We get a lot of weird questions in our inbox about parasites. Most of them, we can’t answer because doing so would amount to giving medical advice…but every so often we get a question completely out of left field. Like this one:

Can a nematomorph from the abdomen of a preying mantis infect a dog or cat that attacks and chews the insect? My cat keeps killing mantises on my deck; it’s mortifying because I love them. A lot of them seem to be infected with nematomorphs. Could it get into my cats digestive system and do harm?

When it comes to doctors and invertebrates, I’m notoriously skeptical because there’s a long history of doctors misdiagnosing brown recuse bites. Rick Vetter-one of my personal heroes-made a long career out of simply pointing out that diagnosis of brown recluse bites do not correlate with the known range of the critters…a series of mistakes which likely masked the rise of community acquired antibiotic resistant flesh-eating bacteria.

It’s not often that I hear a new medical claim, so this was absolutely something I wanted to check out.

So what are Gordian worms?

Gordian worms, or nematomorphs, are parasitoid nematode-like critters which have complicated lifecycles, usually revolving around water.

They typically have two hosts. After they hatch, their larvae seek out an initial insect host. Specifically, they look for insect larvae…usually a fly of some sort. They need a first host which will leave the water as an adult, because it needs to be eaten by an insect on land. Once it’s eaten, it develops into an adult which takes up the entire body cavity of the insect.

The adults aren’t parasitic, so they need to find water to bring about the next generation, and they accomplish this through mind control.

This is probably their best known, and also most misunderstood feature. Once the insect becomes an adult, it begins to wander and doesn’t really seem to recognize environmental dangers anymore. Once the insect encounters a body of water, it usually attempts to cross it by jummping in.

I feel like their ability to manipulate hosts has always been a bit exaggerated. A common misconception is that these parasites cause the host to seek out water, which doesn’t seem to be the case. They just have a tendency to leave their habitats and just walk around, which means they’ll usually find a pool of water somewhere nearby.

Either way, this is a legitimate form of mind control, and that’s what makes them compelling parasites. There is, after all, a long history of mind-controlling parasites being antagonists in sci-fi literature.

What does the medical literature say?

When first looking into stuff like this, my first stop is always Wikipedia, because I can usually count on stuff like this to be discussed…and sure enough:

So I dug up every article I could find (most of them are cited below, although there’s some I can’t access some reports from the early 1900s which aren’t digitized, etc), and it looks like there are actually quite a few case reports of these worms being found in vertebrates. Roughly 40 of these are associated with humans.

So, I guess case closed…right?

Well, it turns out that this isn’t quite accurate. There is apparently some literature which claims these things can infect household pets and people, but these guys simply aren’t capable of parasitizing household pets.

This is a case of something called pseudoparasitism.

Fake…parasitism?

Nematomorphs rely on a few relatively rare events to complete their lifecycle, so they make up for this by having an insane reproductive rate. If you need one host to get eaten, and another host to stumble around long enough to find some water while not getting eaten, you’re gonna need to have a lot of babies in order to make that happen. Because their hosts are just kind of drunkenly stumbling around, a lot of them are going to get eaten and these worms need a backup plan to survive.

These things aren’t super well studied, but my impression is that the adults probably deal with being an unexpected meal by being super tough. Once the insect is eaten, the adults are in a body of liquid and let the host dissolve around them. From here, there’s exactly two exit points…which is why we see a lot of case reports of these worms being found in vomit or poop.

Kids and household pets get their hands on a lot of things, and eat a lot of things they shouldn’t. So it makes sense that if they eat a random bug, they’re going to occasionally eat one with a gordian worm inside. Some of these cases are associated with people drinking unfiltered well water, as well. I’m not sure if the worms would be able to make a predator vomit-again this isn’t well studied-but it’s not uncommon for other animals to make this happen. So it wouldn’t surprise me if they also had some kind of chemical defense just in case.

I know the original question was about household pets, but I felt that one case out of China was worth a discussion because it demonstrates pretty well not only how these myths get started, but also why they can’t live inside people or pets:

I don’t want to get too much into the graphic details, but a lot of these worms were brought in by people who had grabbed them after urinating and brought them in. So the doctors didn’t actually witness them passing the animal, and there’s a long history of people falsely believing they’re infected with parasites bringing in random arthropods to doctor offices.

The issue with these case reports is that they don’t actually line up with the biology of the worms very well. Adult Gordian worms don’t actually eat, and the hosts they develop in generally don’t live longer than a few months even if they’re not infected. Gordian worms spend a few months developing inside hosts, and the adults only live a few weeks outside of the hosts. Their entire lifecycle takes less than a year, and they (presumably) overwinter as eggs.

So because these worms live less than a year, it’s not really possible for them to live inside of another animal for 3-4 years. In cases where other parasites do find their way inside of hosts, they have considerable difficulty navigating the unfamiliar biology of these accidental hosts.

What happens when parasites get into the wrong hosts?

We can pretty easily compare this to the rat lung roundworm Angiostrongylus. Normally, the nematode larvae get into rats after they eat a snail. They navigate to the lungs, where they cause the rat to produce a lot of mucus, which they then swallow. The rat poops them out, and the whole thing is relatively harmless.

When they get into a human, however, the biochemical cues they need to find the lungs aren’t there. So they wander through the body, eventually ending up in the brain, where they cause extensive damage which usually results in death. So while these worms are harmless to the rats, they’re pretty much always fatal to humans.

This is pretty common with nematodes, and although Gordian worms are a completely different phylum, their biology is very similar. If Gordian worms were capable of causing these kinds of infections, we’d expect to see a similar disease associated with these cases. We just don’t see that, though.

So not only are these case reports not consistent with the biology of the Gordian worm, they’re also not consistent with what we’d expect to see with other kinds of accidental parasitism from similar animals.

The Bottom Line

There’s a lot of myths about animals which are thought to be parasites of people, and these are pretty hard to quash once they get started. It’s not all that different than what we’ve seen with the brown recluse, or even the candiru*.

A lot of these myths get started because of the kinds of observations I described above, and these actually have a name…it’s called pseudoparasitism and it means exactly what it sounds like: fake parasitism.

Pseudoparasitism happens when a critter finds itself in a place (well, orifice really) they shouldn’t be and happens to survive the ordeal. It’s a testament to how tough some of these critters really are, but because they’re not living and feeding inside the other animal as a habit, it’s not actually parasitism.

Still pretty cool, though.

I want to caution against cases like this being interpeted as malicious, because they’re usually not. In the cases above the patients were clearly mistaken, and the doctors were probably a bit more eager to interpret these cases as real parasitism than they should have been. The human cases I described were nearly half a century old (hence the inappropriate language at some points), and none appeared to consult with experts on the biology of these animals. Hence, they likely weren’t in a position to realize how inconsistent these cases are with the biology of the animals.

Ultimately, though, at the end of the day, the idea of the Gordian worm as a parasite can really just be chalked up to a series of mistakes.

Works Cited

Ali-Khan, F. E. A., & Ali-Khan, Z. (1977). Paragordius varius (Leidy)(Nematomorpha) infection in man: a case report from Quebec (Canada). The Journal of parasitology, 63(1), 174-176.

Anaya, C., Hanelt, B., & Bolek, M. G. (2021). Field and laboratory observations on the life history of Gordius terrestris (Phylum Nematomorpha), a terrestrial nematomorph. The Journal of Parasitology, 107(1), 48-58.

Dexiang, W., & Wenyuan, Y. (1981). Parachordodes sp.(Nematomorpha) human infestation of the lower urinary tract: the first case report in China. Acta Academiae Medicinae Wuhan, 1(2), 40-45.

Hanelt, B., Bolek, M. G., & Schmidt-Rhaesa, A. (2012). Going solo: discovery of the first parthenogenetic gordiid (Nematomorpha: Gordiida). PLoS One, 7(4), e34472.

Herter, C. D., & Nesse, R. E. (1989). Pseudoparasitism with Gordius robustus. American family physician, 39(3), 139-142.

Hinkle, N. C. (2000). Delusory parasitosis. American Entomologist, 46(1), 17-25.

Hong, E. J., Sim, C., Chae, J. S., Kim, H. C., Park, J., Choi, K. S., … & Park, B. K. (2015). A horsehair worm, Gordius sp.(Nematomorpha: Gordiida), passed in a canine feces. The Korean journal of parasitology, 53(6), 719.

Lee, K. J., Bae, Y. T., Kim, D. H., Deung, Y. K., Ryang, Y. S., Im, K. I., & Yong, T. S. (2003). Gordius worm found in a three year old girl’s vomitus. Yonsei medical journal, 44(3), 557-560.

斉藤康秀, 井上巖, 林文夫, & 板垣博. (1987). A hairworm, Gordius sp., vomited by a domestic cat. 日本獸醫學雜誌 (The Japanese Journal of Veterinary Science), 49(6), 1035-1037.

Son, H. Y., Chae, J. S., Kim, H. C., & Park, B. K. (2009). Morphological study of the horsehair worm, Chordodes koreensis (Nematomorpha: Gordiida), isolated in canine vomitus. Journal of Veterinary Clinics, 26(4), 348-352.

Thomas, F., Schmidt‐Rhaesa, A., Martin, G., Manu, C., Durand, P., & Renaud, F. (2002). Do hairworms (Nematomorpha) manipulate the water seeking behaviour of their terrestrial hosts?. Journal of Evolutionary Biology, 15(3), 356-361.

Vetter, R. S. (2008). Spiders of the genus Loxosceles (Araneae, Sicariidae): a review of biological, medical and psychological aspects regarding envenomations. The Journal of Arachnology, 36(1), 150-163.

Yamada, M., Tegoshi, T., Abe, N., & Urabe, M. (2012). Two Human Cases Infected by the Horsehair Worm, Parachordodes sp.(Nematomorpha: Chordodidae), in Japan. The Korean journal of parasitology, 50(3), 263.

*Note from Joe:

The Candiru is a small catfish which is rumored to wedge itself into human penises and feed on blood. There’s actually no evidence it does this, and the one “confirmed” case report is sketchy to the point of violating the laws of both physics and biology. In the biology sphere, it’s largely thought to be one of many myths started by colonialist explorers.

I was going to use that as an example of how these parasite myths get started, but it turns out that Candirus do occasionally attack people. There are case reports of larger species attacking children, although this is thought to be extremely rare.

However, in 2013, there was actually a new species described which was actually discovered while feeding on a human. However, it feeds more like a leech and the penis isn’t actually involved because it just can’t…fit inside there. They can feed on people, but not at all like the rumors claim they do. There’s photographic evidence in both these links, and they are a bit graphic…so content/trigger warning there.

This is a bit more of a complicated one, and the entire story about this myth could be a post in and of itself. However, since it’s the most famous and pervasive human parasite myth out there, I thought I’d give it a shout-out.

https://meridian.allenpress.com/journal-of-parasitology/article-abstract/107/1/48/453991

https://onlinelibrary.wiley.com/doi/pdf/10.1046/j.1420-9101.2002.00410.x

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0034472