Disgusting parasites

They eat human brains and eyeballs, turn snails into pulsating zombies, grow up to 25 feet long in people's intestines and eat the tongues of fish. Symbiotic relationships may be common in nature, but that doesn't mean that all parasites are beneficial – many are downright horrifying. These 13 disgusting parasites will have you immediately jumping in the shower to quell that itchy paranoid feeling.

Tongue-Eating Cymothoa exigua

It's like something out of a bad horror movie: a disgusting little critter that invades a fish's body, eats its tongue and proceeds to live in its mouth. The fish is able to survive just fine with its new ugly parasitical tongue, and while this creature will give you the creeps, it isn't a threat to us. A fisherman who found a live one, describing it as "a bit of a nasty beast" said, "It doesn't affect humans other than if you do actually come across a live one and try and pick it up – they are quite vicious, they will deliver a good nip."

Human Botfly

There are all kinds of botflies – from the Horse Stomach Botfly to the Sheep Nose Botfly and indeed, the Human Botfly. It's not hard to guess why they're given such names. The adult female human botfly captures a mosquito and attaches its eggs to the mosquito's body, so that they drop off into the wound created when the mosquito feeds. The larvae develop inside the human body for about eight weeks before dropping out to pupate. The wounds swell to outrageous proportions in the meantime, and the larvae can move throughout the body and even cross the blood-brain barrier.

Crab Hitchhiker: Sacculina Carcini

Once a crab picks up a freeloading Sacculina carcini, which starts its life as a tiny slug, it's over. The crab has now been doomed to a lifetime of watching helplessly as the slug forces itself into the crab's body armor, taking over its body and extending its root-like extensions into the crab's organs, nervous system, gonads and appendages. The crab can't molt or reproduce on its own – its body is basically serving as a shell for the Sacculina carcini, which eventually pops its 'head' out of the crab's shell to better attract mates. It then forces the crab to do a kind of parody of its own reproduction habits, only with the hitchhiker's eggs.

Tick

There are few things more revolting than the sight of a tick engorged with another creature's blood. These little arachnids are external parasites that lurk in tall grasses, just waiting for a potential host to pass by, whether it's a mammal, bird, reptile or amphibian. They carry a number of diseases including Lyme disease, Q fever, Colorado tick fever and several more.

Filarial Worm

Filariasis is one of the nastiest parasites you can imagine, with effects that are absolutely devastating. Elephantiasis – a disfiguring disease characterized by the thickening of the skin and underlying tissues – is a classic sign of late-stage infection with these microscopic, thread-like worms, which live in the human lymphatic system and prevent normal function. Filariasis is transmitted through the bite of an infected mosquito.

Screwworms

The screwworm larvae eat the living tissue of warm-blooded animals, burrowing into the surrounding flesh as they feed. Is the most nightmarish thing about this parasite the fact that it can enter animals through their navels and nostrils, or that they 'screw' deeper into the flesh if you disturb them?

Half a century ago, these nasty little maggots were a serious threat – especially to pets and livestock, though they were known to invade the wounds of humans as well. They were eradicated in the U.S. through the farming and release of sterile adult screwworm flies in the 1960s, but are still a problem in many parts of the world.

Candiru, the Urethra-Invading Fish

Tales have been told so many times of humans being invaded by tiny fish that swim up through their urine streams and into their urethras when they urinate into certain bodies of water, it's hard not to believe it. While some are skeptical, there's a recent documented case– photos and all – of a small fish removed from a man's scrotum after it entered through the urethra and bit its way through the scrotal wall. Though the species wasn't positively identified, it seems likely that it was the Candiru, which is certainly capable of the feat.

Sushi Worms: Anisakis

There's a reason that raw fish is supposed to be frozen before humans consume it, whether in the form of sushi, sashimi, fermented herrings, cod livers or ceviche. That reason is the Anisakis, a genus of parasitic nematodes that often infect fish. The life cycle of Anisakis involves being eaten first by a crustacean, then a fish, and is complete when consumed by a mammal – and that mammal might just be you.

And, don't worry, you'll know if you do become infected – because you'll be doubled over in violent abdominal pain almost immediately.

Tapeworm

Even if you manage to escape Anisakis, there are always tapeworms to contend with – and they're no picnic, either. One Chicago man told ABC about the time he went to the hospital with a 9-foot worm – and he didn't find it in his garden. Getting a tapeworm from raw or undercooked fish, beef or pork may be rare, but it happens. Food blogger Helen Rennie says, "Chefs sometimes joke if the worms are moving then the fish is fresh."

Tapeworms can reach up to 25 feet, and it can take months or years for them to reach full size. Symptoms are generally mild, so you might not even realize you have one.

Insane Snail Parasite

A poor little snail is gliding along, minding its own business, when it accidentally infects itself with Leucochloridium paradoxum by eating the parasite's eggs in bird excrement. In the snail's digestive gland, these parasites hatch and produce sporocystes, long green-banded tubes that stretch into the snail's tentacles and begin to pulsate. Those little worms are intentionally trying to attract the attention of birds, so they'll get eaten and end up back in the birds' bodies to complete their life cycle.

Guinea Worm

This worm is called Dracunculiasis, or "affliction with little dragons", because sufferers say it feels like they've got fiery serpents under their skin. Europeans gave it the name "Guinea worm" because they first saw it on the Guinea coast of Africa in the 17th century. It's contracted when a person drinks stagnant water contaminated with the worm's larvae, or when they walk unprotected in infected waterways.

Once inside the body, the larvae find their way into a body cavity and breed. Then, the male is absorbed into the body and the female, full of eggs, burrows into connective tissues or long bones. About a year after infection, the worm attempts to leave its human host's body by creating a blister on the skin of the person's leg or foot.

Cat Parasite Turns Humans into Zombies

Ever wonder what compels crazy cat people to collect so many animals? As out-there as it sounds, their brains might just be acting on the orders of a parasite called Toxoplasma gondii, which is commonly found in cat feces. Experts say infection by a Toxoplasma gondii can make some people more prone to some forms of neuroticism, and may cause them to slide in their hygiene and housekeeping habits – thus, allowing the parasite to spread and flourish. Some 60 million people are estimated to be infected in the United States alone.

Eyeball Eater: Acanthamoeba

Washing your contact lenses doesn't just remove irritants – it prevents little parasites from hitching a ride into your delicious eyeball, where they'll not only feed on bacteria present in the eye, but on your corneas themselves. Luckily, only about one out of a million contact lens wearers are susceptible to infection by the Acanthamoeba parasite, but this free-living amoeba can also get into your eye when you swim in pools or lakes. Early diagnosis is essential for treatment, or victims will eventually go blind.