Most Horrific Parasitic Worms

Hookworm images

Loathed wherever they go, parasitic worms are some of the animal kingdom's true lowlifes. They feed off living hosts, sapping them of energy and nutrients while causing a range of horrible diseases. After worming their way into people's lives, particularly in developing countries, these suckers can lie undetected for months or even years. They just take, take, take, and when they reveal themselves, gracious me, are they hideous creatures to lay your eyes on! We've picked five of the worst known to eat us humans from the inside out – but try to be good hosts as we welcome in the parasitic worms.

1. Hookworm

They say you shouldn't judge by appearances, but the Hookworm's kisser is a pretty fair indicator of what to expect from this parasitic little monstrosity. While living in its host's gut, the Hookworm lodges its well developed, toothy mouth into the intestinal lining and voraciously begins sucking out blood. This thirsty vampire drinks so much of the red stuff that there is a serious risk of anemia caused by iron deficiency. Rashes, nausea and diarrhea are among the symptoms those infected by Hookworm can expect, though part of the danger lies in the fact that its damage is so insidious and easy to miss.

Foot infested by Hookworm larvae

As larva, the Hookworm forces entry by burrowing into its host's skin, usually through bare feet, and from there it travels through the body to the intestine, at which point it grows into an adult worm. Although all but eradicated in America's South, where it infected a large proportion of the population during the early 20th century, the Hookworm remains a major threat to children in the tropics, causing retarded growth as well as cognitive and intellectual impairment. As many as 740 million individuals are thought to be infected by Hookworm today. With something that gets under the skin like this, rarely has a better case been made for good footwear.

2. Ascaris

The next worm to grace our list is arguably the most disgusting of the lot. A larger cousin of the Hookworm, Ascaris is a giant sized roundworm that can reach as long as 40cm, as opposed to little over 1cm. It too sets up shop in its host's small intestine, using its characteristic mouth, which is surrounded by three less than luscious lips. Ascaris is in fact the parasite most familiar to us humans, though the fact that up to 25% percent of the world's population is infected certainly doesn't make it any more welcome in our bowels. Sickness, fever, and heavy infestations with severe intestinal blockages kill up to 20,000 people a year.

Massive Ascaris infection in a child

Like the Hookworm, the Ascaris likes warm, damp conditions with poor hygiene, and is a particularly severe pain in the backside for young people. Yet while the larva of Ascaris' smaller relative tends to penetrate the skin of prospective hosts, infection in this case occurs through consuming food contaminated with faeces containing Ascaris eggs. The larva then hatches and migrates through the gut and respiratory system, before eventually being re-swallowed and allowed to mature, anchored snugly to the intestinal wall. The female Ascaris may then lay hundreds of thousands of eggs a day. Distinctly unsanitary food for thought.

3. Guinea Worm

Another elongated roundworm of ill repute, the Guinea Worm is one of the best documented of human parasites – and at around a meter in length, as thick as a spaghetti noodle, and with some decidedly unsavoury habits, it's easy to to see why. When humans drink stagnant water contaminated with this nasty blighter's eggs, it eventually gives rise to a slender, fulsome female Guinea Worm that burrows its way along to the arms or more likely the legs. There it assumes a position under the skin, before boring its way out through a blister that brings excruciating pain to the unfortunate host.

Guinea worm having emerged from a foot

The burning sore often leads the afflicted individual to dunk their limb in cool water, much to the delight of the Guinea Worm, which proceeds to release hundreds of thousands of larvae, infecting the water supply and starting the cycle all over again. Meanwhile, the host must wind the Guinea worm around a stick and slowly tease it out over several agonising, debilitating weeks. Still, it's better to endure the ghastly Guinea Worm hanging there, for if it breaks apart it is more likely to cause a potentially fatal infection. Despite efforts to eradicate it, largely through education, this remains one strand of evolutionary adaptation that definitely gets too close to the bone.

4. Tapeworm

If Ascaris and the Guinea Worm thought they had it sewn up in the length stakes, the Tapeworm puts them both to shame. Typically 3-5m long, this odious form of parasitic flatworm can grow to over 12m in some situations – situations that usually involve the digestive tracts of humans, livestock or other animals. Armed with powerful suckers and revolting teeth, the Tapeworm hunkers down and grows. And grows. Despite its size, the common Beef Tapeworm is not especially dangerous: it can by avoided by having your steak well done, and its symptoms are limited to sickness and intestinal obstruction. Not so with its cousin Echinococcus.

Hydatid cyst being removed

An Echinococcus like the Hyper Tapeworm carries with it a rather nasty surprise in the shape of hydatid disease. This occurs when the Tapeworm eggs are ingested in fecal-contaminated food, and the embryos take a ride in the bloodstream to hook themselves onto an organ like the lungs or liver. There, they grow into hydatid cysts, which act as a nursery for the next generation of Tapeworm larva – but which, less maternally, also put pressure on the host's organs, and can cause shock if they rupture. These large, potentially fatal cysts need to be surgically removed. Nobody told the Tapeworm size isn't everything.

5. Filarial Worm

Last but not least – although it may actually be the most microscopic of the bunch – meet the Filarial Worm, another repugnant roundworm, and one with a rather remarkable life cycle allied to some pretty gruesome costs. Once injected into its host by that harbinger of disease and destruction, the mosquito, this threadlike little horror needles its way into its host's lymphatic system, where it lodges itself and can cause blockages. This in turn may lead to elephantiasis, a grotesque swelling of the skin and tissue usually in the limbs. The Filarial Worm is thought to be one of the world's leading causes of disability.

Elephantiasis of legs due to filariasis

One of the things that makes the Filarial Worm so ingenious, as well as so abhorrent, is the way in which it has so successfully perpetuated its population over thousands of years. What's so special? Well, after the Filarial larva has been born in the lymphatic system, it slowly migrates to the lungs, where it rests by day, only to move to the peripheral blood vessels by night, in anticipation of being sucked up by a thirsty mosquito. Thus the cycle is kept going. Recently, we humans have stepped up our efforts to eliminate this crippling critter, but until that day the Filarial Worm looks set to remain among the diminutive big hitters that are parasitic worms.