Modelling anti-Ov16 seroprevalence for the control and elimination of onchocerciasis

Abstract

The World Health Organization aims to verify elimination of onchocerciasis transmission in 12 endemic countries by 2030. Serological assays detecting IgG4 antibodies against the Onchocerca volvulus Ov16 antigen guide decision-making regarding start- and stop-mass ivermectin treatment, post-treatment and post-elimination surveillance. Transmission models are crucial for understanding anti-Ov16 IgG4 seroprevalence dynamics. Uncertainty associated with the parasite stage(s) stimulating seroconversion and duration of antibody persistence has hitherto hindered progress. We use the individual-based stochastic transmission model, EPIONCHO-IBM, to test hypotheses regarding the timing of seroconversion and seroreversion. We fit the model to data from (treatment-naïve) Gabon and validate it using data from Togo (four decades of control). The best-fit hypothesis is that seroconversion is elicited by near-patent/patent infection. The duration of antibody persistence remains uncertain. Our findings support the use of mechanistic transmission models to refine serological thresholds for start- and stop-treatment, and aid interpretation of post-treatment and post-elimination surveillance serological signals. Ov16 antibody tests are used for onchocerciasis surveillance. Fitting a transmission model to age-stratified seroprevalence data identified patent infection as the most likely parasite stage leading to seropositivity, indicating ongoing transmission.