Whenever I hear the word “parasite”, I tend to think of lamprey fish or wasp larvae eating a caterpillar from the inside out. However, these are the most extreme examples of parasites – it is to the benefit of most parasites to leave the host alive so that they can continue to feed and reproduce at the expense of their host. In fact, scientists believe that mutualisms like coral reefs may have originally evolved from parasitic relationships. Parasites are usually one of two general kinds: ectoparasites (attached to the outside of the body) or endoparasites (inside the body). The common ectoparasites that most of us are familiar with are ticks, mosquitoes and bedbugs, while human endoparasites include tapeworms, nematodes and trematodes. Other animals, including amphibians, suffer from many of these same nonlethal parasites.
The most common parasites found in frogs and toads are trematodes and nematodes. Both of these kinds of parasites have complex life cycles that involve two or more hosts and have an environmental component, which typically involves freshwater in trematodes and is more terrestrial in nematodes. Trematodes, also known as parasitic flatworms, have been linked to some of the amphibian deformities and malformations that have been reported in recent years. Many species of trematodes encyst in the hind limb buds of tadpoles, resulting in extra legs or missing legs when the tadpoles metamorphose. It is thought that these malformations may make the frogs more likely to fall prey to predators. In addition, it was found that in waters infiltrated by agricultural pesticides there is a higher chance of trematode infection and frog deformities.
Another interesting, though much less common parasite, is a flesh-eating fly that will sometimes lay its eggs on amphibian skin (scroll down to Haptophrya virginiensis at the bottom of this page to see some pictures). This fly usually lays its eggs on carrion, but will opportunistically infect toads and has been recorded on wood frogs (Lithobates sylvaticus) as well. The eggs on the amphibian’s skin hatch and then the larvae burrow into the frog. Within a matter of days, these fly larvae can consume an entire wood frog, leaving only the bones. This is another rare example of a lethal parasite. Most amphibian parasites don’t directly kill their hosts, but they may weaken or stress them so that they are more susceptible to predators or disease. More studies on parasites in amphibians are still needed in order to broaden our understanding of the impact that they have on amphibian ecology and conservation.