|Top: Adult female L. appendiculatum (scale bar 500 μm)|
Bottom: Adult male L. appendiculatum (scale bar 200 μm)
From Figure 2 of the paper
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.