"So, naturalists observe, a flea has smaller fleas that on him prey; and these have smaller still to bite ’em; and so proceed ad infinitum."
- Jonathan Swift

November 8, 2018

Leidynema appendiculatum

A while ago, I wrote a post about a tadpole pinworm, and pinworms are found in the hindgut of a variety of different vertebrate animals. But there are also many pinworms which live in animals without backbone. The reason why pinworms make their home in the gut of those animals is because they are after bacteria which dwell in the the gut of animals which are hindgut fermenters. And there are some insects which provide just the right environment for them to thrive.

Top: Adult female L. appendiculatum (scale bar 500 μm)
Bottom: Adult male L. appendiculatum (scale bar 200 μm)
From Figure 2 of the paper
Pinworms (Oxyurida) are separated into two main groups - the Oxyuroidea which consist of species that infect vertebrate animals (including humans), and the Thelastomatoidea, which consist of species that infect invertebrates such as cockroaches and millipedes. Leidynema appendicaulatum is in the latter group, and it lives in the hindgut of cockroaches. It has been reported from many different species of cockroaches around the world. In Japan, this parasite is most commonly found in the smokybrown cockroach Periplaneta fuliginosa and it is the only parasitic roundworm known to infect that cockroach species.

So is this pinworm also found other cockroach species in Japan? A pair of scientists decided to investigate the presence and development of this parasite in both wild and captive cockroaches. They looked at a mix of cockroaches collected from the wild, from lab-reared colonies, and some that were bought from petshops where they are commonly sold as food for pet reptiles.  They examined 14 species of cockroaches in total, and of those, the smokybrown cockroach was usually found to have L. appendiculatum as expected, but the rest were largely free of the pinworm in question. So what's stopping it from infecting those other cockroaches considering that it has no trouble infecting different cockroach species from around the world?

To find out, the scientist did some infection experiments using colonies of cockroaches which had been raised in the laboratory and never been exposed to any nematode parasites. They fed them food which had pinworm larvae mixed in to see which ones were susceptible. In contrast to what they found for the wild cockroaches, L. appendicaulatum had no trouble making themselves at home in those other cockroach species, going through the same developmental cycle at the usual rate as those living in the smokybrowns. One of the cockroach species which they successfully infected was Blattella nipponica - which was found to be free of the pinworms in the wild.  So why is it that L. appendiculatum can infect captive B. nipponica, but not those out in nature? This might have something to do with those cockroaches' feeding habits.

Pinworms have a relatively simple transmission pathway - the host becomes infected when they ingest fecal matter which are contaminated with the parasite's eggs. So in order for a cockroach to become infected with L. appendiculatum, at some point they have to ingest another cockroach's feces. Unlike the smokybrown cockroach which are common in urban areas and readily feed on detritus, including feces from other cockroaches, B. nipponica mainly live in forests and grasslands, on a more discerning diet of mostly of plant matter, including forest fruits. So out in the wild, B. nipponica would rarely come into contact with the parasite.

Given the right set of circumstances, L. appendiculatum can easily establish itself in many cockroach species, it's just that for some cockroaches, those kind of circumstances just doesn't happened in nature. In the end, you are infected by what you eat.

Ozawa, S., & Hasegawa, K. (2018). Broad infectivity of Leidynema appendiculatum (Nematoda: Oxyurida: Thelastomatidae) parasite of the smokybrown cockroach Periplaneta fuliginosa (Blattodea: Blattidae). Ecology and Evolution 8: 3908-3918.