Long Black Worms and Dogs

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A concerned reader wrote to us recently about a long, black worm he found in his house. He is worried that his dog has “a terrible worm of some kind.” We are of course highly sensitive to his concerns, but the reader makes no explicit connection between his dog and the long, black worm he found. He merely mentions that he came home (and presumably found the worm) and is worried about his dog. He didn’t mention any behavior of the dog that implies it has a worm infection, nor did he say he saw the worm come from the dog’s body. We are therefore slightly puzzled why he immediately linked the worm he found to his dog. In any case, he only asked us to identify the long, black worm, so hopefully in helping with this we can provide some guidance.

Before we get started, however, we must state up front that if the reader does have a good reason to link the worm with his dog – e.g., the dog threw up the worm, or he found the worm right by his sick-looking dog – then he should go to the vet as soon as possible. We are often able to help people identify worms and larvae, but we are not medical doctors or veterinarians, so we cannot offer any sort of advice about the health of humans or pets. With that emphasized, we’ll consider what our reader found. Fortunately, he sent in two good pictures of the worm:

long black worm

And here is the same worm captured from a slightly further distance away:

long black worm far

Based on these pictures, it appears that our reader found a hammerhead worm, which is a type of terrestrial flatworm. (Flatworms make up the phylum Platyhelminthes.) Hammerhead worms take their names from the fact that their heads look a bit like a hammer. As you can see above, particularly in the first picture, the head of the worm does indeed look a bit like a hammer. Hammerhead worms are also often dark in color and relatively long (some can reach 20 inches – or 50 centimeters – in length), just as the worm in the picture appears to be.

If our reader did find a hammerhead worm, then its presence in his house should have nothing to do with his dog, and it isn’t a threat to his pet or himself. In fact, hammerhead worms are only harmful to earthworms because the latter serve as the former’s food source. Interestingly, we have actually written about hammerhead worms and pets before. A reader found one stuck on the side of her kitten, and if it really was a hammerhead worm, we told the reader she had no reason to be concerned because, once again, hammerhead worms are simply not harmful to pets. We can offer similar advice to our reader: if we have correctly identified the worm he found, he has no reason to think his dog is suffering from “a terrible worm of some kind.” He simply found a relatively common type of flatworm whose presence in the house is unrelated to his dog.

As we conclude, though, we should point out that we are not certain our reader found a hammerhead worm. So, again, if our reader is concerned about his pet or has reason to suspect the worm he found is compromising the health of his dog, he should visit the vet. One feature of the photos that our reader sent that gives us pause is the unidentifiable stuff that surrounds the worm, but it is unclear what this stuff even is, and the reader doesn’t mention that it has anything to do with his dog. What we know is that the reader’s find looks like a hammerhead worm, and moreover it doesn’t really look like the common worms that afflict dogs (such as roundworms, hookworms, and whipworms), but we could nevertheless be mistaken. We wish our reader the best of luck, and hope very much that he only found a hammerhead worm.


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