RESEARCH Open Access The impact of a single round of community mass treatment with azithromycin on disease severity and ocular Chlamydia trachomatis load in treatment-naïve trachoma-endemic island communities in West Africa Anna R. Last 1* , Sarah E. Burr 1,2 , Emma Harding-Esch 1 , Eunice Cassama 3 , Meno Nabicassa 3 , Chrissy h. Roberts 1 , David C. W. Mabey 1 , Martin J. Holland 1 and Robin L. Bailey 1 Abstract Background: Trachoma, a neglected tropical disease, is caused by ocular infection with Chlamydia trachomatis (Ct). The World Health Organization (WHO) recommends three annual rounds of community mass drug treatment with azithromycin (MDA) if the prevalence of follicular trachoma in 1–9 year olds (TF 1–9 ) exceeds 10% at district level to achieve an elimination target of district-level TF 1–9 below 5% after. To evaluate this strategy in treatment-naïve trachoma- endemic island communities in Guinea Bissau, we conducted a cross-sectional population-based trachoma survey on four islands. The upper tarsal conjunctivae of each participant were clinically assessed for trachoma and conjunctival swabs were obtained ( n = 1507). We used a droplet digital PCR assay to detect Ct infection and estimate bacterial load. We visited the same households during a second cross-sectional survey and repeated the ocular examination and obtained conjunctival swabs from these households one year after MDA ( n = 1029). Results: Pre-MDA TF 1–9 was 22.0% (136/618). Overall Ct infection prevalence (CtI) was 18.6% (25.4% in 1–9 year olds). Post-MDA (estimated coverage 70%), TF 1–9 and CtI were significantly reduced (7.4% (29/394, P < 0.001) and 3.3% (34/1029, P < 0.001) (6.6% in 1–9 year olds, P < 0.001), respectively. Median ocular Ct load was reduced from 2038 to 384 copies/swab (P < 0.001). Following MDA cases of Ct infection were highly clustered (Moran’s I 0.27, P < 0. 001), with fewer clusters of Ct infection overall, fewer clusters of cases with high load infections and less severe disease. Conclusions: Despite a significant reduction in the number of clusters of Ct infection, mean Ct load, disease severity and presence of clusters of cases of high load Ct infection suggesting the beginning of trachoma control in isolated island communities, following a single round of MDA we demonstrate that transmission is still ongoing. These detailed data are useful in understanding the epidemiology of ocular Ct infection in the context of MDA and the tools employed may have utility in determining trachoma elimination and surveillance activities in similar settings. Keywords: Chlamydia trachomatis, Bacterial load, Spatial clustering, Trachoma, Disease severity, Community mass treatment * Correspondence: anna.last@lshtm.ac.uk 1 Clinical Research Department, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT, UK Full list of author information is available at the end of the article © The Author(s). 2017 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Last et al. Parasites & Vectors (2017) 10:624 DOI 10.1186/s13071-017-2566-x