No Apparent Reduction in Schistosome Burden or Genetic Diversity Following Four Years of School-Based Mass Drug Administration in Mwea, Central Kenya, a Heavy Transmission Area

Abstract

BackgroundSchistosomiasis is a debilitating neglected tropical disease that infects over 200 million people worldwide. To combat this disease, in 2012, the World Health Organization announced a goal of reducing and eliminating transmission of schistosomes. Current control focuses primarily on mass drug administration (MDA). Therefore, we monitored transmission of Schistosoma mansoni via fecal egg counts and genetic markers in a typical school based MDA setting to ascertain the actual impacts of MDA on the targeted schistosome population.MethodsFor 4 years, we followed 67 children enrolled in a MDA program in Kenya. Infection status and egg counts were measured each year prior to treatment. For 15 of these children, for which there was no evidence of acquired resistance, meaning they became re-infected following each treatment, we collected microsatellite genotype data from schistosomes passed in fecal samples as a representation of the force of transmission between drug treatments. We genotyped a total of 4938 parasites from these children, with an average of 329.2 parasites per child for the entire study, and an average of 82.3 parasites per child per annual examination. We compared prevalence, egg counts, and genetic measures including allelic richness, gene diversity (expected heterozygosity), adult worm burdens and effective number of breeders among time points to search for evidence for a change in transmission or schistosome populations during the MDA program.FindingsWe found no evidence of reduced transmission or schistosome population decline over the course of the program. Although prevalence declined in the 67 children as it did in the overall program, reinfection rates were high, and for the 15 children studied in detail, schistosome egg counts and estimated adult worm burdens did not decline between years 1 and 4, and genetic diversity increased over the course of drug treatment.InterpretationSchool based control programs undoubtedly improve the health of individuals; however, our data show that in an endemic area, such a program has had no obvious effect on reducing transmission or of significantly impacting the schistosome population as sampled by the children we studied in depth. Results like these, in combination with other sources of information, suggest more integrated approaches for interrupting transmission and significantly diminishing schistosome populations will be required to achieve sustainable control.

Repeated praziquantel treatments remodel the genetic and spatial landscape of schistosomiasis risk and transmission

“…Since nearly everyone with detectable infection in these communities was treated and re-examined, we were able to calculate accurate incidence and reinfection rates. Both progressively declined following treatment, and in contrast to multiple studies in Africa (Huyse et al, 2013; Lelo et al, 2014), these reductions were maintained over a 4 year period with little intervention apart from retreatment. The relative success of this program was likely due to the communities’ small size, higher success rates for research over operational programs, better sanitation compared with much of Africa, education and sensitization of the population to the problem and treatment of all identified infected individuals regardless of age.…”

“…No change was found in parasite allelic richness, heterozygosity, inbreeding coefficient or fixation index. A 4 year mass drug administration program in Kenya showed no change in parasite genetic diversity in children with no decline in infection intensity (Lelo et al, 2014). These studies may not be contradictory since the dynamics of transmission vary between sites.…”

A call to strengthen the global strategy against schistosomiasis and soil-transmitted helminthiasis: the time is now

“…8,9 Unfortunately, even countries that have successfully implemented the recommended preventive chemotherapy strategy for schistosomiasis and STH—i.e., repeated treatment of school-aged children at WHO-recommended ≥75% coverage—have met challenges in achieving optimal morbidity control or the more ambitious goal of transmission elimination. 8,10,11 This finding is consistent with estimates by the Global Burden of Disease (GBD) study and others that have documented how progress has lagged behind for schistosomiasis and STH relative to many other NTDs.…”

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

“…The authors, however, found no significant changes to allelic richness or expected heterozygosity in S. mansoni before and after two rounds of treatment with PZQ amongst this small sample group (Huyse et al, 2013). In addition no reduction in genetic diversity following four years of MDA was observed in parasites from children in Western Kenya (Lelo et al, 2014). Likewise a microsatellite study in Brazil (Blanton et al, 2011) which compared the similarity of S. mansoni populations that survived PZQ treatment with susceptible worms also found no significant difference according to the differentiation index (Jost, 2008).…”