Schistosomiasis

Schistosomiasis remains an endemic parasitic disease causing much morbidity and, in some cases, mortality. Mass drug administration (MDA), using the drug praziquantel, is the main form of control at present, alongside behavior modification and improvement in sanitation. Current work on potential vaccine candidates is promising, but at an early stage at present.

In January 2021, the WHO issued the 2021-2030 road map for neglected tropical diseases (NTDs). The goal for schistosomiasis is elimination as a public health problem, to be reached by 2023 in 49 countries, by 2025 in 69 countries, and by 2030 in 78 countries. This is to be achieved by reducing the heavy-intensity (eggs per gram ≥400) prevalence in school-age children to less than 1%.

The end goal for schistosomiasis is the elimination of transmission, achieved when the incidence of new infections reduces to zero. The NTD road map suggests this should be reached by 2030 in 25 countries.

Can we move from elimination as a public health problem, to elimination of transmission?

We consider low (school-aged children prevalence <10%), moderate (10-50%) and high (≥50%) baseline prevalence settings, with a low or high adult burden of infection. These reflect different pristine transmission settings as defined by the magnitude of the basic reproductive number, R₀.

We find that treating school-aged children only, cannot achieve elimination of transmission within a feasible time frame, regardless of the transmission setting and observed age-intensity of infection profile. Both models show that community-wide treatment is necessary to interrupt transmission in all endemic settings with low, medium, and high pristine transmission intensities.

The required MDA coverage level to achieve either transmission or morbidity elimination depends on the prevalence prior to the start of treatment and the burden of infection in adults. The higher the worm burden in adults, the higher the coverage levels required for this age category through community-wide treatment programmes. Therefore, it is important that intensity and prevalence data are collected in each age category, particularly from school-aged children and adults so that the correct coverage level can be calculated and administered.

It should be noted that, even when the elimination as public health problem goal has been achieved, higher coverage levels among school-aged children and adults are required to move towards the elimination of transmission (Table 1 and Table 2).

Specifically, the coverage among school-aged children varies between 75-95% and the coverage among adults varies between 30-75%, depending on the baseline prevalence and the worm burden in adults.

This is due to the fact that although the heavy-intensity prevalence in school-aged children is reduced to less than 1% (for the elimination as public health problem), the prevalence in children and adults remains high. A large reduction in the prevalence of heavy intensity infection does not necessarily correspond to a large reduction in the prevalence of infection.

After the elimination as public health problem has been achieved, there might be light to moderate intensity infections in school-aged children and light to heavy intensity infections in the other age categories. Therefore, stopping MDA after this goal has been achieved, will lead to a resurgence in infection across all age classes.

The study featured in this blog post was published in the LCNTDR Collection: Advances in scientific research for NTD control, led by the London Centre for Neglected Tropical Disease Research (LCNTDR). The collection has been publishing in Parasites & Vectors since 2016, and releasing new articles periodically. This series features recent advances in scientific research for NTDs executed by LCNTDR member institutions and their collaborators. It aims to highlight the wide range of work being undertaken by the LCNTDR towards achieving the United Nations Sustainable Development Goals as well as supporting the objectives of the World Health Organization road map for neglected tropical disease 2021-2030.

The LCNTDR was launched in 2013 with the aim of providing focused operational and research support for NTDs. LCNTDR, a joint initiative of the Natural History Museum, the London School of Hygiene & Tropical Medicine, the Royal Veterinary College, the Partnership for Child Development, the SCI Foundation (formerly known as the Schistosomiasis Control Initiative) and Imperial College London, undertakes interdisciplinary research to build the evidence base around the design, implementation, monitoring and evaluation of NTD programmes.

You can find other blog posts in the series here.

Table 1: Model recommended treatment strategies for achieving elimination as a public health problem and elimination of transmission (after the achievement of EPHP) for low adult burden of infection. Results for low transmission settings are generated using the ICL model.

Table 2: Model recommended treatment strategies for achieving elimination as a public health problem and elimination of transmission (after the achievement of EPHP) for high adult burden of infection. Results for low transmission settings are generated using the ICL model.