The Road to Elimination: Current State of Schistosomiasis Research and Progress Towards the End Game

Abstract

The new WHO Roadmap for Neglected Tropical Diseases targets the global elimination of schistosomiasis as a public health problem. To date, control strategies have focused on effective diagnostics, mass drug administration, complementary and integrative public health interventions. Non-mammalian intermediate hosts and other vertebrates promote transmission of schistosomiasis and have been utilized as experimental model systems. Experimental animal models that recapitulate schistosomiasis immunology, disease progression, and pathology observed in humans are important in testing and validation of control interventions. We discuss the pivotal value of these models in contributing to elimination of schistosomiasis. Treatment of schistosomiasis relies heavily on mass drug administration of praziquantel whose efficacy is comprised due to re-infections and experimental systems have revealed the inability to kill juvenile schistosomes. In terms of diagnosis, nonhuman primate models have demonstrated the low sensitivity of the gold standard Kato Katz smear technique. Antibody assays are valuable tools for evaluating efficacy of candidate vaccines, and sera from graded infection experiments are useful for evaluating diagnostic sensitivity of different targets. Lastly, the presence of Schistosomes can compromise the efficacy of vaccines to other infectious diseases and its elimination will benefit control programs of the other diseases. As the focus moves towards schistosomiasis elimination, it will be critical to integrate treatment, diagnostics, novel research tools such as sequencing, improved understanding of disease pathogenesis and utilization of experimental models to assist with evaluating performance of new approaches.

Assessment of the accuracy of 11 different diagnostic tests for the detection of Schistosomiasis mansoni in individuals from a Brazilian area of low endemicity using latent class analysis

“…Data from the records of the Brazilian national schistosomiasis control programme-the Sistema de Informação do Programa de Controle da Esquistossomose (SISPCE), indicate that the prevalence of schistosomiasis in Brazil between the years 2009 and 2019 ranged from 3.22 to 5.20%, with an average of 4.29% ( BRASIL M da S, 2021 ). Although these percentages may suggest that Brazil is due to meet the World Health Organization (WHO) target of eliminating schistosomiasis as a public health problem (defined as a prevalence of heavy infections lower than 1%) by 2030 ( WHO, 2021 ), it is likely that these data are underestimated because of the microscopy-based diagnostic method used, namely the Kato-Katz (KK) test ( Siqueira et al, 2011 ; Weerakoon et al, 2018 ; Ogongo et al, 2022 ). The KK test consists of the microscopic observation of eggs in fecal samples ( Katz et al, 1972 ) and has a high analytical specificity, low cost, and relatively simple execution.…”

“…For these reasons, this is the method recommended by both the WHO and the Brazilian Ministry of Health for the diagnosis of intestinal schistosomiasis and use in epidemiological surveys. However, the performance of the KK test is limited by the intensity of the host infection, the daily variation of parasite egg excretion and the uneven distribution of eggs within fecal samples ( Ogongo et al, 2022 ). This is especially critical in low endemicity settings, where the KK test may overlook 25–30% of the infected people, underestimating the true prevalence of the disease ( Berhe et al, 2004 ; Enk et al, 2008 ; McManus et al, 2018 ).…”

Development of real-time and lateral flow recombinase polymerase amplification assays for rapid detection of Schistosoma mansoni

“…Determining the precise prevalence of schistosomiasis in a specific area is conditional to a sensitive, specific, reproducible, and accessible diagnostic method. However, diagnostics improvements are very much needed for detection of light infections in low prevalence settings, and for the verification of interruption (Ogongo et al, 2022;World Health Organization, 2022). If good diagnostic tools are not available, the true prevalence of a specific area may be underestimated limiting the efficiency of schistosomiasis control programs (Panzner, 2022).…”

“…It has been extensively used for epidemiological surveys presenting a satisfactory performance in high prevalence settings. However, the KK's sensitivity varies depending on the period of infection, daily fluctuation of egg excretion, uneven distribution of eggs in the stool, endemicity, and/or co-endemicity of the area (Bärenbold et al, 2017;Cavalcanti et al, 2019;Diego et al, 2021;Ogongo et al, 2022). These limitations are mainly observed in moderate and low endemic areas, where 25-30% of positive cases can be missed (Berhe et al, 2004;Enk et al, 2008;McManus et al, 2018).…”

The Role of Praziquantel in the Prevention and Treatment of Fibrosis Associated with Schistosomiasis: A Review

“…Worldwide, it is estimated that over 140 million people are thought to have the disease, with a further 779 million at risk of infection [2]. Although the elimination of this tropical parasitic disease requires a multidisciplinary integrated approach [3][4][5], the control of schistosomiasis still relies on chemotherapy, which has been proven to be e ective in reducing the prevalence and intensity of the parasitic infection [6][7][8].…”