Answer to Case of the Week 540

Answer to Parasite Case of the Week 540: Macracanthorhynchus species, one of the acanthocephala ("thorny-headed worms"). M. hirudinaceus or M. ingens would be the most likely culprits, although as mentioned by Florida Fan, we would need the head of the worm to morphologically identify this to the species level. The acanthocephala are common parasites of animals, but only rarely infect humans. Infection is acquired through ingestion of an infected arthropod such as a millipede or beetle.

The eggs are very striking. They are ovoid and measure 80-100 micrometers in length. As described by Bernardino Rocha, they are dark brown egg with a thick rugged textured shell. Within the egg is the characteristic larval form (acanthor) with rostellar hooks (which you can sort of make out in this egg).

Interestingly, Macracanthorhynchus eggs are rarely seen in human stool. Blaine Mathison mentioned that this worm can occasionally reach sexual maturity in the human host and can even produce mature eggs; for some reason, however, the eggs are either not readily shed from the female, or are shed in such low numbers that they aren't commonly seen. Blaine also speculated that the eggs could be missed in O&P exams due to lack of technologist familiarity with them. Finally, there is the possibility of spurious passage from ingestion of soil or pig intestines containing the eggs/worms.

Of note, this particular patient had a follow-up specimen several days later which still contained Macracanthorhynchus eggs. Thus, my hypothesis is that this represents true infection rather than spurious passage.