Assessing morbidity and burden due to neglected tropical diseases at different geographical scales

Background: The neglected tropical diseases (NTDs) comprise of a group of infections, that primarily occur in impoverished communities and may impair the course and outcome of pregnancy, childhood growth, intellectual development, education and worker productivity. Long-term chronic infection may result in disfigurement and stigmatization. Hence, the NTDs may exacerbate poverty, but because there are only limited commercial opportunities, the fight against these conditions has been widely neglected. Amongst others, helminth infections such as schistosomiasis, soil-transmitted helminthiasis and food-borne trematodiasis belong to the NTDs.
For better quantifying and prioritizing the health burden caused by diseases, injuries and risk factors, the World Health Organization and the World Bank developed the disability-adjusted life year (DALY) metrics. The increasing popularity of DALYs and burden of disease estimates sparked also renewed interest in the underlying epidemiological parameters. For conditions like the NTDs, which mainly cause morbidity rather than mortality, the assessment of the average disability incurred by a diseased individual is crucial for correctly compiling global burden estimates. Interestingly, anamnestic questionnaires to assess morbidity due to helminth infections had been used before in attempts to better gauge helminth control efforts.
Objectives: The overarching goal of this PhD thesis was to develop, validate, and apply tools for assessing the morbidity and burden caused by NTDs at different geographical scales. Specific objectives were: (i) to participate in the global burden of diseases, injuries and risk factors study 2010 (GBD 2010) and assess – for the first time – the global burden of food-borne trematodiasis; (ii) to generate new evidence on the disability incurred by individuals infected with schistosomes and/or soil-transmitted helminths in field-based epidemiological investigations in rural Côte d’Ivoire and compare the results with the most recent disability weights of the global burden of disease study; and (iii) to explore the potential of simple and low-cost anamnestic questionnaire tools for the assessment of morbidity due to schistosomiasis and soil-transmitted helminthiasis in the recently established Taabo health demographic surveillance site (Taabo HDSS) in Côte d’Ivoire in order to identify high-risk groups and guide control measures.
Methods: To tackle the first objective, a systematic review and meta-analysis was conducted and the global burden of food-borne trematodiasis assessed according to the GBD 2010 guidelines. Regarding the second objective, two cross-sectional surveys were carried out in 2010, one with 156 schoolchildren in south Côte d’Ivoire and one with 187 adults in the Taabo HDSS, south-central Côte d’Ivoire. Participants were parasitologically tested for helminth and Plasmodium infections, clinically examined, interviewed with quality of life questionnaires and the schoolchildren also invited to participate in a maximal multistage 20 m shuttle run test. The different test results were then juxtaposed to each other. For the third objective, 187 adults participating in the second annual parasitological survey of the Taabo HDSS in 2010, and 146 children and 439 adults participating in the third annual parasitological survey in 2011 were interviewed with anamnestic questionnaires containing questions about risk factors, signs and symptoms. The questionnaire results were compared with the participants’ parasitological results to evaluate the diagnostic properties of single responses and response combinations.
Results: The estimates on the global burden of human food-borne trematodiasis indicate that 56.2 million people were infected with food-borne trematodes worldwide in 2005, 7.9 million suffered from severe sequelae and 7,158 died due to cholangiocarcinoma and cerebral infection. These figures result in a global burden of 665,352 DALYs. Regarding the disability measurements, no effect of schistosomiasis and soil-transmitted helminthiasis on schoolchildren’s physical fitness could be identified, irrespective of whether objectively measured shuttle run test results or questionnaire results on self-reported physical fitness were considered. However, statistically significant correlations between the children’s shuttle run test and questionnaire results were found. In contrast, infected adults reported a significantly lower quality of life than non-infected adults. The results of the anamnestic questionnaire in the 2010 study revealed no promising diagnostic properties. Analyses of the 2011 data are still underway.
Conclusions: Despite disclosing many unsolved issues, food-borne trematodiasis was identified as an important cluster of NTDs and in most endemic areas, control efforts are urgently needed. The results on schistosomiasis and soil-transmitted helminthiasis associated disability are ambiguous, but the significant correlation between objectively measured and self-reported physical fitness of schoolchildren encourages further application of quality of life questionnaires. The reduction in adults’ quality of life indicates a stronger negative effect of these helminthiases than currently assumed in most burden compilations. The findings regarding the anamnestic questionnaires should be interpreted as preliminary results and even though the results were not promising, the potential benefits of refined anamnestic questionnaire tools warrant further efforts. The need for further innovation, validation and application of systematic, truly interdisciplinary, field-based, epidemiological methods to gather additional evidence was emphasized and potential perspectives outlined throughout the thesis.