Twenty years of fighting soil-borne helminths and schistosome infections in Ethiopia

The most common neglected tropical diseases are soil-borne helminths and schistosomiasis. In Ethiopia alone, they infect around 36 million and 5 million people, respectively. Ethiopia has set ambitious elimination targets. A new systematic review analyzes national progress in this regard over the past 20 years.

This blog post is one in a series of new articles published in the LCNTDR Collection: Advances in Scientific Research for the Control of NTDs, led by the London Center for Neglected Tropical Diseases (LCNTDR). Stay tuned for updates on Twitter @bugbittentweets and @NTDResearch. You can find other articles in the series here.

Soil-borne helminths and schistosomiasis

Soil-transmitted helminths (STH) and schistosomiasis (SCH) are the most common neglected tropical diseases (NTDs) in the world, with 1.5 billion and 240 million people infected worldwide. In Ethiopia, 36 million and 5 million people are currently infected with STH and SCH. STH collectively refers to the roundworm (Ascaris lumbricoides; AL), whipworm (Trichuris trichiura; TT), hookworm (Necator americanus, Ancylostoma duodenale and Ancylostoma ceylanicum; HW) and threadworm (Strongyloides stercoralis; SS). the Schistosoma the blood flukes cause either urogenital (Schistosoma haematobium; SH) or intestinal (Schistosoma mansoni; SM) SCH. Usually, STH and SCH eggs are excreted in the faeces of infected individuals and contaminate soil and water sources. Their presence can therefore be considered as a surrogate marker for poor sanitation and hygiene.

Ethiopia has set ambitious national goals to eliminate STHs and PPBs as public health problems by 2020 and to interrupt their transmission by 2025. To support national efforts to achieve these goals , a recent review presented epidemiological data on the prevalence of HRT and SCH, stratified by region, species, age and diagnosis.

Ethiopian soil-transmitted helminths and schistosomiasis endemicity figures. Prevalence of HRTs and SCH in Ethiopia by age group, number of endemic woreda (districts) and people requiring treatment according to data published in 2017. Based on Maddren, Phillips, Ower et al. (2021).

Breaking down the worms: STH & SCH trends in different parts of Ethiopia

The systematic review of studies published between 2000 and 2020 identified 267 datasets suitable for data extraction. All nine Ethiopian regions reported the prevalence of HRTs and SCH, the majority of which was reported in Amhara (38%). Interestingly, the focus on the sample population has shifted over time from a community perspective to that of school-aged (SAC) and pre-SAC children.

A total of 402,189 stool samples extracted from the 267 datasets informed the resulting parasite prevalence and intensity measurements used for the analysis. The prevalence of parasites has declined over time for most species, as shown in the following graph.

Evolution of the prevalence of HST and SCH between 1994 and 2019. Prevalence of Ascaris lumbricoides (AL; a), Trichuris trichiura (TT; b), hookworm (HW; c), Strongyloides stercoralis (SS; d), Schistosoma mansoni (SM; e) and Schistosoma haematobium (SH; f) infections between 1994 and 2019. Pearson’s correlation coefficient (r; upper right corner of each graph with associated p-value) was used to measure linear correlation between the prevalence of the parasite and the date of the study. Study populations, differentiating preschool children (pre-SAC), SAC populations and study populations at the community level, are indicated by different colors, while the point size indicates the size of the study population. (see legend) (g). The trend line has not been weighted by the sample size. Note that the range of the y-axis of the SS graph is from 0 to 70 and differs from that of the other plots.

The most statistically significant decrease was demonstrated by TT (34–2%), while a significant reduction was also observed in SM (45–14%), AL (34–11%) and SH (35–6 %).

The second graph below shows the prevalence over time stratified geographically, highlighting regional successes and those that require more attention. The greatest parasite reduction is observed in Amhara, where TT, SM and AL were reduced by 99%, 83% and 80% respectively, and SNNPR where HW, TT, SM and AL were reduced by 98%, 94%, 86 %, and 80% respectively. A significant increase in prevalence was observed only in Tigray for HW.

a – e Regional change in prevalence in Amhara (yellow), Oromia (red), Southern Nations, Nationalities and Peoples Region (SNNPR; turquoise) and Tigray (gray). Change in prevalence of AL (a), TT (b), HW (c), SS (d) and SM (e) infections between 1994 and 2019, stratified by region for Amhara (n = 102), Oromia (n = 67), SNNPR (n = 59) and Tigray (n = 21). Pearson’s correlation coefficient (upper right corner of each graph with associated p-value) was used to measure the linear correlation over time between parasite prevalence and date of study.

Breaking down the worms: STH & SCH trends in different age groups in Ethiopia.

In particular, the historical SAC-centric mass drug delivery (AMM) is reflected in the age-specific distribution of parasite burden prevalence. LA and TT showed the most pronounced deviation from the typical distribution of infection, as higher parasite loads were observed in adults compared to pre-SAC, SAC and adolescents. HW showed higher infection levels than expected in SAC, but remained generally high in adults. This mainly demonstrates the interaction of natural distributions of prevalence by age on the effectiveness of MAID. AL and TT are normally found in higher loads in the pre-SAC and SAC age groups, which decreases in adulthood. Therefore, SAC-based MDA has been successful in reducing the parasite load of SAC while simultaneously creating a reservoir of infection in adults.

Bringing down worms: the diagnostic need to progress towards elimination

The overall decrease in prevalence is a testament to the intensive efforts of the Ethiopian Institute of Public Health to eliminate HRTs and SCH. However, as parasite prevalence levels decline, the sensitivity of the diagnostics and epidemiological measures employed will need to increase in order to accurately report national progress. Due to the non-linear relationship between prevalence and mean intensity of infection, in which the prevalence changes negligibly at high intensities and dramatically at lower intensities, the use of prevalence as a parameter predominant epidemiologic should be re-examined. Despite this, throughout the reviewed literature, intensity was reported in 20-35% of studies to be parallel with prevalence. In order to improve monitoring of Ethiopia’s control program, the Ethiopian Institute of Public Health may consider updating its key statistical definitions to include the intensity measurements recommended by the WHO, and foster an environment where intensity is systematically measured alongside prevalence.

Overall, 19 different diagnostic methods were noted throughout this review. Kato – Katz (35%) and formal ether concentration technique (MADE) (5%) were mainly used alone or in combination (6%). Kato – Katz sensitivity depends on STH intensity. The precision of single slides goes from 74-95% to 53-80% between high and low intensity settings. This sensitivity can increase by double the slides read per sample, 50 to 80% in low intensity settings. A third of the studies reviewed used unique Kato-Katz slides, demonstrating the need to update Ethiopia’s diagnostic protocol to accurately report the low prevalence desired for elimination.

What are the next steps to bring down worms?

Epidemiological data provided by this review suggests that the current CTS-based treatment and control strategy should be extended to community-wide interventions. In doing so, the infection reservoir of adult populations will be targeted, thus reducing reinfection of treated SACs. This will also allow the predominantly adult-centered HW infection to reflect the decreased prevalence observed for LA and TT. As the prevalence of HRTs and SCHs declined, current surveillance would benefit from the use of intensity measurements and better diagnostic sensitivity to accurately capture the success of Ethiopia’s control efforts. It will be crucial to further stratify them by age and sex for the assessment by allowing the identification of pockets of infection within communities. This in turn will generate specific guidelines targeted at age / gender groups to reduce interaction with infectious material through parasite control, WaSH behavior change activities and targeted communication materials. Implementing these changes has the potential to help Ethiopia meet its elimination goals in the near future.

The study featured in this blog post was published in the LCNTDR Collection: Advances in Scientific Research for the Control of NTDs, led by the London Center for Neglected Tropical Diseases (LCNTDR). The collection was publish in Parasites and vectors since 2016, and periodically publishing new articles. This series presents recent advances in NTD scientific research carried out by LCNTDR member institutions and their collaborators. It aims to highlight the wide range of work undertaken by LCNTDR to achieve the United Nations Sustainable Development Goals and to support the goals of the World Health Organization Roadmap for Neglected Tropical Diseases 2021-2030. .

The LCNTDR was launched in 2013 with the aim of providing targeted operational and research support to NTDs. LCNTDR, a joint initiative of Natural History Museum, the London School of Hygiene and 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 control programs.

You can find other blog posts in the series here.

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