Finding an eight-legged frog, most of us would assume there’s got to be a nuclear plant somewhere nearby. But in at least 17 states, radiation or other toxic waste isn’t responsible for frogs with extra legs and skin flaps. Instead, the mutations are the work of a parasite — the flatworm Ribeiroia ondatrae, which gets all up in frogs’ business and makes them totally weird.

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High Country News explains:

[The flatworm] breeds inside the common freshwater ramshorn snail, burrows into tadpoles’ limbs, and creates cysts that interfere with development, explains Johnson. The adult frogs have trouble feeding themselves and become easy prey.

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Humans aren’t entirely free of blame for this state of affairs. According to HCN, all the tinkering we’ve done with the environment has made frogs more vulnerable to infection. Plus, nutrient runoff from agriculture leads to more algae leads to more snails leads to more parasites leads to more terrifying spiderfrogs. The story ends just like the tale of the old lady that swallowed a fly: eventual doom.