The Decoy Effect in the Snail-Schistosome System Public

Abstract

The presence of non-host species can divert parasites away from susceptible host individuals, reducing parasite transmission, a process termed the “decoy effect”. Previously, we found that the body size of host snails susceptible to human schistosomes influenced their exposure rate and susceptibility to infection. Therefore, we investigated whether the body size of non-host decoy snails could affect their exposure rates, thereby influencing transmission in snail communities. We examined two hypotheses: (1) that non-host snails would significantly act as decoys to disrupt schistosome transmission to host snails and (2) that the strength of the decoy effect would increase with increasing body size of the non-host species. We conducted transmission trials with Biomphalaria glabrata as a host species and Helisoma anceps as a non-host species to quantify these decoy effects. Populations with greater body sizes of non-host snail species had lower prevalence of infection in host snails. We then created a size-dependent transmission model for non-host snails that incorporated body size of non-host snails, susceptibility of host snails, and exposure of host snails. Fitting this model to our data indicated that (1) schistosomes cannot effectively distinguish host and non-host snails, resulting in decoy effects, and (2) larger decoys exerted stronger decoying, consistent with our predictions. This study demonstrates the potential for body size-dependent decoying in the snail-schistosome system and raises the possibility that the body size structure of host communities could influence parasite transmission more broadly.

Table of Contents

Introduction…………………………………………………………….…………………………1

Methods……………………………………………………………….………………...…………4

Snail Maintenance…………………………………………………………..……4 Transmission Trials……………………..……………………..…………………4 Infection Diagnosis……………………………………………………..………...5 Generalized Linear Models……………….………………………….………….6 Transmission Model………….……………………………………..……………6

Results………………………………………………………………………………………….....10

GLMs.………………………………………….….……………………..………..10 Transmission Model…………………………………………………...………...10

Figures…………………………………………………………………………………..………...11

Figure 1. Prevalence in Host Snails Decreases as Diameter of Decoys Increases, with Uniform Small Host Snails (Decoy Size of 0 mm)………..……..………………………..11 Figure 2. Frequency of the Ratios of Exposure Rates of Non-Hosts Compared to the Exposure Rates of Hosts…………...……………………………...….….….....……………12 Figure 3. Posterior Density Estimates for Body Size Dependence of Exposure Rates of Decoys.……….……………………………………………….….……...………………….13 Figure 4. Parameter Estimates for Non-Host Relative Exposure Rates Compared to Non-Host Body Size…………………………………………………………….……………..14

Discussion……………………………………………………………………………………….15

References……………………………………………………………………………………….18

About this Honors Thesis

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School	Emory College
Department	Human Health
Degree	B.A.
Submission	Honors Thesis
Language	English
Research Field	Biology, Ecology Health Sciences, General Biology, Parasitology
Mot-clé	schistosomiasis decoy effect parasite transmission size structure
Committee Chair / Thesis Advisor	Civitello, David, Emory University
Committee Members	de Roode, Jaap, Emory University Noble, Don, Emory University

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