The risk of onchocerciasis infection by human population movements between high and low transmission settings in Ghana

TL;DR

This study shows that human movement between high and low-risk onchocerciasis areas in Ghana increases infection risk, suggesting the need to extend treatment to low-risk communities for effective elimination.

Contribution

The study provides empirical evidence on human movement patterns and infection levels between high and low-risk onchocerciasis communities in Ghana.

Findings

Onchocerciasis prevalence was significantly higher in high-risk than low-risk areas for both sero-prevalence and microfilariae prevalence.

Over 60% of participants moved between high-risk and low-risk communities daily or weekly, indicating strong connectivity.

Infection levels were higher among individuals moving from high-risk to low-risk areas, though not statistically significant.

Abstract

Onchocerciasis control strategies have focused on mass drug administration (MDA) to reduce morbidity in high-risk (HR) areas (sites close to blackfly breeding sites). However, with the current drive towards elimination, treatment must be extended to low-risk (LR) areas. It is uncertain how well HR and LR communities are connected for decision making in recommending treatment strategies to include the LR areas. We provided data on current onchocerciasis infection status, connectivity between HR and LR communities and rates of human movement within some endemic communities in Ghana.

Selected communities were 5km (HR) and 15km (LR) from breeding sites. Questionnaires were administered to obtain data on demographics and human movement patterns. Samples were collected from participants and tested for O. volvulus infections using Ov16 RDT and presence of microfilariae (mf) in skin snips using microscopy and quantitative PCR.

We observed a significantly higher onchocerciasis prevalence in HR than LR sites for both sero-prevalence (42.5% vrs 16.0%) and mf prevalence (15.2% vrs 4.1%) [P < 0.0001]. There was a high connectivity between the HR and LR with about 64–82% people who moved from HR to LR and 3.2-10% from LR to HR daily or weekly. Infection levels in those who moved from HR to LR communities were higher than those who moved from LR to HR, although these were not statistically significant (P > 0.05). Some individuals in Lancha (a LR community) who tested positive for infection frequently visited the HR communities for farming.

A strong connectivity existed between HR and LR communities by human movement. On the average >60% of participants moved between endemic communities (between HR and LR) either daily, within the week or weekly. This supports the need to initiate treatment in LR areas; hence, such movement data would be useful during assessment of onchocerciasis elimination and delineation of transmission zones.

The main control of onchocerciasis is by MDA to reduce morbidity targeting only communities close to blackfly breeding sites where infection levels are high (high-risk, HR) sites. However, with a change from control to elimination, MDA should be extended to other sites within the endemic region which were not previously included, especially distant communities from breeding sites (low-risk, LR). It is not clear how people in the HR migrate to LR areas and vice versa. Such information will help formulate treatment strategies to include the LR sites towards the elimination agenda. To support such decisions, we collected human data on demographics and movement patterns in HR and LR endemic communities in Ghana. We also screened participants for onchocerciasis infection. Our findings show a higher onchocerciasis prevalence in HR than LR sites for both sero-prevalence (42.5% vrs 16.0%) and mf prevalence (15.2% vrs 4.1%) [P < 0.0001]. There was a strong connectivity between the HR and LR with about 64–82% who moved from HR to LR and 3.2-10% who moved from LR to HR daily or weekly. This highlights the need to equally treat LR areas; hence this movement information should be considered during assessment of onchocerciasis elimination and delineation of transmission zones.

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