Pinworm
Encyclopedia
The pinworm (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.
s 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.
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 micrometer
s 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
. Inside the host, the larvae grow to 140–150 micrometers in length.
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
.
The entire life cycle—from egg to adult—takes place in the human gastrointestinal tract of a single human host. Cook et al. (2009) and Burkhart & Burkhart (2005) disagree over the length of this process, with Cook et al. stating two to four weeks, while Burkhart & Burkhart states that it takes from four to eight weeks.
The life cycle begins with eggs being ingested
. The eggs hatch in the duodenum
(i.e., first part of the small intestine
). The emerging pinworm larva
e grow rapidly to a size of 140 to 150 micrometers in size, and migrate through the small intestine towards the colon
. During this migration they moult
twice and become adults. Females survive for 5 to 13 weeks, and males about 7 weeks. The male and female pinworms mate in the ileum
(i.e., last part of the small intestine), whereafter the male pinworms usually die, and are passed out with stool. The gravid female pinworms settle in the ileum
, caecum (i.e., beginning of the large intestine
), appendix
and ascending colon
, where they attach themselves to the mucosa and ingest colonic contents. Almost the entire body of a gravid female becomes filled with eggs. The estimations of the number of eggs in a gravid female pinworm ranges from about 11,000 to 16,000. The egg-laying process begins approximately five weeks after initial ingestion of pinworm eggs by the human host. The gravid female pinworms migrate through the colon
towards the rectum
at a rate of 12 to 14 centimeters per hour. They emerge from the anus
, and while moving on the skin near the anus, the female pinworms deposit eggs either through (1) contracting and expelling the eggs, (2) dying and then disintegrating, or (3) bodily rupture due to the host scratching the worm. After depositing the eggs, the female becomes opaque
and dies. The reason the female emerges from the anus is to obtain the oxygen necessary for the maturation of the eggs.
, by ingesting (i.e., swallowing) infectious pinworm eggs and/or by anal sex. 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 −8 degrees Celsius (18 °F).
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.
, whose primary symptom is itching in the anal
area.
Genus
In biology, a genus is a low-level taxonomic rank used in the biological classification of living and fossil organisms, which is an example of definition by genus and differentia...
Enterobius), also known as threadworm (in the United Kingdom) or seatworm, is a nematode
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...
(roundworm) and a common human intestinal parasite
Intestinal parasite
Intestinal parasites are parasites that populate the gastro-intestinal tract in humans and other animals. They can live throughout the body, but most prefer the intestinal wall. Means of exposure include: ingestion of undercooked meat, drinking infected water, and skin absorption...
, especially in children. The medical condition associated with pinworm infestation is known as enterobiasis
Enterobiasis
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...
, or less precisely as oxyuriasis in reference to the family
Family (biology)
In biological classification, family is* a taxonomic rank. Other well-known ranks are life, domain, kingdom, phylum, class, order, genus, and species, with family fitting between order and genus. As for the other well-known ranks, there is the option of an immediately lower rank, indicated by the...
Oxyuridae
Oxyuridae
Oxyuridae is an family of nematode worms of the class secernentea. It consists of eight genera, one of which contains the human pinworm ....
.
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). ChimpanzeeChimpanzee
Chimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
s are host to Enterobius anthropopitheci, which is morphologically
Morphology (biology)
In biology, morphology is a branch of bioscience dealing with the study of the form and structure of organisms and their specific structural features....
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
Spicule
Spicules are tiny spike-like structures of diverse origin and function found in many organisms, such as the copulatory spicules of certain nematodes or the grains on the skin of some frogs.In sponges, spicules perform a structural function....
(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
Nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of 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 micrometer
Micrometer
A micrometer , sometimes known as a micrometer screw gauge, is a device incorporating a calibrated screw used widely for precise measurement of small distances in mechanical engineering and machining as well as most mechanical trades, along with other metrological instruments such as dial, vernier,...
s 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
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
or a fully developed pinworm larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
. Inside the host, the larvae grow to 140–150 micrometers in length.
Distribution
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 bitingNail biting
Onychophagia, or nail biting, is a common oral compulsive habit in children and adults.-Classification:Nail biting is considered an impulse control disorder in the DSM-IV-R; the ICD-10 classifies it as "other specified behavioral and emotional disorders with onset usually occurring in childhood and...
has been similarly associated. Because it spreads from host to host through contamination
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
, 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
Social class
Social classes are economic or cultural arrangements of groups in society. Class is an essential object of analysis for sociologists, political scientists, economists, anthropologists and social historians. In the social sciences, social class is often discussed in terms of 'social stratification'...
, 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
Coprolite
A coprolite is fossilized animal dung. Coprolites are classified as trace fossils as opposed to body fossils, as they give evidence for the animal's behaviour rather than morphology. The name is derived from the Greek words κοπρος / kopros meaning 'dung' and λιθος / lithos meaning 'stone'. They...
, carbon dated to 7837 BC at western Utah
Utah
Utah is a state in the Western United States. It was the 45th state to join the Union, on January 4, 1896. Approximately 80% of Utah's 2,763,885 people live along the Wasatch Front, centering on Salt Lake City. This leaves vast expanses of the state nearly uninhabited, making the population the...
.
Life cycle
The life cycle begins with eggs being ingested
Ingestion
Ingestion is the consumption of a substance by an organism. In animals, it normally is accomplished by taking in the substance through the mouth into the gastrointestinal tract, such as through eating or drinking...
. The eggs hatch in the duodenum
Duodenum
The duodenum is the first section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms anterior intestine or proximal intestine may be used instead of duodenum...
(i.e., first part of the small intestine
Small intestine
The small intestine is the part of the gastrointestinal tract following the stomach and followed by the large intestine, and is where much of the digestion and absorption of food takes place. In invertebrates such as worms, the terms "gastrointestinal tract" and "large intestine" are often used to...
). The emerging pinworm larva
Larva
A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians typically have a larval phase of their life cycle...
e grow rapidly to a size of 140 to 150 micrometers in size, and migrate through the small intestine towards the colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
. During this migration they moult
Moult
In biology, moulting or molting , also known as sloughing, shedding, or for some species, ecdysis, is the manner in which an animal routinely casts off a part of its body , either at specific times of year, or at specific points in its life cycle.Moulting can involve the epidermis , pelage...
twice and become adults. Females survive for 5 to 13 weeks, and males about 7 weeks. The male and female pinworms mate in the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
(i.e., last part of the small intestine), whereafter the male pinworms usually die, and are passed out with stool. The gravid female pinworms settle in the ileum
Ileum
The ileum is the final section of the small intestine in most higher vertebrates, including mammals, reptiles, and birds. In fish, the divisions of the small intestine are not as clear and the terms posterior intestine or distal intestine may be used instead of ileum.The ileum follows the duodenum...
, caecum (i.e., beginning of the large intestine
Large intestine
The large intestine is the third-to-last part of the digestive system — — in vertebrate animals. Its function is to absorb water from the remaining indigestible food matter, and then to pass useless waste material from the body...
), appendix
Vermiform appendix
The appendix is a blind-ended tube connected to the cecum , from which it develops embryologically. The cecum is a pouchlike structure of the colon...
and ascending colon
Ascending colon
The ascending colon is smaller in caliber than the cecum.It passes upward, from its commencement at the cecum, opposite the colic valve, to the under surface of the right lobe of the liver, on the right of the gall-bladder, where it is lodged in a shallow depression, the colic impression; here it...
, where they attach themselves to the mucosa and ingest colonic contents. Almost the entire body of a gravid female becomes filled with eggs. The estimations of the number of eggs in a gravid female pinworm ranges from about 11,000 to 16,000. The egg-laying process begins approximately five weeks after initial ingestion of pinworm eggs by the human host. The gravid female pinworms migrate through the colon
Colon (anatomy)
The colon is the last part of the digestive system in most vertebrates; it extracts water and salt from solid wastes before they are eliminated from the body, and is the site in which flora-aided fermentation of unabsorbed material occurs. Unlike the small intestine, the colon does not play a...
towards the rectum
Rectum
The rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
at a rate of 12 to 14 centimeters per hour. They emerge from the anus
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
, and while moving on the skin near the anus, the female pinworms deposit eggs either through (1) contracting and expelling the eggs, (2) dying and then disintegrating, or (3) bodily rupture due to the host scratching the worm. After depositing the eggs, the female becomes opaque
Opacity (optics)
Opacity is the measure of impenetrability to electromagnetic or other kinds of radiation, especially visible light. In radiative transfer, it describes the absorption and scattering of radiation in a medium, such as a plasma, dielectric, shielding material, glass, etc...
and dies. The reason the female emerges from the anus is to obtain the oxygen necessary for the maturation of the eggs.
Transmission
Pinworms spread through human-to-human transmissionTransmission (medicine)
In medicine and biology, transmission is the passing of a communicable disease from an infected host individual or group to a conspecific individual or group, regardless of whether the other individual was previously infected...
, by ingesting (i.e., swallowing) infectious pinworm eggs and/or by anal sex. 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 −8 degrees Celsius (18 °F).
After the eggs have been initially deposited near the anus, they are readily transmitted to other surfaces through contamination
Contamination
Contamination is the presence of a minor and unwanted constituent in material, physical body, natural environment, at a workplace, etc.-Specifics:"Contamination" also has more specific meanings in science:...
. 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
Plumbing fixture
A plumbing fixture is an exchangeable device which can be connected to an existing plumbing system to deliver and drain away water but which is also configured to enable a particular use.-Common fixtures:The most common plumbing fixtures are:*Bathtubs...
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
Gastrointestinal tract
The human gastrointestinal tract refers to the stomach and intestine, and sometimes to all the structures from the mouth to the anus. ....
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
Autoinfection
Autoinfection is the infection of a primary host with a parasite, particularly a helminth, in such a way that the complete life cycle of the parasite happens in a single organism, without the involvement of another host. Therefore, the primary host is at the same time the secondary host of the...
(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.
Effect on host
In humans, enterobius vermicularis causes the medical condition enterobiasisEnterobiasis
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...
, whose primary symptom is itching in the anal
Anus
The anus is an opening at the opposite end of an animal's digestive tract from the mouth. Its function is to control the expulsion of feces, unwanted semi-solid matter produced during digestion, which, depending on the type of animal, may be one or more of: matter which the animal cannot digest,...
area.