The loa loa is a roundworm species that lives in mammals’ eyes

The larvae develop in horseflies and are passed to mammals like humans through a fly bite. If it gets into the eye tissue, hosts can feel it moving in the eye and can see it floating in there if it’s big enough. The worm is easily killed using antibiotics or is extracted through minor surgery, though it can live inside a human for up to 17 years if left untreated.

Video of a 19cm worm being removed from a man’s eye: here