A strange parasite with even stranger behavior

Most parasites are simply organisms that inhabit other organisms in order to steal the host’s nutrients for their own benefit. While they are certainly not the most friendly of creatures out there, most of the time, as they depend on their host for survival, they are not killers. However, some parasites exhibit some pretty over-the-top behavior, taking over their host’s body and mind to make them do some absurd stuff.

The horsehair worm is one example of a parasite that uses excessive and horrifying means to ensure their survival. They infect arthropods mainly, like crickets, grasshoppers, cockroaches, and beetles, and when they do, things get ugly. Horsehair worms take over and control their host’s mind to the point that the host that will eventually take a seemingly suicidal leap into water. However, there is no danger for the horsehair worm, as they will easily make their escape from holes in their host’s exoskeleton.

What is a horsehair worm?

Horsehair worms, also called Gordian worms, are part of the class Nematomorpha, also Gordiacea. The class encompasses about 350 known freshwater species, though there are probably more like 2,000 different species of them globally. These are superficially similar to nematodes, or roundworms, except a lot larger and thinner. Horsehair worms typically grow to at least four inches long, though sometimes up to 39 inches. but remain only one-tenth of an inch or less in diameter, about the same as a kite string.

Horsehair worm and cricket
Wikimedia Commons

A horsehair worm’s body is so thin, it is hair-like, hence their name. In fact, back in the day, people actually thought they were horses’ hairs that came to life when submerged in water. Their other name, Gordian word, comes from the fact that they tend to tie themselves up into complex knots, like the Gordian knot. They live either directly in water or near water, such as beside puddles, streams, cisterns, or any outdoor open container of water.

While horsehair worms are fairly innocuous to humans, pets, and livestock, they are quite harmful to some other insects. As they are parasites, they develop in the bodies of their hosts, oftentimes crickets. Once they reach maturity, they exit their host to mate and then lay eggs. Before that happens, however, they take over their host’s mind in order to create an ideal and safe habitat for themselves.

The lifecycle of horsehair worms

Mature horsehair worms mate in water, where the females also lay their eggs. The water temperature will determine how long the eggs take to hatch, but it is typically between two weeks and three months. Larva horsehair worms are microscopic in size, and their early life remains largely a mystery. Somehow, within the 24 hours after they are born, the larva seems to develop some kind of a protective covering. They do this in order to get eaten a certain kind of insect, an arthropod.

Every once in a while though, a huge crowd of horsehair worms will pack into one very unfortunate host.

Once inside their host, their protective covering dissipates and the larva bores through their host’s gut to reach its body cavity. It is curious how they do this, seeing as they don’t have a mouth to eat or chew their way through. They also don’t seem to have a recognizable digestive system. In any case, this is where they make themselves at home, absorbing the host’s surrounding tissue. Once the horsehair worm is fully developed, it exits the host to mate and begins the process once more.

Normally, just one horsehair worm lives in a host at a time. Occasionally, two or three will inhabit a single host. Every once in a while though, a huge crowd of them will pack into one very unfortunate host.

The over-the-top parasitic behavior of horsehair worms

As mentioned, a horsehair worm takes over the brain of its host. When inhabiting a cricket, for instance, it somehow manipulates the creature known for its chirping to stop chirping. They do this so the cricket expends less energy so it maintains more nutrients to absorb and also keep it safer. After all, a chirping cricket is more likely to get eaten, and that is the opposite of the worm’s plan.

Once the horsehair worm is nice and mature, that is when it gets the cricket or other arthropod to really act outside of its typical behavior. Most crickets try to stay away from the water, so much so that they typically hydrate themselves from food or dew drops. Put an arthropod like a cricket near water and it will quickly exit that scene because their chances of drowning or getting eaten are much too high to risk it.

Under the mind-control of the horsehair worm, a cricket always finds itself attracted to the water, suddenly taking a suicidal dive right in.

However, an infected cricket exhibits different behavior. Under the mind-control of the horsehair worm, a cricket always finds itself attracted to the water, suddenly taking a suicidal dive right in. Then, as soon as it hits the water, the worm, knowing its host may not live long in those conditions, makes a swift exit through the porthole it had previously bored.

The plan is really perfect for the horsehair worm, as the water is exactly where it needs to be to mate. Males, once delivering their sperm, die soon thereafter. Apparently, their only purpose in life is to drive crickets mad and then procreate. The female then lays her eggs, often up to 15 million of them, which she secures underwater, pasted to a stone or a stick. Then, just like the male having fulfilled his life’s mission, she dies too.

How and why a horsehair worm takes a cricket’s brain hostage

Horsehair worms are able to take over a cricket’s brain by producing an exceedingly large amount of neurotransmitters, the chemicals in the brain that make the transmission of signals between neurons possible. Plus, those extra neurotransmitters also cause the crickets to act how the worm wants it to. The only thing missing is that scientists still don’t know which neurotransmitters are being used and how exactly they are causing the crickets’ behavior to change. On top of producing extra neurotransmitters themselves, horsehair worms also seem to be increasing the cricket’s production of neurotransmitters as well, which definitely plays into their mind-control scheme.

Horsehair worms are able to take over a cricket’s brain by producing an exceedingly large amount of neurotransmitters, the chemicals in the brain that make the transmission of signals between neurons possible.

Scientists only have theories at this point regarding why horsehair worms somehow evolved to behave like such horrifyingly excessive parasites. The most convincing one is simply that inhabiting a nutrient-rich insect is far easier than finding their own food, especially if they’re living in a region with scarce resources (which could easily be and often is the case). Perhaps horsehair worms originated in a habitat without many resources and had to make do with what was around.

It’s too bad for the crickets and other anthropods though, because their chances of survival once jumping into a pond or a lake are pretty grim. However, if they do end up in a puddle or another, safer body of water, they could make it, depleted of nutrients, but still alive

Should humans be worried about them?

Horsehair worms are harmless to humans. They infect insects, not humans nor anything they would care about, including their pets, livestock, or plants. On the contrary, perhaps humans should be grateful for the existence of horsehair worms. They kill pests that might otherwise infest their spaces. Even if you find these worms around, nothing needs to be done to control them, as they cause no danger to us.

Human cases

Horsehair worms are harmless to vertebrates, as they are incapable of parasitizing or infecting them even if ingested.

There are two known cases of horsehair worms infecting humans, though using the word “infect” might be a bit of an exaggeration. Both cases were found in Kyoto, Japan, one the vomit of an 80-year-old woman, and the other in the mouth of a one-year-old boy. The cases were in November 2009 and December 2009 respectively.

The 80-year-old woman discovered the horsehair worm when gargling with saline solution after feeling she had something caught in her throat. Her report states that she had eaten vegetables from a private garden. In the other case, the boy’s mother found the worm in his mouth, when she promptly removed it. Both situations likely involved the humans accidentally swallowing insects, perhaps a beetle or cricket, which were infected by the worms.

Neither case is cause for any concern. Horsehair worms are harmless to vertebrates, as they are incapable of parasitizing or infecting them even if ingested. The worst that could possibly happen would be very mild intestinal tract discomfort.

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