Enterobius vermicularis

     

Pinworm findings in archaeological material outside the New World have been scarce for unknown reasons, and it is hypothesized that this parasite did not originate in the Americas, but rather arrived via land route through the Beringia. Pinworm is one of the few helminths that could have possibly arrived through this route, as most helminths require a particular soil temperature to progress to the infective stage, a stage pinworms do not require and would not have been possible to attain through migration in the cold northern territories.

 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.

      Pinworm is a cosmopolitan parasite with particularly high prevalence in countries with a temperate climate. It has the widest distribution of any parasitic helminth, and it is estimated that approximately 200 million people are infected internationally. The most common helminth infection in the USA and Western Europe, it has become the most common intestinal parasite seen in a primary care setting, regardless of factors such as race, socioeconomic status, and culture. As such, pinworm serves as an exception to the general rule that intestinal parasites are uncommon in affluent societies.

In the United States alone, prevalence is estimated to be between 20-40 million, and a CDC surveillance study conducted in 1992 in 35 states found that 11.4% of 9597 tests for pinworm infection were positive. While it is mainly seen in children, pinworm cases have been documented in adults, especially in households where infected children transmit the infection to the rest of the family. Prevalence in children in certain communities has been found to be as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark. 

 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.

 Humans are the only natural host of pinworms, and there currently is no confirmed pinworm reservoir. However, in 2004, Chan et. al documented the existence of pinworm and Trichinella in cockroaches from hospitals and grade schools in Hawaii. While cockroaches are known carriers of bacteria and fungi that produce disease in humans, the link between cockroaches and pathogenic helminths has not yet been elucidated, and this was the first report of pinworm infestation in naturally occurring cockroaches.

 In 2005, Tatfeng et. al isolated pinworm ova from cockroaches (Diploptera punctata), as well as the ova, cysts, oocysts, and mature parasites of several other parasites. This team trapped 234 cockroaches from areas in and around households in Ekpoma, Africa. The cockroaches were placed in containers of formol saline, which were then shaken vigorously to detach the parasites from the insects. The fluid was then spun and examined under a microscope.

The findings from both studies have great public health implications. Cockroaches are ubiquitous, and both studies detected pinworm in cockroaches taken from human-occupied settings. If they are indeed reservoirs for pinworms, control of cockroach populations and limiting contact with cockroaches could possibly greatly minimize not only the spread of pinworm infection, but of many other infectious diseases as well.

As with many other GI nematodes, pinworms do not need to rely on a vector for transmission. Pinworm infection usually occurs via ingestion of infectious eggs by direct anus-to-mouth transfer by fingers. This is facilitated by the perianal itch (pruritis ani) induced by the presence of pinworm eggs in the perianal folds, and commonly occurs as a result of nail biting, poor hygiene, or inadequate hand-washing.
Retroinfection is also possible, where some of the pinworm larvae which hatch on the anus return to the gastrointestinal tract of the original host, leading to a very high parasitic load as well as ensuring continued infestation.
However, the transfer can also occur by touching contaminated surfaces, such as clothing, bed linen, and bathroom fixtures followed by ingestion, or even through inhalation or ingestion of aerosolized eggs from the aforementioned surfaces. As such, pinworm infections are easily spread among young children with the habits of nail biting and or poor hygiene, and infected children can easily spread the infection to other family members through the mechanisms listed above.

Eggs are deposited at night by the gravid females.
Eggs are ingested via person-to-person transmission through the handling of contaminated surfaces (such as clothing, linen, curtains, and carpeting), or airbourne eggs may be inhaled and swallowed. Self-infection may also occur if eggs are transferred from to the mouth by fingers that have scratched the perianal area.
After ingestion, larvae hatch from the eggs in the small intestine. The adults then migrate to the colon. The life span of the adults is about two months. Adults mate in the colon, and the males die after mating.
Gravid females migrate nocturnally to the anus and ovideposit eggs in the perianal area. The females die after laying their eggs. The time period from ingestion of infective eggs to the ovideposition of eggs by females is approximately one month.
The larvae develop and the eggs become infection within 4-6 hours. Newly hatched larvae may also migrate back into the anus, and this is known as retroinfection.