Reduced transmission of human schistosomiasis after restoration of a native river prawn that preys on the snail intermediate host

Abstract

Eliminating human parasitic disease often requires interrupting complex transmission pathways. Even when drugs to treat people are available, disease control can be difficult if the parasite can persist in nonhuman hosts. Here, we show that restoration of a natural predator of a parasite's intermediate hosts may enhance drug-based schistosomiasis control. Our study site was the Senegal River Basin, where villagers suffered a massive outbreak and persistent epidemic after the 1986 completion of the Diama Dam. The dam blocked the annual migration of native river prawns (Macrobrachium vollenhoveni) that are voracious predators of the snail intermediate hosts for schistosomiasis. We tested schistosomiasis control by reintroduced river prawns in a before-after-control-impact field experiment that tracked parasitism in snails and people at two matched villages after prawns were stocked at one village's river access point. The abundance of infected snails was 80% lower at that village, presumably because prawn predation reduced the abundance and average life span of latently infected snails. As expected from a reduction in infected snails, human schistosomiasis prevalence was 18 ± 5% lower and egg burden was 50 ± 8% lower at the prawn-stocking village compared with the control village. In a mathematical model of the system, stocking prawns, coupled with infrequent mass drug treatment, eliminates schistosomiasis from high-transmission sites. We conclude that restoring river prawns could be a novel contribution to controlling, or eliminating, schistosomiasis.

Keywords: control; disease; ecology; elimination; neglected tropical disease.

Fig. 1.

(A) Adult M. vollenhovenii prawn. (B) Evidence of prawn predation via characteristic damage on snail shells (arrows). (C) Net enclosure for prawns at Lampsar village.

Fig. 2.

Relative density of snails after prawns were installed at the intervention site (w/prawns) and control site (no prawns) from October 2012 to July 2013; (A) total Bulinus globosus, (B) total Bulinus truncatus, (C) B. globosus shedding schistosome cercariae, and (D) B. truncatus shedding schistosome cercariae.

Fig. 3.

BACI of prawns on the intensity of S. hematobium infection (GM egg burdens) among participants at the intervention and control sites.

Fig. 4.

Effect of prawn density on number of shedding snails (A), human worm burden (B), total snail population (C), and human infection prevalence (D) in response to an average density of prawns ranging from 0 to 100 prawns per 200 m².

Fig. 5.

Trajectory of human schistosomiasis worm burdens and prevalence in a local population of 1,000 individuals under three control scenarios: MDA using praziquantel alone yearly for 5 y with 80% coverage (A and D), prawns maintained at a density of 0.25 per square meter for 20 y (B and E), or prawns maintained at a density of 0.25 per square meter and MDA applied in years 2 and 4 only (C and F). Thick, solid lines indicate the treated population (those participants who received praziquantel); thin, solid lines indicate the untreated population (those participants who did not receive praziquantel); and dashed lines indicate the mean population (including treated and untreated individuals). Arrows show the timing of MDA.