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Adults of the flatworm Ribeiroia ondatrae are gut parasites of aquatic birds. The eggs of the worms are released to the water through bird's feces and then hatch in a free living form that infects water snails. After a series of stages inside the snail, another swiming form –the cercaria– is shed and infects tadpoles. The tadpoles eventually develop in adult frogs which usually have malformed limbs that make them more vulnerable to predators. Aquatic birds eat such frogs with encysted parasites and then the cycle proceeds.

There is a growing concern that human-induced environmental changes are increasing the incidence of malformation in amphibians caused by Ribeiroia ondatrae. For instance, Johnson et al. (2007) showed how eutrophication can boost tadpole infection. The addition of dissolved nutrients to controled aquatic mesocosm enhanced the algae on which the snails feed. The surplus of food increased growth, reproduction and survival of snails, and thus the availability of intermediate hosts for Ribeiroia ondatrae. Also, nutrient addition enhanced the rate of cercaria shed by each snail.

Propose an epidemiological model that describes the proposed effects of eutrophication on the incidence of tadpole infection by Ribeiroia ondatrae.

• What is the minimal model that can predict the effects proposed by Johnson et al. (2013)?
• Does this model provide cues on how to control the disease?
• Does this model predict the risk of epidemics? Does eutrophication increase this risk?

• Johnson, P. T., Chase, J. M., Dosch, K. L., Hartson, R. B., Gross, J. A., Larson, D. J., … & Carpenter, S. R. (2007). Aquatic eutrophication promotes pathogenic infection in amphibians. Proceedings of the National Academy of Sciences, 104(40), 15781-15786. link

• group_3.pdf