Hammerhead Worms: Are They Harmful or Dangerous?

A reader from South Africa (Durban, South Africa, more precisely) wrote to us about two hammerhead worms that he found in his home. He was wondering if hammerhead worms are harmful or dangerous to animals or babies, and he was also keen to discover what is causing them to enter his home. So, the question before us is this: are hammerhead worms harmful to humans or animals (like pets), and how do you get rid of hammerhead worms? (Technically, the reader didn’t ask how to get rid of hammerhead worms, but he was wondering how they got into his house, and in answering the latter we also answer the former.)

First, for those unfamiliar with hammerhead worms, here is a little background. Hammerhead worms are flatworms, which means they are members of the phylum Platyhelminthes. Working further down the taxonomic ladder, they belong to the family Geoplanidae (i.e., the “land planarians”). There are several species of hammerhead worms, so the term “hammerhead worm” actually ranges over multiple types of similar worms that belong to the genus Bipalium. Ok, enough taxonomy.


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Hammerhead worms can grow to be quite large, up to 20 inches (about 50 centimeters) in length, in fact. They are nocturnal, so it is easiest to find them at night, but they are commonly seen in the morning too. They are most often found on the top of soil – like in a garden or on one’s lawn – after it has rained. Like earthworms, hammerhead worms live in the soil because they need a moist environment to survive.

Now, onto our reader’s specific questions. Hammerhead worms are not harmful to humans, nor are they harmful to animals like household pets. In fact, they are only harmful to earthworms, which serve as their source of food. (Earthworms are technically animals, so we can’t make the blanket statement that hammerhead worms are harmless to all animals.) Because they feed on earthworms, hammerhead worms are carnivorous. Hammerhead worms will follow an earthworm’s trail to capture it, and then it holds its victim in place with its muscles and a sticky secretion. Hammerhead worms eat earthworms in the most gruesome of ways. A hammerhead worm will push its pharynx out of its mouth and then secrete enzymes on the worm. The enzymes will dissolve the earthworm’s body, and once this occurs, the hammerhead worm sucks the liquefied tissue into its body. Because the earthworm is dissolved before being consumed, the hammerhead worm’s digestion process largely occurs outside of its body.

Why are hammerhead worms entering our reader’s house? Of course, we can’t be sure, but they probably just ended up there by accident. Since hammerhead worms eat earthworms, there is presumably no source of food inside the house that is attracting them. Moreover, hammerhead worms depend on moist soil for survival, so they are generally only above ground after it rains. So, again, we have to assume the hammerhead worms simply entered the reader’s house by chance. Maybe there are some cracks in the foundation of the reader’s house that let them through, or perhaps they managed to get through a crack at the bottom of a door. If the reader wants to keep the hammerhead worms out, he should seal up any gaps in his house that he finds, but it seems unlikely that they will keep coming into the house. It’s not an environment they want to be in.

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So, we can tell our reader that hammerhead worms are not harmful to humans or animals that aren’t earthworms. It might have been a little starling to find hammerhead worms in his house, but the reader shouldn’t worry about them harming anyone, and if he wants to keep them out, we can recommend nothing more than sealing up any gaps or crack that may allow them into the house.

19 thoughts on “Hammerhead Worms: Are They Harmful or Dangerous?

  1. Earth worms are, in fact, an invasive species.If they were ever in North America they were killed off during the ice age 10,000 years ago. They were introduced to North America by European immigration in ballast used in ship’s and plant’s imported from Europe.
    Earth worms cause much damage to hardwood forests. While they can’t kill mature tree’s their digging kills seedlings and wildflowers, ferns and other small plant’s.
    You can find many articles on the internet attesting to what I have related here.
    As to the way Hammerhead Worms digest their food while it might be strange to humans that is the way nature made them. No one finds it grotesque that most spiders digest the flies they catch in a very similar manner.
    I have no problem with Hammerhead Worms and as I live in a hardwood forest and would like the seedlings to grow into mature tree’s that future generations can enjoy. With no seedlings to replace the mature tree’s, that will one day die, all that will be on this land will be a giant hay field.

  2. Are these the same as arrowhead worms? Arrowhead worms also prey upon slugs, which is good, but they can also carry the rat lung disease which they get from the slugs in Hawai’i, which is bad. During rainstorms, I have found several in my carport. A little rubbing alcohol, and they permanently leave my earthworms alone and help to stop spreading the disease. How do we prevent them?

  3. I live in st. Lucia in the Caribbean and today, for the first time I saw one of those in my yard. My mother is in her 60s and had never seen it before either. I must admit I was a bit scared at first but from what I read here I feel much better.

  4. Thanks for giving this valuble information,one day I found that on my dress that time I was afraid of that ,the sticky thing which is secreat on that is adher on my skin,so I think that it would create any skin disease ,but here after I never get fear of that .thank you .

  5. Thanks for the info, I’m a plumber in northeast Texas , and today I covered a hole I had dug up to fix a leak, got in my truck and was driving away looked at my forearm and liked to of had a wreck 10 inches long and a 1/2 inch wide head scared the s–t out of me, it looked like some kind of creature

  6. I’ve seen them in NC now… one more thing to worry about… I will be stocking up on salt for these because I need the worms in our soil.

  7. Glad to have found this site thanks for the information been seeing these buggers past week because of heavy rains (Pennsylvania, USA) I have 3 kids and a outdoors cat so was worried about how harmful it was. Btw table salt kills them on contact.

  8. I detest these disgusting creatures. There seems to be two kinds hear in southeast Texas. The typical nasty land planarian with the hammerhead and stripes running along its brown body and the black, leech looking ones that do not have the hammerhead. These things seem to like crawling in animals and people. Even though they are harmless to people and pets, it’s still pretty disturbing. I can’t stand to even look at the hammerhead variety ::shudders:: seems the black leech like ones have become more common the past decade or so. Used to be the other way around. Now I rarely see the hammerhead and mostly see the black ones. Maybe the black ones are eating the hammerhead? Probably be the other way around though as the black ones don’t seem to get as big as it’s hammerheaded cousins. Ick! I love earthworms, most insects…even spiders and roaches don’t bother me…but something about the hammerheads…just makes my skin crawl…almost a phobia.

  9. If they eat earth worms they obviously not great to have in the garden. Earth worms are important.

  10. I found a hammerhead worm on my dog’s paw after a heavy rain. It was about 1 inch long and light brown with a black stripe. I live in Los Angeles

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  11. I live in Appling,Ga. I found this hammerhead worm crawling up the glass storm door after raining all morning. Are they harmful other than eating earth worms?

  12. Could someone in the Western Cape help me getting hold of a Hammerhead worm or two. Found one in my garden many years ago but never since. Please mail me as soon as you locate one which I can come collect

  13. Two or three of these nasty things were (slime included) were crawling on the leg of an outdoor cat, which must have crawled on him while asleep on the ground. I managed to remove two with a paper towel before he ran off to deal with the rest

  14. I find these things in my garage every week and it’s always in the same spot. The first one I saw was the only live one, sitting in a pile of slime. But ever since then, they have been dead. I would see a slime trail and a puddle of slime with a dried up/hard black thing… Not sure if it’s the dead worm or a piece of it.

  15. So glad to get this info on the hammerhead. Fearing for my animals,I took a newly discovered worm to my vet to get answers. The seasoned vet was baffled. Still needing answers, I tracked down an Agriculture agent. He too was in awe, but continued to research. He identified this slimy critter which enabled me to search the web to find your helpful info. Thank you so much. I am relieved, this hammerhead is no harm to humans or pets! Now, I would like to save my earthworms….. Is there anything more effective than orange oil? My backyard is very rich with humus and earthworms, surely a hammerheads haven. Any advise on getting rid of these hammerheads would be appreciated!

  16. I found a hammerhead worm in my barrel composter on March 7, 2015. I suppose it has been feasting on the earthworms that I left in there. Thinking it was just another type of earthworm I put it back. Next time I see it I am putting it into the trash. I need all the earth worms in my garden. :-)

  17. I found one of these critters on a dam wall in Benoni after rain trying to negotiate its way across dry concrete. I noticed its strange head and since I was fishing decided to keep it in the same jar as my earthworms. A day later I went online to search for it and discovered it eats earthworms and originates from Asia as it was the species that only has the one black line down its back. This write up indicates that they are not harmful but on another site I have found that they do carry toxins and because they eat earthworms it is suggested that they are cannibals and are possibly their own worst enemy.

  18. Thank you for this information. I found one of these works around my dog’s food bowl so I picked it up with a napkin and put in our compost bin. If its not dangerous than i could have left it alone but i wanted to be sure for my dog. Im sure the worm will be happy enough with the earth worms in the compost. Next time i see one ill just move it away from my dogs food and water.

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