Onchocerciasis-associated epilepsy pathogenesis and diagnostic tools for onchocerciasis elimination

Onchocerciasis, caused by the filarial nematode Onchocerca volvulus and transmitted by black flies, is known to cause skin disease and river blindness. It is also epidemiologically linked to Onchocerciasis-associated epilepsy (OAE). The pathogenesis of OAE remains unknown. While elimination efforts reduced the OAE burden, lack of optimal onchocerciasis diagnostic test hinder its complete eradication. Current rapid diagnostic tests (RDTs) lack accuracy, and new RDTs like DDTD biplex and GADx still require field validation. In this dissertation we investigated the biology of O. volvulus biology to explore the pathogenesis of OAE and examined the role of HLA as a potential co-factor for OAE. We also field-tested novel RDTs, DDTD and GADx, and compared them with existing diagnostic options. In Chapter 2.1., we investigated the O. volvulus virome using both in silico identification and metagenomics. This study identified the rhabdovirus O. volvulus RNA-virus 1 (OVRV1) as part of the O. volvulus virome. This virus was found to elicit a significant antibody response in several different sub-Saharan African countries, suggesting interaction with the human host. OVRV1 was found in all developmental stages of O. volvulus and in the closely related O. ochengi, which infects cattle. Further research is needed to confirm OVRV1’s link to OAE. In Chapter 2.2., in a case-control study (98 epilepsy cases and 112 controls) in Mahenge, an onchocerciasis-endemic area in Tanzania, we examined the role of HLA as a potential co-factor for OAE. An association between certain HLA haplotypes and nodding syndrome, as was previously suggested, was not confirmed. Additionally, no association between HLA haplotypes and OAE or anti-OV16 seropositivity were observed. However, prior to multiple testing corrections, some potential HLA associations were identified that warrant further investigation in larger case control studies. In Chapter 3.1, we assessed the level of ongoing transmission of O. volvulus in Maridi, Western Equatoria State in South Sudan. In Maridi the onchocerciasis elimination program had been recently strengthened by switching from annual to bi-annual community directed treatment with ivermectin (CDTI) combined with establishing a community-based vector control method. We used the anti-OV16 (Ov16 SD Bioline RDT) seroprevalence among three- to nine-year-old children as marker for O. volvulus transmission. Resulting a 6 seroprevalence of 30.2%, suggesting ongoing transmission with the highest prevalence observed in children living closest to the Maridi Dam. In Chapter 3.2. and 3.3., we evaluated novel RDTs, including DDTD biplex type A and C and GADx, comparing them to the commercially available OV16 SD Bioline. In a first study among three- to nine-year-old children, we assessed the feasibility of the DDTD biplex A RDT prototype. After which the test was found to be feasible, acceptable, and easy to use in a field setting. In a second study among pregnant and post-partum women in Maridi Hospital, we assessed the performance of the monoplex RTDs (Ov16 SD Bioline and GADx) and the biplex DDTD A and C. Using the monoplex tests the O. volvulus antibody prevalence was ±60% while it remained below 50% using DDTD A and C. The calculated sensitivity of the test ranged for GADx between 58.3% and 80.8%, for SD Bioline RDT between 54.2 and 76.1%, for DDTD A between 36.4 and 65.6%, and for DDTD C between 39.1 and 60.6%. Overall specificity remained under 73.2% for all tests. These results reveal that the novel RDTs demonstrated an estimated sensitivity of approximately 60%, which represents the lowest sensitivity threshold of WHO, comparable to the commercially available Ov16 SD Bioline RDT. However, none of the RDTs met the WHO specificity threshold of 99.8%. In conclusion, the mechanism underlying the pathogenesis of OAE remains unknown. However, the recent discovery of a virome within O. volvulus, incl. OVRV1, has led to novel, credible hypotheses that direct future research. While our study could not confirm an association between HLA and OAE, identifying potential genetic co-factors warrants further investigation. The novel RDTs, DDTD and GADx, do not reach the required WHO performance thresholds for onchocerciasis mapping or for deciding to stop mass distribution of ivermectin. To improve diagnostic accuracy, we propose investigating novel diagnostic targets including OVRV1 proteins.