Parasite Found Guilty of Limb Mutilation in Pacific Chorus Frogs

When looking at the organisms of the world, one might not expect that over 50% of them survive by being parasitic either in part or through their whole life. These organisms range from the mundane free-loaders like the Indian pipe flower to the exotic organ replacing tongue louse, which consumes and becomes the tongue of its host. Some even manipulate their hosts so as to allow for their transmission to more preferred hosts. Such parasites include the infamous trematode, which manipulates its intermediate host (e.g. a frog) both physically and behaviourally so as to increase its host’s risk of predation. Once eaten, the parasite breeds within its final host (e.g. a bird) and its eggs are dispersed through its host’s waste. The waste is then consumed by an initial host (e.g. a snail), initiating the start of a new cycle.

Ribeiroia ondatrae Photography Courtesy Pieter Johnson

Current research has shown that exposure to Ribeiroia ondatrae, a type of trematode, causes limb malformations in Pacific chorus frogs (Pseudacris regilla). In studies conducted by Johnson et al. (2011) it was found that the body location and developmental stage at which the frogs were infected significantly affected the degree and type of malformation exhibited, whereas the amount of parasite exposure did not. While the parasite did not recognize frog eggs as a host, infection in the early stages of frog growth resulted in the most dramatic effects such as missing or multiple limbs. In later developmental stages, the effects were less pronounced, and included malformations such as skin fusions and small bone projections. These results were obtained by placing healthy frogs at different developmental stages into parasite-infected waters and monitoring the effects of the parasite on their growth.

Photograph courtesy Dave Herasimtschuk, Freshwaters Illustrated

These malformations affected how the frogs made use of their surrounding environments, as well as their performance within them. Frogs with malformations were found to have reduced jumping ability and accuracy, and thus, used ground habitats with less angled perches instead of their preferred higher and more angled perches, which forced them to live in areas with higher sun exposure. As a result of living in hotter areas, these malformed frogs were likely subjected to temperatures outside of their preferred temperature range, which ultimately increased their likelihood of death through either overheating, dehydration, or predation. Additionally, they allowed simulated predators to approach closer before trying to escape, and covered less distance in their attempts to escape, as compared to normal frogs. Moreover, malformations decreased their ability to swim and forage effectively. These changes made the frogs easier prey, which ultimately increased the likelihood of trematode transmission through host predation. This was shown by sampling both normal and malformed frogs from two ponds and observing their relative performances on these tasks, both in their natural habitat and within a laboratory setting.

The effects of trematodes have also been noted in human populations, and while they may not increase a human’s risk of predation, they can significantly affect their normal bodily functioning. For example, schitosomiasis, caused by the trematode Schistosoma, is the third most prevalent tropical disease in the world.  It causes internal bleeding and organ damage, and similar to the effects of Ribeiroia ondatrae on frogs, the severity of the damage depends on the host’s developmental stage, with more severe effects seen at younger ages. Thus, while the effects of parasites have been well researched with respect to humans, their effects on other organisms have been neglected. In order to fully appreciate the role of parasites in our world we must attempt to understand how they affect not only humans, but other organisms as well. Research into parasitism in animals has provided insight into how these parasites negatively affect their hosts with respect to habitat use and survival ability, ultimately increasing their own survival.  Given that frog populations are declining significantly, it is imperative that attention be paid to the role of parasites in amphibians, for it is only with this knowledge that we can hope to protect amphibian populations from such life-threatening diseases.

See Also

• Whats with the Frogs? Short Documentary on the Malformations
• Colorado University Amphibian Parasite Observatory
• Goodman, B.A., Johnson, P.T.J., 2011a. Disease and the extended phenotype: Parasites control host performance and survival through induced changes in body plan. PLoS ONE 6, 1-10.
• Goodman, B.A., Johnson, P.T.J., 2011b. Ecomorphology and disease: cryptic effects of parasitism on host habitat use, thermoregulation, and predator avoidance. Ecology 92, 542-548.
• Johnson, P.T.J., Kellermanns, E., Bowerman, J., 2011. Critical windows of disease risk: amphibian pathology driven by developmental changes in host resistance and tolerance. Functional Ecology (DOI: 10.1111/j.1365-2435.2010.01830.x).