Seasonal Patterns in Typhoid Environmental Surveillance in Ibadan, Nigeria

Abstract

Introduction: Typhoid continues to be of public health concern with a pre-pandemic estimated incidence of 36 per 100,000 person-years in Ibadan, Nigeria where we conduct surveillance. The epidemiology and burden of typhoid fever elsewhere in Nigeria is poorly understood due to inherent diagnostic challenges and low availability of blood culture (BC) services. Environmental surveillance (ES) has been proposed as a useful alternative tool to BC-based surveillance that could be deployed more broadly, but evidence on concordance of these approaches is lacking. The University of Ibadan Typhoid Environmental Surveillance Study (TESS) is piloting typhoid ES, benchmarked against on-going BC surveillance. Methods: ES is being conducted in five urban Local Government Areas (LGAs) of Ibadan metropolis, catchment areas for on-going BC surveillance, and three peri-urban LGAs where perforation cases are commonly recruited. Geographic Information System based on sewage and water drainage patterns was used to identify sampling locations to supplement three existing polio ES sites. Moore swabs and grab samples were collected monthly from selected sites and processed for molecular identification of S. Typhi according to recently developed protocols. Results: Following identification of 45 sampling sites, 805 ES samples were collected between 1st June 2023 and 29th February, 2024. Two (0.5%) of 406 grab samples and 101 (25.3%) of 399 Moore swabs processed were positive for S. Typhi. The median number of positive ES samples overall was 14 in the June-October wet season months but only 7 in the November-February dry season months, with the lowest number of wet-season positives (8) recorded during rainy season's ‘August break’. Salmonella Typhi was detected in every LGA at some point and repeatedly at >2 sites in three urban LGAs sites (Ibadan North, Ibadan Northeast and Ibadan Southwest) during the study period except during the dip in August, 2023. There was no apparent temporal or spatial concordance between ES and blood cultures or perforations but the number of clinical cases was very low with only 22 (2.4%) of 926 blood cultured patients positive for S. Typhi during the same period. Discussion: The high ES positivity rate (25.3%) of Moore swab samples suggests a greater prevalence than can be inferred for blood culture and positive samples are detected more commonly in the wet season. ES does not reflect blood culture-determined temporal and spatial typhoid epidemiology, but benchmarking is confounded by poor access to blood culture and current low positivity rates. Conclusion: ES in Ibadan demonstrates worrisome levels of S. Typhi at drainage sites, emphasizing that a large population is at risk of infection and there is a need to improve WASH and deploy Typhoid Conjugate Vaccines. More data are required to clarify whether ES can improve on or substitute for blood culture-based surveillance