Parasites have always filled me with fear. I still experience the occasional bout of night terrors when images of Guinea Worms wriggling out of weeping lesions in my flesh flicker across my dream state subconscious. Over the past few days, while working on a piece for the next issue of High Country News, I’ve become familiar with another parasite whose machinations create an equal sense of repulsion, although it hasn’t kept me up at night. Yet.
This parasite goes by the scientific name of Ribeiroia ondatrae and is a type of flatworm found in freshwater systems. It plays a key role in creating malformed frogs (although it is not responsible for all deformities). Frogs affected by Ribeiroia can sprout extra legs (a frog has been found with eight hind legs) or grow additional flaps of skin between their joints. Dragging around an extra set of limbs tends to make adult frogs unwieldy, making them easy prey for predators, as well as slow to hunt their own food.
“Once they emerge onto land they have a pretty rapid death sentence,” says Pieter Johnson, a disease ecologist at the University of Colorado Boulder, who has recorded parasite-related deformed frogs in 17 states, including six Western states, since he began studying the phenomenon in 1996.
Ribeiroia needs three hosts to complete its life cycle. Its first port of call is the water-dwelling ramshorn snail. While inhabiting the snail, it clones itself through asexual reproduction to produce larval minions that swim off to find a tadpole to infect.
“It sounds bad to be a frog, but you really don’t want to be a snail infected with this parasite because then your entire reproductive system gets eaten and transformed, basically into thousands of parasites,” explains Johnson.
Then, Ribeiroia larvae burrow into tadpoles around their limb areas, creating cysts in which they continue to grow. The cysts interfere with limb development, sometimes resulting in the growth of extra appendages. Once a predator, like a heron, has swallowed an infected frog the parasite reproduces inside it, laying eggs the bird expels in its feces, often spreading the little critter to new locations where it starts its lifecycle all over again.
Understanding how the parasite impacts frog populations is important because amphibians are the most threatened group of vertebrates in the world. In particular, Johnson is researching how nutrient run-off from agricultural fertilizers and cattle grazing can promote algae growth, creating more food for the snail and upping the number of parasite hosts in a system. A loss of biodiversity can also make frogs more vulnerable to infection. This is because some frog species are resistant to the parasite and when the worm attacks them it dies. Having such species in a system means fewer parasites manage to spread to other, more sensitive frogs. Some fish and insects, such as larval dragonflies, eat the parasite so when there are less of these around there could be more parasites too.
Possible ways to keep parasite levels down include planting vegetation around ponds to buffer them from nutrient run-off, and applying fertilizers more efficiently to reduce run-off, says Johnson. As he notes in this video: “I really want to understand how we manage the landscape in such a way as to reduce the types of diseases that can affect and kill amphibians. And if we can figure out how to manage the land in such a way that we can all live together without causing increased amounts of diseases in our wildlife, I think that has enormous applied benefit for keeping these populations viable for generations to come."
Brendon Bosworth is a High Country News intern.