Post-Validation Survey in Two Districts of Morocco after the Elimination of Trachoma as a Public Health Problem

Abstract

Trachoma is the leading infectious cause of blindness. In 2016, Morocco was validated by WHO as having eliminated trachoma as a public health problem. We evaluated two previously endemic districts in Morocco for trachomatous inflammation—follicular (TF), trachomatous trichiasis (TT), and antibodies against Chlamydia trachomatis, the causative agent of trachoma. Community-based cross-sectional surveys in the districts of Boumalene Dades and Agdez included 4,445 participants for whom both questionnaire and serology data were available; 58% were aged 1–9 years. Participants had eyes examined for TF and blood collected for analysis of antibodies to the C. trachomatis antigen Pgp3 by both a multiplex bead assay (MBA) and lateral flow assay (LFA). Seroconversion rates (SCR) per 100 people per year were used to estimate changes in the force of infection using Bayesian serocatalytic models. In Agdez, TF prevalence in 1–9-year-olds was 0.3%, seroprevalence ranged from 9.4% to 11.4%, and SCR estimates ranged from 2.4 to 3.0. In Boumalene Dades, TF prevalence in 1–9-year-olds was 0.07%, and modeling data from the different assays indicated a decrease in transmission between 20 and 24 years ago. The TF data support an absence of active trachoma in the two districts examined. However, seroprevalence and SCR in younger people were higher in Agdez than Boumalene Dades, showing that there can be differences in serology metrics in areas with similar TF prevalence. Data will be included in multicountry analyses to better understand potential thresholds for serological surveillance in trachoma.

Monitoring transmission intensity of trachoma with serology

“…We compiled serological, molecular, and clinical measurements collected between 2012 and 2019 across nine studies in five countries (Supplementary Table 1) 4,[25][26][27][28][29][30][31][32] . When possible, measurements within the same study were stratified into populations corresponding to trachoma evaluation units, resulting in 14 study populations.…”

“…We excluded clusters with fewer than 15 children measured to ensure sufficient information to estimate cluster level means (n=22 clusters excluded, reduced from 481 to 459). Details of serological, clinical, and PCR measurements can be found in the published reports for each study 4,[25][26][27][28][29][30][31][32] . In all but one study, dried blood spots were analyzed for IgG antibodies to Pgp3 using a multiplex bead assay on a Luminex platform and were reported as median fluorescence intensity minus background (MFIbg), and seropositivity cutoffs were generated using receiver operator characteristic (ROC) methods 14 .…”