Many studies have suggested that onchocerciasis might be associated with epilepsy. Therefore, we did a cohort study to assess the incidence of epilepsy relative to Onchocerca volvulus skin microfilarial density (MFD) measured during childhood and to assess the possibility of a temporal relationship.

During onchocerciasis surveys undertaken in 25 villages in Cameroon during 1991–93, we measured MFD in individuals aged 5 years or older. In 2017, we revisited seven of these villages. With a standardised five-item questionnaire, we collected information on the occurrence of epilepsy in 856 individuals who were aged 5–10 years in 1991–93, and had MFD determined during the original surveys. We did multivariable analyses to assess the overall incidence and incidence ratios taking into account age, sex, individual MFD in 1991–93, and onchocerciasis endemicity level in the village.

In 2017, we obtained data on the history of epilepsy for 85% (729 of 856) of individuals. Among these individuals, we classified 60 as being suspected cases of epilepsy. The overall incidence of epilepsy was 350 per 100 000 person-years (95% CI 270–450). The adjusted incidence ratio for developing epilepsy was 7·07 (95% CI 0·98–51·26; p=0·0530) in individuals with initial MFD of one to five microfilariae per skin snip (mf per snip), 11·26 (2·73–46·43) in individuals with six to 20 mf per snip, 12·90 (4·40–37·83) in individuals with 21–50 mf per snip, 20·00 (3·71–108·00) in individuals with 51–100 mf per snip, 22·58 (3·21–158·56) in individuals with 101–200 mf per snip, and 28·50 (95% CI 3·84–211·27; p=0·0010) in individuals with more than 200 mf per snip, compared with that of individuals without detectable densities of skin microfilariae.

Individual O volvulus MFD in childhood was associated with the risk of either seizures or epilepsy in an onchocerciasis focus in Cameroon. This temporal relationship suggests a potential causal link between onchocerciasis and epilepsy.

European Research Council, NSETHIO Project.