modelling work suggests that low endemic areas can act as a source to re-initiate transmission in MDAicontrolled onchocerciasis endemic areas (de Vos et al, 2021;McCulloch et al, 2023).…”

Oncocercose: a doença tropical negligenciada que acomete a Etnia Yanomami

“…Logo, destacando-se na tribo Yanomami, uma vez que esse grupo indígena constrói suas aldeais próximas a rios, sendo o maior exemplo de região endêmica. Adicionalmente, observa-se que as larvas dos simulídeos exigem alto teor de oxigênio no meio para o seu desenvolvimento, ocorrendo, mais frequentemente, em locais/porções altas e encachoeiradas dos rios, onde se observa uma maior aeração das águas (Moraes, 1991;McCulloch et al, 2023).…”