Parasite of the month

Ribeiroia ondatrae

This trematode reproduces asexually in ram's horn snails before infecting amphibians where it causes limb malformation. This in turn is supposed to enhance predation by the final hosts, i.e. birds.

Human-induced eutrophication (artificial enrichment of water bodies with nutrients, typically nitrogen and phosphorus) has been shown to facilitate its transmission by altering the interactions between the parasite and the snail (Johnson et al. 2007 PNAS). Indeed, eutrophication promotes the growth of algae and in turn favours populations of grazers such as ram's horn snails. In these conditions, snails occur at high densities and grow bigger. This provides more resources for the parasite and thus facilitate its transmission to and development in the snail. Moreover infected snails were better able to tolerate infection for longer periods of time. This has caused the production of cercariae (stages infective to amphibians) to double in eutrophied waters, generating outbreaks in amphibian populations.

Recently, the impact of global warming has been investigated concerning the transmission of this parasite (Paull & Johnson 2014 Ecology Letters). Using mesocosms, researchers showed that increases in temperature extend the time window for parasite transmission from snails to tadpoles. Yet, the study also revealed that less cercariae are produced during the summer at warmer temperatures compared to under normal conditions. This generates an asynchrony in the presence of cercariae and of amphibian hosts and lowers the overall transmission of the parasite in heated mesocosms. These results indicate that global warming might not be beneficial to all parasites.

It would be interesting to investigate the joined effects of eutrophication and global warming. Indeed, eutrophication is often associated with higher water temperatures. The negative effects of global warming on parasite transmission could then be reduced by the positive effects of eutrophication on the snail-trematode interaction.


source for picture: http://www.science-art.com