PINWORM (Intestinal Parasites)

The pinworm (in the United States of America) (genus Enterobius), also known as threadworm (in the United Kingdom) or seatworm, is a nematode (roundworm) and a common human intestinal parasite, especially in children. The medical condition associated with pinworm infestation is known as enterobiasis, or less precisely as oxyuriasis in reference to the family Oxyuridae.Throughout this article the word pinworm refers to Enterobius. In British usage, however, pinworm refers to Strongyloides while Enterobius is called threadworm.

CLASSIFICATION

The pinworm (genus Enterobius) is a type of roundworm, and three species of pinworm have been identified with certainty. Humans are hosts only to Enterobius vermicularis (formerly Oxyuris vermicularis). Chimpanzees are host to Enterobius anthropopitheci, which is morphologically distinguishable from the human pinworm. Hugot (1983) claims there is another species affecting humans, Enterobius gregorii, which is supposedly a sister species of E. vermicularis, and has a slightly smaller spicule (i.e., sexual organ). Its existence is controversial however; Totkova et al. (2003) consider there to be insufficient evidence, and Hasagawa et al. (2006) contend that E. gregorii is a younger stage of E. vermicularis. Regardless of its status as a distinct species, E. gregorii is considered clinically identical to E. vermicularis.

MORPHOLOGY

The pinworm appears as a white, small and delicate nematode (i.e., roundworm). The adult female has a sharply pointed posterior end, is 8 to 13 millimeters long, and 0.5 millimeter thick. The adult male is considerably smaller, measuring 2 to 5 millimeters long and 0.2 millimeter thick, and has a curved posterior end. The eggs are translucent and have a surface that adheres to environmental objects. The eggs measure 50 to 60 micrometers by 20 to 30 micrometers, and have a thick shell that is flattened on one side. The small size and colorlessness of the eggs make them invisible to the naked eye, except in barely visible clumps of thousands of eggs. Eggs may contain a developing embryo or a fully developed pinworm larva. The larvae grow to 140?150 micrometers in length.

DISRTIBUTION

The pinworm has a worldwide distribution, and is the most common helminth (i.e., parasitic worm) infection in the United States and Western Europe. In the United States, a study by the Center of Disease Control reported an overall incidence rate of 11.4% among people of all ages. Pinworms are particularly common in children, with prevalence rates in this age group having been reported as high as 61% in India, 50% in England, 39% in Thailand, 37% in Sweden, and 29% in Denmark. Finger sucking has been shown to increase both incidence and relapse rates,and nail biting has been similarly associated. Because it spreads from host to host through contamination, pinworms are common among people living in close contact, and tends to occur in all people within a household. The prevalence of pinworms is not associated with gender, nor with any particular social class, race, or culture. Pinworms are an exception to the tenet that intestinal parasites are uncommon in affluent communities. The earliest known instance of pinworms is evidenced by pinworm eggs found in coprolite, carbon dated to 7837 BC at western Utah.

LIFE CYCLE

After the eggs have been initially deposited near the anus, they are readily transmitted to other surfaces through contamination. The surface of the eggs is sticky when laid,and the eggs are readily transmitted from their initial deposit near the anus to fingernails, hands, night-clothing and bed linen. From here, eggs are further transmitted to food, water, furniture, toys, bathroom fixtures and other objects. Household pets often carry the eggs in their fur, while not actually being infected. Dust containing eggs can become airborne and widely dispersed when dislodged from surfaces, for instance when shaking out bed clothes and linen. Consequently the eggs can enter the mouth and nose through inhalation, and be swallowed later. Although pinworms do not strictly multiply inside the body of their human host, some of the pinworm larvae may hatch on the anal mucosa, and migrate up the bowel and back into the gastrointestinal tract of the original host. This process is called retroinfection. According to Burkhart (2005), when this retroinfection occurs, it leads to a heavy parasitic load and ensures that the pinworm infestation continues. This statement is contradictory to a statement by Caldwelli (1982), who contends that retroinfection is rare and not clinically significant. Despite the limited, 13 week lifespan of individual pinworms, autoinfection (i.e., infection from the original host to itself), either through the anus-to-mouth route or through retroinfection, causes the pinworms to inhabit the same host indefinitely.

TRANSMISSION

Pinworms spread through human-to-human transmission, by ingesting (i.e., swallowing) infectious pinworm eggs. The eggs are hardy and can remain viable (i.e., infectious) in a moist environment for up to three weeks. They do not tolerate heat well, but can survive in low temperatures: two-thirds of the eggs are still viable after 18 hours at negative 8 degrees Celsius. After the eggs have been initially deposited near the anus, they are readily transmitted to other surfaces through contamination. The surface of the eggs is sticky when laid, and the eggs are readily transmitted from their initial deposit near the anus to fingernails, hands, night-clothing and bed linen. From here, eggs are further transmitted to food, water, furniture, toys, bathroom fixtures and other objects. Household pets often carry the eggs in their fur, while not actually being infected. Dust containing eggs can become airborne and widely dispersed when dislodged from surfaces, for instance when shaking out bed clothes and linen. Consequently the eggs can enter the mouth and nose through inhalation, and be swallowed later. Although pinworms do not strictly multiply inside the body of their human host, some of the pinworm larvae may hatch on the anal mucosa, and migrate up the bowel and back into the gastrointestinal tract of the original host. This process is called retroinfection. According to Burkhart (2005), when this retroinfection occurs, it leads to a heavy parasitic load and ensures that the pinworm infestation continues. This statement is contradictory to a statement by Caldwelli (1982), who contends that retroinfection is rare and not clinically significant. Despite the limited, 13 week lifespan of individual pinworms, autoinfection (i.e., infection from the original host to itself), either through the anus-to-mouth route or through retroinfection, causes the pinworms to inhabit the same host indefinitely.

ENTEROBIASIS: HUMAN INFECTION WITH PINWORMS

A pinworm infection or enterobiasis is a human parasitic disease and one of the most common childhood parasitic worm infections in the developed world. It is caused by infestation with the parasitic roundworm Enterobius vermicularis, commonly called the human pinworm. Infection usually occurs through the ingestion of pinworm eggs, either through contaminated hands, food, or less commonly, water. The chief symptom is itching in the anal area. The incubation time from ingestion of eggs to the first appearance of new eggs around the anus is 4 to 6 weeks. Pinworms are usually considered a nuisance rather than a serious disease. Treatment is straightforward in uncomplicated cases, however, elimination of the parasite from a family group or institution often poses significant problems either due to an incomplete cure or reinfection. Pinworm infection has no association with any socioeconomic level, race or culture.

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