Soil borne helminths and schistosomiasis are the most commonly neglected tropical diseases. In Ethiopia alone, they infect around 36 million and 5 million people, respectively. Ethiopia has set ambitious goals for eradication. A new systematic review analyzes national progress over the past 20 years.

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

Soil borne helminths and schistosomiasis

Soil borne helminths (STH) and schistosomiasis (SCH) are the most widespread neglected tropical diseases (NTD) worldwide. It is estimated that 1.5 billion and 240 million people are infected worldwide. In Ethiopia, 36 million and 5 million people are currently infected with STH and SCH. STH collectively refers to roundworm (A.scaris lumbricoides;; AL), whipworm (Trichuris trichiura;; TT), hookworm (Necator americanus, Ancylostoma duodenale and Ancylostoma ceylanicum;; HW) and roundworm (Strongyloides stercoralis;; SS). The Schistosoma Leeches either cause urogenital (Schistosoma haematobium;; SH) or intestine (Schistosoma mansoni;; SM) SCH. In general, STH and SCH eggs are excreted in the feces of infected individuals and contaminate soil and water sources. Their presence can therefore be seen as a substitute for poor hygiene and sanitation.

Ethiopia has set ambitious national targets to eradicate STH and SCH as public health problems by 2020 and to suspend their transmission by 2025. To support national efforts towards these goals, a recent review presented epidemiological data on GH and SCH prevalence stratified by region, type, age, and diagnosis.

Ethiopian soil borne helminths and schistosomiasis endemicity numbers. STH and SCH prevalence in Ethiopia by age group, number of endemic woreda (districts) and people in need of treatment, according to data published in 2017. From Maddren, Phillips, Ower et al. (2021).

Killing the worms: STH & SCH trends in different regions of Ethiopia

The systematic review of published studies between 2000 and 2020 identified 267 data sets that are suitable for data extraction. All nine Ethiopian regions reported STH and SCH prevalence, most of which were reported in Amhara (38%). Interestingly, over time, the focus of the sample population has shifted from community-wide perspectives to school-age children (SAC) and pre-SAC.

A total of 402,189 stool samples from the 267 data sets provided information on the resulting parasite prevalence and intensity measurements that were used for the analysis. Parasite prevalence decreased over time in most species, as shown in the graph below.

Change in the prevalence of STH 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. The Pearson correlation coefficient (r; upper right corner of each plot with its p-value) was used to measure the linear correlation between parasite prevalence and study date. Study populations that differentiate between pre-school children (Pre-SAC), SAC, and community-wide study populations are indicated by different colors, while the point size indicates the size of the study population (see legend) (g). The trend line was not weighted according to sample size. Notice that the y-axis range of the SS chart is from 0 to 70 and is different from that of the other charts.

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

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

a – e Regional change in prevalence in Amhara (yellow), Oromia (red), southern nations, nationalities and peoples (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). The Pearson’s correlation coefficient (top right corner of each graph with the associated p-value) was used to measure the linear correlation over time between the parasite prevalence and the study date.

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

In particular, the historical SAC-focused mass drug administration (MDA) is reflected in the age prevalence distribution of the parasite load. AL and TT showed the greatest deviation from the typical infection distribution, since higher parasite loads were observed in adults compared to pre-SAC, SAC and adolescents. HW showed higher than expected rates of infection in SAC but typically remained high in adults. This mainly shows the interaction of natural age prevalence distributions with MDA effectiveness. AL and TT usually occur at higher levels in age groups prior to SAC and SAC, which decrease in adulthood. Therefore, SAC-focused MDA has successfully reduced the SAC parasite load while creating a reservoir of infection in adults.

Knock down the worms: the diagnosis must progress to elimination

The general decline in prevalence is evidence of the intensive efforts of the Ethiopian Public Health Institute to eliminate STH and SCH. However, if the parasite prevalence decreases, the sensitivity of the diagnostic and epidemiological measurements used needs to be increased in order to accurately report national progress. Because of the non-linear relationship between prevalence and mean infection intensity, with prevalence changing negligibly at high intensities and dramatically changing at lower intensities, the use of prevalence as the dominant epidemiological parameter should be re-examined. Nevertheless, in all of the literature reviewed, the intensity was given alongside the prevalence in 20-35% studies. To improve monitoring of the control program in Ethiopia, the Ethiopian Public Health Institute could consider updating its key statistical definitions to include WHO-recommended intensity measurements and creating an environment where intensity is routinely measured alongside prevalence becomes.

A total of 19 different diagnostic methods were identified during this review. Kato-Katz (35%) and the formal ether concentration technique (FECT) (5%) were mainly used either alone or in combination (6%). The Kato-Katz sensitivity depends on the STH intensity. The accuracy of individual slides decreases between high and low intensity settings from 74–95% to 53–80%. This sensitivity can be increased by doubling the slides read per sample from 50-80% at low intensity settings. A third of the studies examined used single 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 kill the worms?

The epidemiological data from this review suggest that the current SAC-based treatment and control strategy should be extended to community-wide interventions. In this way, the reservoir of infection in adult populations is targeted, thereby reducing re-infection of treated SAC. This will also allow the predominantly adult-oriented UTI infection to reflect the decline in prevalence observed for AL and TT. As GH and SCH prevalence declines, current surveillance would benefit from the use of intensity measurements and improved diagnostic sensitivity to pinpoint the success of Ethiopia’s control efforts. Further stratification by age and gender will be critical to the assessment by enabling the identification of sources of infection within communities. This in turn will generate specific guidelines targeting age / gender groups to reduce interaction with infectious material through parasite control, activities to modify WasH behavior, and targeted communication materials. Implementing these changes has the potential to help Ethiopia achieve its elimination goals in the near future.

The study presented in this blog post was published in the LCNTDR Collection: Advances in Scientific Research on NTD Control, directed by the London Center for Neglected Tropical Disease Research (LCNTDR). The collection was Publication in Parasites & Vectors since 2016 and regular publication of new articles. This series highlights the recent advances in scientific research for NTDs carried out by LCNTDR member institutions and their staff. The aim is to highlight the broad spectrum of work done by the LCNTDR to achieve the United Nations Sustainable Development Goals and to support the goals of the World Health Organization’s Roadmap for Neglected Tropical Diseases 2021-2030.

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

More blog posts in the series can be found here.


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