Authors

Discussion

Background: 

The association between onchocerciasis and epilepsy has been documented for nearly a century; yet both the epidemiological evidence and aetiology of onchocerciasis-associated epilepsy (OAE) remain inconclusive. A retrospective cohort study recently conducted in the Mbam Valley of Cameroon (25 years after a baseline epidemiological study) established, for the first time, a dose-dependent relationship between OAE incidence and Onchocerca volvulus microfilarial (mf) load during childhood (5-10 years of age). We, therefore, modified the EPIONCHO-individual based model (IBM), a stochastic age-and sex-structured model of onchocerciasis transmission, to include this relationship, compare model outputs with those observed in Cameroon, and investigate the impact of mass drug administration (MDA) with ivermectin on OAE prevalence and incidence.

Methods: 

The model was calibrated to reflect the baseline transmission conditions of the Mbam Valley and capture the age distribution of mf load in the retrospective cohort. An Annual Biting Rate (ABR) of 41,922 vector bites/person/year was chosen as the minimum value representative of such conditions. A probability of OAE onset was assigned to those aged 3-10 years in the simulated model population. Twenty-five years of annual ivermectin treatment (to those aged ≥5 years) were simulated, both with standard therapeutic coverage (65%) and high coverage (80%) for a range of ABRs.

Results/Discussion: 

When run to equilibrium, EPIONCHO-IBM (ABR = 41,922) yielded an OAE prevalence of 5.6% in the overall population and of 7.2% in the age group (30-35 years) investigated 25 years after the baseline epidemiological study. Sensitivity analyses on both input ABR and individual exposure heterogeneity indicated that an ABR of approximately 125,000–150,000 would be needed to match the 8.2% OAE prevalence recorded in the Mbam Valley. Upon implementation of ivermectin MDA, the incidence of OAE decreased in the treated population, but elimination (at representative ABR values) was not achieved. This was in part due to the OAE burden held by children aged 3-4- years, who are not currently eligible for ivermectin treatment.

Conclusion:

Considering recent studies demonstrating safety and efficacy of ivermectin in young children, this work contributes to calls for the need of lowering the age for ivermectin treatment if OAE elimination is sought to be achieved in hyperendemic communities.