The Horsehair Worm. Pretty Gross Parasite, Super Cool Story.

phil torres nematomorpha










While collecting aquatic insects last week in this pleasant stream in Malibu, CA (left) I came across this little wriggler, commonly called the Horsehair worm but more fancily called Nematomorpha (right). I placed the worm next to a pen so you can get a better perspective, it comes in at over 13″ long. That’s one heck of a parasitic worm. Luckily for me, it is no parasite of humans, it prefers the much smaller prey. Here’s a video of me moving it around a bit:

These worms have an amazing story. They are endoparasites of arthropods, which means that they grow up feeding off the insides of things like crabs, crickets, cockroaches, and beetles. One particularly cool species is a grasshopper parasite. Once the worm has grown to its massive adult size, it infects the brain of the grasshopper, takes over some of its natural instincts, and ‘instructs’ the grasshopper to jump into water, an otherwise very unnatural behavior.

Once in the water, the grasshopper drowns, the worm escapes, and it goes on to live happily as a free-living adult in the stream/pond/puddle that the grasshopper jumped in to. There it meets other horsehair worms, mates, lays a goopy mass of eggs, and the story goes on.

Here’s where people can get scared though: sometimes, these worms end up in your toilet. This can cause an immediate alarm in people, thinking they might have some parasitic worm living in their intestine. Luckily, there is an easy way to distinguish our harmless horsehair worms from the potentially nasty human parasite. Horsehair worms have a distinctly clefted/forked rear end, as seen in the picture below, while human parasites have a hooked/blunt end.

So, if that worm in your toilet has a forked end you’re likely in luck, it probably contributed to one less cockroach running around your house by parasitizing it. However, if there is no forked end, I’d recommend the not-so-fun task of putting it in a jar and visiting your doctor.

These worms got their name from people in the olden days believing that they formed from horse hairs coming to life when they fell into a stream. As cool as that would be, I think their actual natural history story is even better.

12 Comments to "The Horsehair Worm. Pretty Gross Parasite, Super Cool Story."

  1. April 9, 2012 - 3:08 am | Permalink

    Hey! This is my first visit to your blog! We are a collection of volunteers and starting a new initiative in a community in the same niche. Your blog provided us useful information to work on. You have done a extraordinary job!

  2. Danielle's Gravatar Danielle
    January 5, 2013 - 1:07 pm | Permalink

    Thanks so much for putting my fears to rest. We kept seeing these worms show up on our carport following a heavy rain. My family unknowingly kept calling them tape worms. I knew better but still feared that they were harmful to my kids. Then I searched and found your site. Now, I can once again feel safe if my children happen to slip outside without shoes on again. Thanks.

  3. G C's Gravatar G C
    February 26, 2013 - 10:53 pm | Permalink

    I killed a cockroach & 6 inch worn came out his ass.
    It looks like horsehair worm.

  4. Cherith Chapman's Gravatar Cherith Chapman
    March 2, 2013 - 11:22 pm | Permalink

    We live in Ecuador and had a wierd, long worm with a distinct head on it on our cutting board in the kitchen this morning. Anyhow, what you have here might be what we had. What caught my attention was what looks like a “head”, but you say it is a tail. I’m hoping this is what we had. I’d really love for it not to have been something that would like to live in me. There is enough other gross stuff here that wants to live inside us.

  5. Candy Venincasa's Gravatar Candy Venincasa
    May 31, 2013 - 8:12 pm | Permalink

    I had noticed a beetle about an inch in length-dark in color with an iridescent prism on its back-and also noticed a long projection coming from its back became really long and then started to retreat..noticed the beetle pacing on the rail on my interesting to find out what it is..feel awful for the beetle, knowing its going to be succumbing to its death..was also wondering, if as a neat experiment, if the beetle was placed in a cup of water would the parasite release itself into the water?( if beetle only placed in briefly) and if the parasite did, would the beetle live or die? Trying to save its life! and also perform an experiment!! Thanks for solving my mystery-never heard of a horsehair worm interesting..

  6. ruth evans's Gravatar ruth evans
    July 24, 2013 - 9:10 am | Permalink

    I went to fueling my car the other day and the attendant had just put water in the windshield washing deal and had ran it over, I looked down and here were these long tiny snake like acting little worms. We put one of them in a cup and they took it to the county extension office and they told us what it was so I googled it and found your sit. What an interesting thing. Thanks for the info

  7. Think again!'s Gravatar Think again!
    July 24, 2013 - 8:03 pm | Permalink

    I know a human that has just tested positive for this parasite! So think again!!!

  8. Geri's Gravatar Geri
    July 24, 2013 - 10:57 pm | Permalink

    I found a worm that looks like this in the toilet after I threw a cock roach in the water but did not flush it. I thought it was a tape worm but looked it up and it didn’t resemble one, so I kept looking. Thank you so much for this info. If you need a video of this, I have one. Thanks

    • Anonymous's Gravatar Anonymous
      August 20, 2013 - 10:48 pm | Permalink

      this is exactly what happened at our house. I saw a dead spider, got a tissue, and threw it in the toilet, I did NOT flush it down. Hours later when the family got home from school/work, the worm was swimming around in the toilet. Everyone thought the worst until we figured it out using sites like this one.

  9. Stacy's Gravatar Stacy
    August 6, 2013 - 6:54 pm | Permalink

    Can you please give more info on the ‘potentially nasty human parasite’ that can be mistaken for the horse hair parasite? All the info you can give would be greatly appreciated.

  10. Colin Evans's Gravatar Colin Evans
    October 4, 2013 - 9:03 am | Permalink

    Thanks for this as we have one in a jar. It’s about 6inches long. We have video off it. How does the horsehair worm get into the Grasshopper ? We are in Wales. UK. A reply would be appreciated. Thank you. Colin Evans.

  11. Desirae Sensat's Gravatar Desirae Sensat
    May 31, 2014 - 11:49 pm | Permalink

    I was outside on my front porch and I noticed a cricket with a worm hanging off of it. I instantly freaked out because I was barefoot and didn’t know what it was. My neighbor told me it was one of these worms. It was about 7 inches long. I’ll never go outside barefoot again. Anything creepy crawly or wiggly like a worm freaks me out!

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