Mapping Schistosoma mansoni endemicity in Rwanda: a critical assessment of geographical disparities arising from circulating cathodic antigen versus Kato-Katz diagnostics

Abstract

BackgroundSchistosomiasis is a neglected tropical disease caused by Schistosoma parasites. Intervention relies on identifying high-risk regions, yet rapid Schistosoma diagnostics (Kato-Katz stool assays (KK) and circulating cathodic antigen urine assays (CCA)) yield different prevalence estimates. We mapped S. mansoni prevalence and delineated at-risk regions using a survey of schoolchildren in Rwanda, where S. mansoni is an endemic parasite. We asked if different diagnostics resulted in disparities in projected infection risk.MethodsInfection data was obtained from a 2014 Rwandan school-based survey that used KK and CCA diagnostics. Across 386 schools screened by CCA (N = 19,217). To allow for uncertainty when interpreting ambiguous CCA trace readings, which accounted for 28.8% of total test results, we generated two presence-absence datasets: CCA trace as positive and CCA trace as negative. Samples (N = 9,175) from 185 schools were also screened by KK. We included land surface temperature (LST) and the Normalized Difference Vegetation and Normalized Difference Water Indices (NDVI, NDWI) as predictors in geostatistical regressions.FindingsAcross 8,647 children tested by both methods, prevalence was 35.93% for CCA trace as positive, 7.21% for CCA trace as negative and 1.95% for KK. LST was identified as a risk factor using KK, whereas NDVI was a risk factor for CCA models. Models predicted high endemicity in Northern and Western regions of Rwanda, though the CCA trace as positive model identified additional high-risk areas that were overlooked by the other methods. Estimates of current burden for children at highest risk (boys aged 5–9 years) varied by an order of magnitude, with 671,856 boys projected to be infected by CCA trace as positive and only 60,453 projected by CCA trace as negative results.ConclusionsOur findings show that people in Rwanda’s Northern, Western and capital regions are at high risk of S. mansoni infection. However, variation in identification of environmental risk factors and delineation of at-risk regions using different diagnostics likely provides confusing messages to disease intervention managers. Further research and statistical analyses, such as latent class analysis, can be used to improve CCA result classification and assess its use in guiding treatment regimes.

Schistosoma haematobium infection and environmental factors in Southwestern Tanzania: A cross-sectional, population-based study

“…also found a positive association between S . mansoni and LST using geostatistical models [ 67 ], negative associations were found in other studies [ 20 , 68 ].…”

“…Regarding LST day in the univariable analysis, we found a positive significant effect only after including the spatial correlation structure. While Clark et al also found a positive association between S. mansoni and LST using geostatistical models [67], negative associations were found in other studies [20,68].…”

“…Sturrock et al [75] also observed significantly decreased odds of S. haematobium infection with increasing EVI in their model without spatial component, which however vanished after including the spatial component into the analyses. Some studies have found a significant positive association between S. haematobium and/or S.mansoni and the Normalized Difference Vegetation Index (NDVI), which is very similar to EVI [19,67,76], whereas in other studies NDVI was not associated with schistosomiasis in the preliminary analyses and thus excluded from further analyses [77,78]. Another study observed a non-linear association between schistosomiasis and NDVI [20].…”

Parasite associations predict infection risk: incorporating co-infections in predictive models for neglected tropical diseases

“…Our findings supported this hypothesis by showing that increased household electricity access correlated with decreased risk of infection for both hookworms and A. lumbricoides. Male children were more likely to be infected with S. mansoni, a result that was also found in a study comparing different diagnostic methods for detecting this parasite in Rwandan children [68]. Whether this reflects increased risky behaviour by male children, or perhaps different cultural expectations regarding contributions to household farming, requires further investigation.…”

Nationwide Remapping of Schistosoma mansoni Infection in Rwanda Using Circulating Cathodic Antigen Rapid Test: Taking Steps toward Elimination

“…The risk zones used proximity to water as a stratifying factor, although data on man-made wetlands and water dams, which may play an important role in transmission, 41 were not available. Other factors such as local sanitation, biological transmission factors, 42 or number of previous rounds of MDA may also contribute to the prevalence observed. Finally, although we used a number of different tests, no test is perfect, with each differing in sensitivity and specificity, and the use of multiple tests can complicate interpretation.…”