Modelling for New Schistosomiasis Treatment Thresholds

Oliver Bärenbold and other Swiss TPH scientists developed a modelling approach t
Oliver Bärenbold and other Swiss TPH scientists developed a modelling approach to translate currently used schistosomiasis prevalence thresholds based on stool microscopy to a new point-of-care urine test. (Photo: Swiss TPH)
Schistosomiasis is a parasitic worm infection affecting 250 million people globally. The current prevalence thresholds for preventive chemotherapy of intestinal schistosomiasis are based on the Kato-Katz method using stool samples. A new more sensitive point-of-care urine test is now available in particular for settings with low prevalence. Swiss TPH was mandated by the World Health Organization (WHO) to translate the existing thresholds to the new diagnostic test using a modelling approach. The findings and recommended thresholds were published today in the open-access journal PLoS Neglected Tropical Diseases.

Schistosomiasis is a neglected tropical disease caused by parasitic flatworms. Globally, 250 million people are infected and almost 800 million people are at risk. For high-risk groups, the World Health Organization (WHO) recommends mass treatment with the drug praziquantel. Depending on the prevalence of the disease, the WHO guidelines recommend different preventive chemotherapy approaches. For example, when prevalence of schistosomiasis is above 50% in a certain setting, all school-aged children should be treated once a year with preventive chemotherapy. In areas of such high prevalence, also adults considered at risk (e.g. fishermen) should be treated.

The prevalence thresholds have been defined for prevalence measured by microscopy using the Kato-Katz technique. Kato-Katz measures the number of Schistosoma mansoni eggs in the stool and thus may not necessarily detect schistosomiasis when there are only a few parasitic worms in the body. The new point-of-care circulating cathodic antigen (POC-CCA) urine test looks at the antigens instead of the number of eggs and can therefore detect even light infections.

New prevalence thresholds recommended

Given the availability of the new diagnostic test, Swiss TPH was mandated by WHO to translate the Kato-Katz thresholds to POC-CCA thresholds. "To relate the two tests, we developed a modelling approach using large data sets and taking into account the diagnostic errors of Kato-Katz and POC-CCA," said Penelope Vounatsou, Head of the Biostatistics Unit at Swiss TPH.

The 50% threshold for duplicate Kato-Katz translates for example to a 60% threshold for POC-CCA. The 10% threshold for Kato-Katz translates to a 30% threshold for POC-CCA. "Given our findings, one might recommend to use POC-CCA in settings of low prevalence which are close to elimination," said Vounatsou. The results and suggested thresholds were published today in the open-access journal PLoS Neglected Tropical Diseases .

"The findings of this elegant modelling work have direct implications for mapping, control and surveillance of schistosomiasis," said Jürg Utzinger, Director of Swiss TPH. "We look forward to continuing the close collaboration with WHO and other partners in designing the best possible approaches to fight parasitic worms globally."

Centre of expertise in parasitic worm infections

Swiss TPH is active in a broad range of activities related to research and control of parasitic worm infections. The work comprises basic research, drug discovery and clinical trials, evaluation of new diagnostic tools, modelling and mapping of risk areas and burden of disease estimates, support of countries in building their control and elimination programmes as well as teaching and training both in Switzerland and in endemic areas. Swiss TPH is a WHO Collaborating Centre for Epidemiology and Control of Helminth Infections.

About the publication

The study was conducted by Swiss TPH in collaboration with the Department of Control of Neglected Tropical Diseases at WHO, the University of Georgia, the Schistosomiasis Control Initiative and other scientists in Europe and Africa.

Bärenbold O. et al. (2018) Translating preventive chemotherapy prevalence thresholds for Schistosoma mansoni from the Kato-Katz technique into the point-of-care circulating cathodic antigen diagnostic test. PLOS Negl Trop Dis. DOI: 10.1371/journal.pntd.0006941