Evolutionary concepts in predicting and evaluating the impact of mass chemotherapy schistosomiasis control programmes on parasites and their hosts

Abstract

SummarySchistosomiasis is a parasitic disease of significant medical and veterinary importance in many regions of the world. Recent shifts in global health policy have led towards the implementation of mass chemotherapeutic control programmes at the national scale in previously ‘neglected’ countries such as those within sub-Saharan Africa. Evolutionary theory has an important role to play in the design, application and interpretation of such programmes. Whilst celebrating the rapid success achieved to date by such programmes, in terms of reduced infection prevalence, intensity and associated human morbidity, evolutionary change in response to drug selection pressure may be predicted under certain circumstances, particularly in terms of the development of potential drug resistance, evolutionary changes in parasite virulence, transmission and host use, and/or competitive interactions with co-infecting pathogens. Theoretical and empirical data gained to date serve to highlight the importance of careful monitoring and evaluation of parasites and their hosts whenever and wherever chemotherapy is applied and where parasite transmission remains.

Evaluation of Intensity of Urinary Schistosomiasis in Biase and Yakurr Local Government Areas of Cross River State, Nigeria after Two Years of Integrated Control Measures

“…Praziquantel is safe and easy to use and it is low cost. The concerns about promoting resistance to Praziquantel by large scale chemotherapy appear to be remote (Utzinger et al, 2009;Webster et al, 2008). Chemotherapy with Praziquantel is the mainstay for the control of the disease in the short term (Useh, 2013).…”

Genetic Consequences of Mass Human Chemotherapy for Schistosoma mansoni: Population Structure Pre- and Post-Praziquantel Treatment in Tanzania

“…2,14,15 There is speculation that untreated parasites or refugia may partly explain the lack of establishment of PZQ resistance, with those parasites present in snails, non-human mammals, and untreated humans providing a pool of susceptible genes that "dilute" any resistance genes in endemic populations. 3 Subdivision in schistosome parasite populations, and the possibility of inherent costs of resistance, have also been suggested as additional explanations for the lack of spread of PZQ resistance. 3 Nevertheless, evidence arising from clinical investigations primarily within Egypt and Senegal, 16 reports of individual failures of PZQ treatment in infected travelers, 17 and results from a range of laboratory studies and artificialselection of resistant lines, 15 give cause for concern regarding potential future changes in the PZQ susceptibility status of schistosome populations in natural foci.…”

“…2,3 Schistosomiasis is one such NTD of profound medical importance across many regions worldwide. 4,5 Schistosomes, the causative agents, are digenean trematodes whose life-cycle alternates sexual reproduction in a mammalian host with asexual reproduction in a molluscan host.…”

Modelling the Effects of Mass Drug Administration on the Molecular Epidemiology of Schistosomes

“…As treatment efforts are stepped-up, stronger selection pressures will be imposed on populations of Schistosoma spp. (Webster et al, 2008.…”

“…Through MDA, intensive and prolonged selective pressures are, and will be, placed on these parasites, which may have implications for the long-term success of campaigns (Webster et al, 2008. Policies to optimise success become crucial as programmes in selected countries shift from morbidity control toward elimination as a public health burden (WHO, 2012).…”