I came home from work on Monday, April 11, 2022, and found nobody home except the cats. And a large note on the table saying, “There is a worm beside the sink. It is from the cat (Saffron). We are grossed out. We went to Tractor Supply[1]” And, in a plastic bag full of water by the sink, I found this big roundworm.

Here it is with the ruler on my Leatherman[2], for scale. I estimate it was about 5 inches long.

Roundworms like this don’t have a lot of detail, but zooming on the head end we can see that it had two flaps (“Alae”) that make it look a bit like an arrowhead.

This is pretty clearly the most common roundworm[3] that infects domestic cats, Toxocara cati[4]. The cat had vomited it up, which is a pretty common thing with roundworm infections. This was the only symptom she had of a worm infection, she otherwise acted perfectly healthy. Although, we should have expected it. The cat in question was this one,

which any of you who read this blog back in December 2021 may recognize as the stray cat that came crying at our back door to come in from the cold. She had clearly picked up worms while she was out fending for herself, probably by eating an infected rodent.

Toxocara cati is very common in cats, particularly outdoor cats, and infected mother cats pass them on to their kittens to such an extent that all kittens should be assumed to have worms. They cause slow growth, “pot-belly”, and diarrhea in kittens. Normally they don’t cause really noticeable symptoms in adult cats, and it is pretty common for the first sign of infection in a full-grown cat to be when it vomits one or more up on the floor. Like our cat did.

The life cycle starts with an adult worm in the small intestine of a cat. It lays eggs that are excreted in the cat’s feces, and if environmental conditions are appropriate the eggs mature to the infective stage (stage 3) after about 2-4 weeks. The stage-3 worms are then eaten by small rodents, where the worms embed themselves in the rodent’s body tissues. They don’t mature further in the rodent, but if it is killed and eaten by a cat, then the worms get into the cat’s body tissues, migrate around inside them until they move into the cat’s intestines, and mature there, where they commence laying eggs. The ones that a mother cat transfers to her kittens apparently are the new worms coming in with things she eats and that migrate into her mammary glands and milk, not ones hatching from eggs laid by the adult worms she already has.

It is possible for humans to catch worms from cats, but apparently not directly. The eggs have to hatch out and get to stage 3 before they infect humans, which means that if you clean out the litter boxes and dispose of the contents every few days, the eggs don’t have a chance to get infectious. Handling cat feces that are more than about 2-4 weeks old, or gardening where outdoor cats have a chance to use the garden as a giant litter box, sound like the most likely way to pick up the worms. If you do, most of the time there are no symptoms, but sometimes humans get toxocariasis, where the larvae migrate around in the body tissues (particularly the liver, lungs, sometimes heart, and eyes(!!!)) causing a variety of unpleasant problems. But, they can’t mature in humans, so they clear out of the system eventually.

The dewormer we used on our cat was one of the Piperazine-based ones. Piperazine supposedly doesn’t actually kill the worms, it just paralyzes them so that they can be excreted. We haven’t seen any worms in the litter box yet, though. I suppose it is possible that she only had the one.

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[1] Tractor Supply sells general agricultural supplies, including over-the-counter veterinary medications. They had gone there to get some dewormer.

[2] I use my Leatherman Wave multitool for multiple things on a daily basis. Not usually for measuring parasitic worms, but it fits right in there. I tell my Chemical Engineering students that every engineer should carry a multitool, because You Never Know When You’ll Want It, and The Best Tool For Any Job Is The One You Have With You (as opposed to being back home in your garage).

[3] Parasitic roundworms are a small set of the Nematode phylum. Nematodes are a different phylum from segmented worms like earthworms, and nematodes of various types are everywhere. There is argument about even approximately how many nematode species there are, with the low estimates being around 40,000 species and the high estimates being over 1 million species. The uncertainty is mainly because they are all mostly featureless worm-shaped things with not much in the way of appendages. Historically the only way to distinguish species is by differences in their habitats, but now DNA sequencing is making it clear that the 1 million species estimate is probably closer to the correct number. Nematodes live in pretty much every conceivable environment where life can exist, throughout the earth, water, tropics, swamps, deserts, polar regions, and of course inside of other, larger organisms. And yet, this is the first time I have actually had a picture of a nematode.

[4] One would think that if it has “Toxo-” in the species name, that would mean it was toxic in some way. But it turns out that “Toxo” is actually the Greek word for “Bow”, and that the worm was given that name because it resembles an arrow. And, the actual derivation of the word “toxic” comes from the practice of putting poison on one’s arrows, to make them more lethal.