Epidemiological studies of onchocerciasis in Ghana : insights from Ghana's Middle Belt and challenges in attaining elimination
Authors:   , Otabil Kenneth 
Published: February 2025

Onchocerciasis, a disease caused by Onchocerca volvulus and transmitted by blackfly vectors of the genus Simulium, is a neglected tropical disease (NTD) that affects the poorest of the poor. About 99% of the global burden of the disease is borne by Sub-Saharan Africa (SSA). In Ghana, 15 of its 16 regions are endemic for the disease despite Ghana being one of the countries covered by the former Onchocerciasis Control Programme in West Africa (OCP). The World Health Organization (WHO), in its recent 2021–2030 Roadmap for NTDs, has targeted onchocerciasis for verification of elimination of transmission (EOT) in 12 countries (possibly including Ghana), approximately 30% of the endemic countries globally. Despite this ambitious target, Ghana, in its recent country NTD master plan 2021–2026, has identified several challenges that could hamper the achievement of national EOT. These challenges include i) inadequate monitoring of Mass Drug Administration (MDA) with ivermectin to ensure that reported coverages are accurate; ii) the paucity and inadequate quality of epidemiological information on the true picture of onchocerciasis endemicity; iii) the need for vector surveillance and control in some areas, and iv) the need for novel field-friendly and sensitive diagnostic tools that are acceptable in endemic communities. The research presented in this dissertation aims to address these challenges. The first piece of research was a pilot study to assess the effects of climate variability on the relative abundance and biting rates of the blackfly vectors in the study area and hence to evaluate the patterns of exposure to blackfly bites. This study demonstrated that Simulium vectors in the study area preferred higher temperatures, lower humidity and rainfall, more hours of sunshine and relatively larger winds speed for survival. The implication of our findings for onchocerciasis control programmes is that it is essential to assess the temporal distribution of the Simulium vectors in endemic areas and consider scheduling MDA during periods when the abundance of vectors has begun to decline immediately after peaking according to seasonal patterns. This timing is recommended as optimal because that is the period where individuals will have been maximumly exposed to blackfly bites, hence ivermectin treatment at this time will clear most of the microfilariae. The second study investigated the prevalence of onchocerciasis and associated dermatological and ophthalmological manifestations in six endemic communities. This study revealed the persistence of O. volvulus infection and associated dermal and ocular pathologies in the study communities despite decades of MDA with ivermectin. Therefore, onchocerciasis elimination programmes need to consider onchocerciasis-associated morbidities in their planning. A morbidity management and disease prevention strategy, similar to that for lymphatic filariasis, should be developed to reach elimination as public health problem as well as EOT targets for onchocerciasis by 2030. In the third study, the prevalence and clinical characteristics of epilepsy were investigated in the study area to determine if there was a substantial problem of onchocerciasis-associated epilepsy (OAE). The epilepsy prevalence in the Bono region was 1.4%. This is similar to the median epilepsy prevalence in Sub Saharan Africa (SSA) as well as the average prevalence of epilepsy in Ghana which is estimated at 1% of the population. Expectedly, it is lower than the high epilepsy prevalence reported in onchocerciasis-endemic regions with high pre-control microfilarial prevalence and high ongoing onchocerciasis transmission intensity such as in certain areas in South Sudan (5.1%), Cameroon (3.5% and 7.8%, the Democratic Republic of the Congo (4.6%), and Tanzania (2.5%). Also, O. volvulus microfilariae were found in only one of the 12 persons with epilepsy tested during skin-snip examination. Our findings mean that, currently in the Bono region, there is no appreciable OAE problem. However, despite 27 years of ivermectin administration, the overall microfilarial prevalence in the study area was still around 10%. Hence, there is a need for the Ghana Onchocerciasis Elimination Programme (GOEP) to strengthen and sustain its efforts to ensure that the gains of MDA are consolidated and improved to achieve elimination of onchocerciasis and its associated morbidities. The fourth piece of research investigated factors associated with ivermectin intake in the areas where we had previously demonstrated persistence of O. volvulus infection. In line with other studies where onchocerciasis persists despite prolonged community-directed treatment with ivermectin (CDTI), the proportion of eligible people who had missed at least one round of ivermectin was 36.4% whilst 28.8% did not take ivermectin during the last treatment round in March 2021. However, the proportion of systematic non-adherers (around 5%) (after 27 years of MDA) was consistent with that recorded in other SSA endemic areas not co-endemic with loiasis. Reasons for non-adherence included fear/experience of side effects, ineligibility due to pregnancy, absenteeism, and desire to drink alcohol. This cohort of non-adherers to ivermectin threaten the achievement of EOT as they represent a reservoir of re-infection in endemic communities. Finally, the fifth study assessed the utility, acceptability and cost of the Ov16 rapid diagnostic test (Ov16 RDT) developed by SD Bioline for the assessment of transmission status. Ov16 seroprevalence was 23.6% (60/254, 95%CI=18.8%–29.2%) ; microfilarial prevalence 11.7% (11/94, 95%CI = 6.7%–19.8%) and nodule prevalence 5.5% (14/254, 95%CI=3.3%–9.0%). The proportion of Ov16 seropositive females (43/136, 31.6%) was twice that of males (17/117, 14.5%). Among 5–9-year-olds, Ov16 seroprevalence was 11.1% (3/27), microfilarial prevalence 23.1% (3/13) and nodule prevalence 3.7% (1/27). For the 94 participants with all three tests, there was no association between the results of Ov16 RDT, skin-snip microscopy and/or nodule palpation. Most participants and technicians preferred Ov16 RDT because of being less painful and invasive, easier to use and faster. Had 400 participants been tested, the total cost per individual would be US$24 (Ov16 RDT) and US$74 (skin-snip microscopy). Ov16 RDT is more acceptable and affordable (a third of the cost) compared to skin-snipping for surveillance activities in transmission hotspots in Ghana. In conclusion, this thesis provides compelling evidence that O. volvulus microfilaridermia as well as the dermal and ocular manifestations associated with onchocerciasis persist in the study communities despite 27 years of CDTI. Possible reasons for disease persistence may include non-adherence to ivermectin treatment and/or a generally increasing reluctance of communities to participate in control efforts. There is, therefore, a pressing need for sustained and improved efforts by the GOEP if the country is to achieve the target of EOT verification at national level by 2030.