WATCH: These parasites want to take up residence in your eyes

Meet the parasites that can crawl under your skin and into your eyes to leave you blind.

A parasitic worm called Onchocerca volvulus affects 25 million people around the world in one of the most revolting ways possible. The larvae of this tiny worm, found in sub-Saharan Africa and Latin America, can live for more than 10 years in a person’s skin, and that’s not even the worst part - they often make their way into their eyes.

According to Tommy Leung, a lecturer in parasitology and evolutionary biology at Australia's University of New England, nearly a third of the people affected by Onchocerca larvae will end up with one of these parasites in their eye, at which point they’re at serious risk of blind due to the infection.

And unfortunately, Onchocerca aren’t the only eye-loving parasites out there. As Leung explains at the Conversation, parasitic flatworms called Oculatrema hippopotami live under the eyelids of African hippopotamuses, and parasitic flukes called Philophthalmidae are notorious for feeding on the tears of birds. Flukes are a type of primitive flatworm, found all over the world and ranging from a few millimetres to several centimetres long. There are more than 10,000 known species of flukes, and several of them live inside the guts of birds, laying eggs that are distributed in their host’s droppings. When these eggs hatch, in order to reproduce, these new flukes have to somehow make their way back into a bird’s gut.

Which is where the eyes come into play.

While it might seem like a bad idea for a parasite to damage their hosts - if your host goes blind and dies, you’re going to have to find a new host, and no one likes moving - for flukes, it’s a necessary step towards completing their lifecycle. Flukes attempt to make their way into the body of a bird’s prey, such as a fish or a snail, and partially blind them, handicapping their ability to hide from predators. When the bird eats the prey, it also eats the fluke, and the fluke has fulfilled its mission to get to a bird’s gut and have babies.

But for some animals, strangely enough, having a parasite in the eye doesn’t seem to affect them a whole bunch. "The Greenland shark is infected by the parasite Ommatokoita elongata which plunges a pair of modified limbs straight into the shark’s eyeball and feeds by grazing on its cornea. As unpleasant as it sounds, the shark does not seem too affected by the parasite’s intrusive presence,” says Leung at the Conversation.

Watch the video above to see a freshwater fish from New Zealand called a bully dealing with a large larval fluke in its eyeball.