Parasite Name: Enterobius vermicularis (formerly Oxyuris vermicularis )
Synonyms: pinworm, threadworm, seatworm
Classification: Helminth (nematode)
Taxonomy: Animalia Nematoda Secernetea Rhabditida Oxyuridae Enterobius vermicularis
Pinworm has the broadest geographic range of any helminth, and is the most prevalent helminth infection in the USA and Western Europe. It is commonly found in school-aged children, though it is seen in adults as well.
The worms are small, white, and threadlike, with the larger females ranging between 8-13 mm x 0.3-0.5 mm and the smaller males ranging between 2-5 mm x 0.1-0.2 mm. Females also possess a long, pin-shaped posterior end from which the parasite's name is derived. They dwell primarily in the cecum of the large intestine, from where the gravid females migrate at night to lay up to 15,000 eggs on the perineum.
Pinworm eggs are flattened asymmetrically on one side, ovoid, approximately 55 mm x 25 mm in size, and embryonate in six hours. These eggs can remain viable for about twenty days in a moist environment, and viable eggs and larvae were even found in the sludge of sewage treatment plants in Czechoslovakia in 1992.
It has also been recently speculated that pinworms themselves may serve as an intermediate host to Dientamoeba fragilis, a relatively mysterious protozoa that is still struggling to gain recognition as a human pathogen in certain countries. However, an increasing number of studies are incriminating it as a legitimate enteric pathogen, and it has been associated with clinical syndromes such as abdominal pain, diarrhea, nausea, vomiting, and fatigue. However, much about this pathogen, including its transmission, is still being investigated. Most intestinal protozoa are transmitted fecal-orally via a cyst form, but D. fragilis is generally accepted as not having a cyst form. Therefore, researchers have turned to its proposed nearest relative, Histomonas meleagridis, for comparison. H. meleagridis possesses several characteristics comparable to those of D. fragilis, and it is interesting to note that it is transmitted via the eggs of the nematode Heterakis gallinae. Burrows and Swerldlow proposed in 1956 that D. fragilis is transmitted via pinworm eggs based on the analysis of 22 appendices in which D. fragilis was isolated: There was a 20-fold greater incidence of pinworm infection than calculated, and small ameboid bodies bearing great resemblance to the nuclei of D. fragilis were observed in the pinworm eggs. However, it is still worth bearing in mind that D. fragilis has been associated with other intestinal parasites (such as Ascaris lumbricoides), and that the lack of a cyst stage yet to be conclusively proven, as D. fragilis has been found to have a high rate of coinfection with organisms which are transmitted fecal-orally.