Restoring native crustacean predators offers transmission control for a neglected tropical disease, schistosomiasis, in Africa

Background/Question/Methods

Among neglected tropical diseases (NTDs), human schistosomiasis is one of the most widespread and devastating. The disease is easy to treat, using the drug praziquantel, but remains hard to control.  In endemic areas, treated people are rapidly re-infected when they wade or swim in waters that harbor parasite-infected snails. Thus, affordable tools to control transmission are lacking, drugs alone are an unsustainable remedy, and more than 220 million infected people on three continents are in need of a solution.  Here, we present a proof-of-concept for a novel method to control transmission of this NTD by restoring native crustaceans that feed on snails.  We propose that predatory freshwater prawns will lead to substantial disease reductions when restored to the rivers and streams where human-induced landscape changes (like dams and irrigation schemes) have decimated their populations and led to disease outbreaks.  We present field evidence of transmission control by prawns in a Before-After-Control-Impact field experiment and couple this with a model that explicitly incorporates predation on intermediate hosts of the parasites. 

Results/Conclusions

Human schistosomiasis prevalence (presence of eggs in the urine) was significantly lower at the prawn site compared with the control, after prawns were installed; the reverse was true at baseline before prawns (BACI interaction effect, p < 0.0001).  By the study endpoint at 18 months the prevalence of heavy schistosome infections had dropped from 11% to 6% at the prawn site, whereas the rate went up from 4% to 12% at the control site during that same period.  Coupling field results with a mathematical model, we discovered a plausible mechanism by which predation is effective: by reducing the average lifespan of latently-infected snails, thus greatly reducing the probability of their progression into the infectious class and curbing transmission. The model suggests prawns can control transmission by this mechanism, even at stocking densities too low to locally eradicate the snails.  So long as stocking ensures that young, growing prawns remain abundant in the system, our model predicts schistosomiasis could be eliminated from high-transmission sites using prawns coupled with mass drug treatment. This novel ecological remedy could be affordable and sustainable because stocked prawns could eventually be harvested and sold for profit by local communities.  Although we focus on schistosomiasis in Africa, our methods and results may be applicable to other parts of the world and other indirectly transmitted parasites.