The relationship between expelled eggs, morbidity and age in a Schistosoma mansoni endemic setting in Uganda: Implications for current elimination policies

Direct morbidity assessments are rarely included in monitoring and evaluation of Schistosoma mansoni mass drug administration programmes. This is despite morbidity reduction being the leading objective of control and elimination as a public health problem in the World Health Organization (WHO) targets. Instead, the number of eggs-per-gram (EPG) of faeces are used as a morbidity proxy. Furthermore, current WHO guidelines use infection intensity thresholds to determine where and when MDA is to be implemented. However, recent work has begun to question this assumption of a direct association between infection intensity in intestinal schistosomiasis and host morbidity. Here we aimed to examine the potential association between S. mansoni infection intensity and morbidity from pre-school-aged children (PSAC) through to elderly individuals, living in Bugoto, Uganda. Prevalence and intensities of S. mansoni infection were diagnosed by Kato-Katz and point-of-care circulating cathodic antigen tests (POC-CCAs) in 287 individuals aged 3-74 years, from Bugoto, Uganda. In addition to data on anaemia and self-reported symptoms, abdominal ultrasound examinations were conducted to identify liver parenchyma image pattern (IP), portal vein dilation (PVD) and left parasternal line (PSL) enlargement. Malaria status was determined using rapid diagnostic testing. Generalised additive models estimated associations between morbidity outcomes and infection intensity/presence, diagnostic method, co-infections, age and sex. The prevalence of positive IP scores, dilated PVD, enlarged PSL and anaemia were 9%, 34%, 33% and 13% respectively. Neither S. mansoni infection intensity or status were significantly associated with PVD, PSL, or anaemia. Age was the most consistent predictor of morbidity, with the highest burden of PVD, PSL and anaemia in PSAC. Malaria infection was also positively associated with PVD and anaemia. A positive POC-CCA predicted only self-reported blood in stool. Our findings add to growing evidence that current infection intensity is an inappropriate proxy for schistosomiasis morbidity, urging a revaluation of tools and targets. The observed prevalence of morbidities in PSAC evidence a need to elucidate the impact of less-specific morbidities, past S. mansoni and other parasitic infections on host health, and adds urgency to the on-going roll out of treatment to this age group.