We received a very interesting question recently about, essentially, parasitic worms that infect other worms. Most of us have heard of parasitic worms that infect other hosts, including humans, but are there parasitic worms that take worms themselves as their host? More broadly, the reader was wondering if there is any “example of a parasitic animal whose host is a worm,” so in addition to looking for parasitic worms that infect worms, we are also looking for any sort of parasite that takes a worm as a host.
As is often the case, the precise answer to this question depends on how one wants to use the word “worm.” We adopt a somewhat narrow definition of the word, but lots of people define “worm” quite broadly, using it to refer not only to creatures that are worms by anyone’s reckoning (e.g., annelids like earthworms), but also a variety of insect larvae. On this more expansive understanding of the word “worm,” creatures like caterpillars, the larval form of moths and butterflies, are worms, and so are a great deal of other insect larvae. Caterpillars can definitely suffer from parasitic infections, and these parasites are often themselves larvae (e.g., wasp larvae). So, a larva (caterpillar) can be infected by a parasitical larva (wasp larva), and thus a “worm” is infected with another “worm.” (Technically, a wasp larvae that infects a caterpillar is a parasitoid, not a parasite, because the larvae ultimately kills the host. Basically, a parasitoid is a particularly devastating parasite.)
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There are also worms that infect insects; for instance, horsehair worms infect creatures like crickets and grasshoppers, and Guinea worm larvae infect fleas. Although we consider horsehair worms and Guinea worms actual worms, their hosts are insects, not worms, and thus these creatures are perhaps of less interest to our reader.
However, there are instances of true worms becoming infected by other true worms, and this is perhaps the information our reader is after. For instance, earthworms (which, again, are annelids) can become infected with a range of parasites. Some of these infections are caused by single-cell organisms, but earthworms are also known to become infected by platyhelminthes and nematodes, or flatworms and roundworms, respectively. The parasitic worms use the earthworm host like any other, sustaining themselves as they settle into the earthworm’s blood, intestines, or other body cavity. Thus, an annelid can be infected by something like a nematode, and since both of these are worms, a worm is infecting a worm.
So, worms are both parasites and parasite victims, and this is true regardless of how broadly you want to define the word “worm.”
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