An 8-year-old child is brought in by her mother with complaints of crampy abdominal pain, nausea, and mild diarrhea for approximately 2 weeks. The day before this visit, she vomited up a cylindrical white worm 30 cm in length, which the mother preserved in a jar. Which of the following is the most likely means by which this organism was acquired?

This is a case of Ascaris lumbricoides infection, the only helminth infection caused by a cylindrical white worm of this remarkably large size. It is acquired directly, with no intermediate host, from fecal contamination from other human beings, and the stage of the life cycle that is transmitted is the egg. Autoinfection is a rare occurrence in helminthic infections, possible only with Strongyloides stercoralis, the threadworm, and Taenia solium, the pork tapeworm. Ingestion of cysts in contaminated water is the normal mechanism of infection used by intestinal protozoa. Ingestion of eggs from an infected pet occurs in visceral larva migrans, when the ascarids of dogs and cats infect the human, an aberrant host. It would never result in the introduction of an adult worm into the vomitus, as described here. Ref: Brooks G.F. (2013). Chapter 46. Medical Parasitology. In G.F. Brooks (Ed),Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.