To ‘eradicate’ a disease is to reduce, through deliberateefforts, the incidence of infection to zero worldwide (Box 1; Keenan et al., 2013). The possibility ofdisease eradication was first acknowledged in 1801, when Edward Jennerdeveloped a vaccine which would provide immunity to smallpox. Yet it took 170years for smallpox to be certified as ‘eradicated’, and to date, it is the onlyhuman disease to have been successfully eradicated, despite efforts toeradicate others such as Guinea Worm and Poliomyelitis (CDC, 2008).
Thishighlights two important points:(1) Worldwide eradication of a disease is not a quick process.(2) Worldwide eradication of a disease is not an easy process. Amongst the diseases currently targeted for eradication isSchistosomiasis. In fact, the 65th World Health Assembly set 2025 asthe target date for elimination of Schistosomiasis as a public health problem(WHO, 2012). However eradication of Schistosomiasis has been the subject ofdebate for at least 30 years with little progress, and the disease presentseven greater challenges than did smallpox since there is currently no Schistosomiasisvaccine . This begs the question: Is worldwide eradication of Schistosomiasis arealistic possibility? Three intricately linked concepts, ‘elimination’,’eradication’ and ‘control’, are essential to answering this question and aredefined in Box 1. Box 1 Figure 1 WHAT ISSCHISTOSOMIASIS?Schistosomiasis is the second most important parasiticdisease after Malaria (Lewis and Tucker, 2014) and one of 17 Neglected TropicalDiseases (NTDs) listed by the World Health Organisation (WHO) (Savioli et al., 2017).
The disease is caused bydigenetic trematodes of the genus Schistosoma(Colley et al., 2014). Sixdifferent species of Schistosoma maycause Schistosomiasis, but the majority of human cases are infected with threespecies, namely Schistosoma haematobium,S. mansoni and S. japonicum (Bergquist etal., 2017). WHY ERADICATE THISDISEASE?Schistosomiasis as a substantial health burdenIn 2015, the global burden of Schistosomiasis was estimatedat 2.6 million disability-adjusted life years (DALYs) (GBD, 2015 cited in Bergquistet al.
, 2017). Reports have indicatedthat over 200 million people worldwide are infected (Siqueira et al., 2017) with an additional 779million at risk of acquiring the disease (Molehin et al., 2016). The disease causes an estimated 200,000 deaths everyyear (Lewis and Tucker, 2014).
Consequences of infection include anaemia, stunted growth, impairedcognition, decreased functional work capacity, liver abnormalities, sub-fertility,decreased physical fitness and potentially fatal complications such as portalhypertension, renal failure and bladder cancer (Colley et al., 2014; Burnim, Ivy and King, 2017; Othman and Soliman, 2015).Schistosomiasis as a substantial economic burden Schistosomiasis may be considered a disease of poverty(Figure 2). A World Bank analysis indicated that the majority of thesub-Saharan population survives on $1.25-2 per day (Adenowo et al., 2015). Coincidentally,sub-Saharan Africa accounts for over 90% of the world’s cases ofSchistosomiasis (Siqueira et al.
,2017). Although there is no current estimate of the global economic burden ofSchistosomiasis, a study in 1972 indicated that resource loss attributable toreduced productivity amounted to over US $6 million (Wright, 1972). This figureexcluded the cost of public health programmes, medical care and compensationfor illness, therefore the true economic burden of Schistosomiasis is likely tobe many times higher. Schistosomiasis as a global threatDespite decades of integrated control measures with improvedsanitation and hygiene, the disease is spreading into new areas of the globe,as indicated by recent reports of infection in Europe (Molehin et al., 2016).IS ERADICATION OFTHIS DISEASE A REALISTIC POSSIBILITY?While the ultimate achievement of control is eradication,diseases that are controlled are not always eradicated. Indeed, according to Dowdleand Cochi, eradication of a disease is only feasible if a “constellation” offour conditions exists. These are (1) biological feasibility, (2) adequatepublic health infrastructure, (3) sufficient funding and (4) substantialpolitical/societal will (Dowdle and Cochi, 2011).
Biological feasibility To address recent concerns about the emergence of resistanceto Praziquantel (PZQ), the drug most commonly used for the treatment ofSchistosomiasis (Siqueira et al.,2017), efforts have focused on finding alternative strategies to fightingSchistosomiasis. A number of molluscicides have been developed to help decreasedisease transmission (Duval et al.,2015), since Schistosoma developmentdepends on the presence of its intermediate host, freshwater snails of thegenera Biomphalaria, Oncomelania and Bulinus (Pila et al.
,2017; Stothard et al., 2017).Interestingly, various microbial pathogens, such as Candidatus Paenibacillus glabratella,have been shown to induce snail mortality, and therefore show potential asbiocontrol agents to reduce snail populations (Duval et al., 2015).Although a Schistosomiasis vaccine does not currently exist,evidence in both humans and animal models suggests that developing an effectivevaccine for Schistosomiasis control and elimination is feasible (Molehin et al.
, 2016). In fact, a few vaccinecandidates are currently under development. S.haematobium 28-kd Glutathione-S-transferase (Sh28GST), S. mansoni fatty acid binding protein (Sm14) and S.
mansoni tetraspanin (Sm-TSP-2) haveentered human clinical trials (Tebeje etal., 2016). With modern tools and a renewed interest in the elimination of Schistosomiasis,there is increasing hope that a vaccine will be developed to help eliminate thedisease. Sufficient funding/Political and societal willPolitical and societal will with regard to Schistosomiasis hasincreased substantially in the last 15 years. Established in 2002, theSchistosomiasis Control Initiative (SCI) successfully instigated nationalcontrol programmes in Zambia, Mali, Burkina Faso, Niger and Uganda (Adenowo et al.
, 2015). In 2008, theSchistosomiasis Consortium for Operational Research and Evaluation (SCORE) wasestablished, its main focus being on control and elimination of S. haematobium and S.
mansoni infections in sub-Saharan Africa (Burnim, Ivy and King,2017). Most recently, in 2012, theGlobal Schistosomiasis Alliance (GSA), the biggest partnership yet, was set up (Savioliet al., 2017).
The GSA is apartnership of endemic countries, academic and research institutions,international governmental and non-governmental agencies, foundations,organisations and private sector companies (Savioli et al., 2017). Also in 2012, the 65th World Health Assembly(WHA) adopted resolution WHA65.21 which calls for elimination ofSchistosomiasis through regular treatment of 75% of school-age children inat-risk areas, through intensification of control programs and throughinitiation of elimination campaigns where possible (WHO, 2012). A number of pharmaceutical manufacturers have pledged tocontinue large medicinal donations of PZQ. Amongst them, Merck Serono, aScience and Technology company, has committed to donate PZQ untilSchistosomiasis is eliminated, increasing its annual production of PZQ tabletsfrom 25 million to 250 million (Colley etal.
, 2014). The cumulative value of all these donations, US $17.8 billionbetween 2014 and 2020, represents the greatest growing public health donation (Savioliet al., 2017). Public health infrastructurePublic health infrastructure, particularly provision of safewater and sanitation, is key to any Schistosomiasis control initiative. Indeed,while potable water reduces the risk of transmission, good sanitation andhygiene practices reduce the risk of water contamination by parasite eggs foundin human excreta (WHO, 2012). Unfortunately, lack of public health infrastructure inendemic countries is a major obstacle to successful eradication of Schistosomiasis(WHO, 2012).
However, this does not mean that eradication is impossible, as strongerpolitical and societal support for eradication of Schistosomiasis is likely todrive further funding in this area. Elimination is already under waySchistosomiasis has already been controlled and eveneliminated in previously endemic countries, demonstrating the feasibility andreal possibility of eradication (Figure 2). In China, Schistosomiasis was madea public health priority in the 1950s, as the disease was seen to be a majorobstacle to rural development (Savioli etal., 2017). After more than 60 years of dedicated planning andinterventions, the number of infected humans in the country was reduced fromaround 12 million to less than 100, 000 (Bergquist et al., 2017). Japan also achieved elimination through long-term,uninterrupted activities, the last new human infection being reported in 1977(Bergquist et al., 2017).
Othercountries, such as Morocco, are close to elimination of the disease (Bergquist et al., 2017).Certain countries already stand out as possible targets forelimination due to the restricted geographic distribution of the disease. Forinstance, in Indonesia, the majority of cases of human Schistosomiasis occur inthree, small isolated valleys in Central Sulawesi (Bergquist et al., 2017). The disease is similarlyconstrained in Cambodia and Lao People’s Democratic Republic of Lao. Eliminationin these countries seems highly feasible (Bergquist et al.
,2017). CONCLUDING REMARKSEradication of Schistosomiasis may not be an immediately realisticpossibility, but eradication certainly is possible. For the first time, the establishment of theGSA and the adoption of WHA65.21 is proof that there is gereral, worldwideconsensus regarding the possibility of eradication. Improvements in sanitationand hygiene, and the development of a vaccine will no doubt accelerate theprocess of eradication.