Pathogens2022,11, 2812 of 16Keywords:onchocerciasis; onchocerciasis-associated epilepsy; epilepsy prevalence; incidence;ivermectin;OV16 antibodies; Africa1. IntroductionOnchocerciasis, commonly known as river blindness, is caused by the filarial wormOnchocerca volvulus(O. volvulus) [1]. It is estimated that 99% of the 20.9 millionO. volvulusin-fected individuals live in 31 African countries [2]. Over 70% (14.6 million) of theO. volvulusinfected individuals are considered to have onchocerciasis-induced skin disease and 5.5%(1.15 million) to have vision loss [3]. Moreover, accumulating evidence suggest thatO. volvulusinfection is also able to trigger epilepsy in a manner that is dependent onthe microfilarial (mf) load in the skin [4–6], so-called onchocerciasis-associated epilepsy(OAE) [7].Onchocerciasis-elimination programs rely on community-directed treatment withivermectin (CDTI) and vector control [3]. Using CDTI, the African Programme for On-chocerciasis Control (APOC) has successfully eliminated onchocerciasis as a public healthproblem in several African countries [3,8]. However, in some onchocerciasis-endemic areasin Africa there is still high ongoingO. volvulustransmission and a high prevalence ofonchocerciasis-associated morbidity including OAE due to low CDTI coverage and in someareas resulting from CDTI interruptions during the periods of insecurity [7,9,10].Several new promising drugs for the treatment of onchocerciasis are being tested inclinical trials [11,12], of which moxidectin was shown to reduce and maintain low skinmicrofilarial density for longer than ivermectin [13]. Macrofilaricides, currently only inan early phase of development, will be needed to drastically reduce the elimination timeof onchocerciasis [11,12]. However, today none of these new drugs are available for massdrug administration programmes.The interruption ofO. volvulustransmission is evaluated by screening pooled blackfliesusing the O-150 PCR technique targeting parasite-specific markers and by dissectingthe heads and thorax of blackflies to determine the level of infectiveO. volvuluslarvae(L3 stage) under a binocular microscope [14]. Moreover, the prevalence of anti-Ov16immunoglobulin G4 (IgG4) antibodies in children aged <10 years, determined by anOv16 ELISA test, is also used to assessO. volvulustransmission interruption [14]. Thismethod has been used by the South American onchocerciasis elimination programme todocument the elimination of onchocerciasis in several Latin American countries [15–19],and also in some African countries such as Senegal [20] and Uganda [21]. However thethreshold required to determine when it is safe to stop CDTI and to declare interruption oftransmission is still under debate [22]. According to World Health Organization (WHO)guidelines,2000 childrenunder 10 years of age have to be tested for Ov16 antibodies, anda seroprevalence below 0.1% is required to assume a sufficient reduction ofO. volvulustransmission such that CDTI can be stopped [14]. A modelling study suggested that theOv16 antibody prevalence in children aged 5–14 years would perform better in predictingelimination and that a threshold value for this age group of 2.0% and even higher thresholdvalues would be safe to use in lower-endemic areas [23].While it is important to know when a CDTI program can be stopped, it is also im-portant to identify CDTI programs that are working sub-optimally in order to strengthenthem. To do so, CDTI coverage is assessed and skin snip testing has been used to monitorcommunity microfilarial loads. There are, however, problems with both methods: CDTIcoverage data reported by the community-directed distributors of ivermectin are oftennot very reliable [24]. Independent surveys, as recommended by the WHO, provide morereliable results but are relatively costly. Skin snip testing is also problematic because itrequires punches that are difficult to obtain and that are quite expensive. It also requires anexperienced lab technician and a good microscope to read the skin snips and differentiateO. volvulusmicrofilariae from other filarial larvae, with results only made available the next
Pathogens2022,11, 2813 of 16day. Moreover, as it is an invasive and slightly painful procedure, therefore populationsare increasingly reluctant to be skin snip tested. Therefore, we propose to use the Ov16IgG4 rapid diagnostic testing (Ov16 RDT) of children 6–10 years old as an easier alter-native way to determine the degree of ongoing onchocerciasis transmission. In differentonchocerciasis-endemic foci in sub-Saharan Africa, we investigated how the Ov16 RDTseroprevalence among 6–10-year-old children as a proxy for ongoingO. volvulustrans-mission, together with epilepsy prevalence and ivermectin coverage data, can be used toevaluate the performance of the onchocerciasis-elimination programs.2. Materials and Methods2.1. Epilepsy Surveys in Different Onchocerciasis Endemic FociBetween 2015 and 2021, door-to-door epilepsy surveys were conducted in onchocerciasis-endemic villages across Central African countries: Cameroon (Sanaga valley in the Littoralregion: Kelleng [25], and Mbam valley in the Littoral region: Bilomo, Bayomen, Nyamongo,and Ngongol [25,26]); the Central African Republic (CAR) [27,28]; West African countries:Nigeria (Imo River Basin) [29]; Central and East African countries including the Demo-cratic Republic of Congo (DRC) in Aketi, Bas Uéle [30], and Logo, Ituri [31,32]; Tanzania(Mahenge) [33]; South Sudan, Maridi [9], Mvolo [10] and Mundri, West County [34]. Intotal, eighteen study sites in eight different onchocerciasis foci were included in the study(Figure 1).Pathogens 2022, 11, x 3 of 15 expensive. It also requires an experienced lab technician and a good microscope to read the skin snips and differentiate O. volvulus microfilariae from other filarial larvae, with results only made available the next day. Moreover, as it is an invasive and slightly painful procedure, therefore populations are increasingly reluctant to be skin snip tested. Therefore, we propose to use the Ov16 IgG4 rapid diagnostic testing (Ov16 RDT) of children 6–10 years old as an easier alternative way to determine the degree of ongoing onchocerciasis transmission. In different onchocerciasis-endemic foci in sub-Saharan Africa, we investigated how the Ov16 RDT seroprevalence among 6–10-year-old children as a proxy for ongoing O. volvulus transmission, together with epilepsy prevalence and ivermectin coverage data, can be used to evaluate the performance of the onchocerciasis-elimination programs. 2. Materials and Methods 2.1. Epilepsy Surveys in Different Onchocerciasis Endemic Foci Between 2015 and 2021, door-to-door epilepsy surveys were conducted in onchocerciasis-endemic villages across Central African countries: Cameroon (Sanaga valley in the Littoral region: Kelleng [25], and Mbam valley in the Littoral region: Bilomo, Bayomen, Nyamongo, and Ngongol [25,26]); the Central African Republic (CAR) [27,28]; West African countries: Nigeria (Imo River Basin) [29]; Central and East African countries including the Democratic Republic of Congo (DRC) in Aketi, Bas Uéle [30], and Logo, Ituri [31,32]; Tanzania (Mahenge) [33]; South Sudan, Maridi [9], Mvolo [10] and Mundri, West County [34]. In total, eighteen study sites in eight different onchocerciasis foci were included in the study (Figure 1). Figure 1. Map with the localisation of the villages included in the study; (1) Imo River Valley (Imo State); (2) Kelleng (Littoral Region); (3) Bayomen (Mbam River Valley); (4) Nyamongo (Mbam River Valley); (5) Bilomo (Centre region); (6) Ngongol (Mbam River Valley); (7) Kodjo (Landja Mboko District); (8) Makoko (Bas Uélé Province); (9) Wela (Bas Uélé Province); (10) Maridi (Western Equatoria state); (11) Kuda valley (Ituri Province); (12) Draju (Ituri Province); (13) Mvolo (Western Equatoria state); (14) Mundri center Payam (Western Equatoria state); (15) Amadi Payam (Western Figure 1.Map with the localisation of the villages included in the study; (1) Imo River Valley(Imo State); (2) Kelleng (Littoral Region); (3) Bayomen (Mbam River Valley); (4) Nyamongo (MbamRiver Valley); (5) Bilomo (Centre region); (6) Ngongol (Mbam River Valley); (7) Kodjo (LandjaMboko District); (8) Makoko (Bas UéléProvince); (9) Wela (Bas UéléProvince); (10) Maridi (WesternEquatoria state); (11) Kuda valley (Ituri Province); (12) Draju (Ituri Province); (13) Mvolo (WesternEquatoria state); (14) Mundri center Payam (Western Equatoria state); (15) Amadi Payam (WesternEquatoria state); (16) Mahenge Sub-urban villages (Ulanga district); (17) Mahenge Rural villages(Ulanga district); (18) Kuda valley (Ituri Province).