Enterobius vermicularis are at home in every biome that is home to humans, around the world. They develop and reproduce in the intestines of humans. Eggs are expelled outside the human body to find a new host.
As a nematode, Enterobius vermicularis has a cylindrical body, and a cuticle with three main outer layers made of collagen and other compounds, secreted by the epidermis. The cuticle layer protects the nematode so it can invade digestive tracts of animals. The worms molt four times, the first two before hatching, and then before their adult stage.
Males are 2-5 mm long by 0.1-0.2 mm wide and have a curled tail, where females are 8-13 mm long by 0.3-0.5 mm wide with a pointed tail. This pointed, or "pin" shaped tail is how E. vermicularis received its common name: pin worms. Females are also distinguished by their alae, or wing-like, anterior expansions of the body wall. Both sexes have three lips surrounding the circular mouth.
Eggs are elongate and characteristically flattened on one side, measuring 50-60 µm by 20-30 µm. The eggs have five membranes: one inner, lipoidal layer, three middle layers known as membrana lucida, and one outer, albuminous membrane which coats the egg. This membrane makes the eggs sticky and therefore itchy to the host, which is important in the life cycle. The larvae differ from adults only in that they are smaller and coiled.
As a member of the Secernentea, Enterobius vermicularis has a specialized tubular excretory system system with three canals. The canals are arranged to form an "H". (Barnes, 1987; Bogitsh and Cheng, 1998; Brusca and Brusca, 2003; Chitwood and Chitwood, 1950; Garcia and Bruckner, 1997)
Humans become infected when ingesting the worm's eggs. The eggs travel down to the host's intestines, molting twice. The juveniles remain in the egg until they've developed to their third stage. The eggs hatch and the worms migrate to the large intestine. Most all adult structures except certain reproductive parts are found in the young just after hatching. The worms molt before becoming become adults. As adults, the worms will not molt, but can grow in size. When reaching the right host, it takes one month for eggs to develop into females and begin egg production. (Barnes, 1987; Brusca and Brusca, 2003)
Females may produce a pheromone to attract males. The male coils around a female with his curved area over the female genital pore. The gubernaculum, made of cuticle tissue, guides spicules which extend through the cloaca and anus. Males use spicules to hold the females during copulation. Nematode sperm are amoeboid-like and lack flagella.
Copulation occurs between adults in the large intestine, and each female produces about 10,000 eggs. The female migrates to the perianal skin at night, spurred by the drop in body temperature of the host. She will only oviposit on the perianum, because air seems to be an ovipositing stimulant. The expulsion of eggs is so forceful that the eggs are sent airborne, and can be spread out over the perianum. After ovipositing, the female often dies, but occasionally returns (or attempts to return) to the intestine. The male dies soon after fertilization of the female. The life cycle of E. vermicularis is about two months long.
Enterobius vermicularis is common among children because thumb-sucking is common at this age. Transmission is usually: a child somehow ingests eggs (probably from sheets or clothes of another child); juveniles hatch in small intestine; adults migrate into large intestine; females migrate to perianal area and oviposit; child scratches perianal area (to relieve itching caused by female migration and sticky eggs) and eggs imbed under child's fingernails; child sucks thumb and ingests more eggs. Infection is possible by inhalation of airborne eggs, but it is rare. (Bogitsh and Cheng, 1998; Chitwood and Chitwood, 1950; Despommier, et al., 1994; Donowitz, 1999; Garcia and Bruckner, 1997)
Enterobius vermicularis neither aggregate (clump) nor swarm (coordinated population movements) while in the host, two characteristics that occur somewhat frequently in Nematoda. Enterobius vermicularis' optimal temperature is around 33°C, showing reduced activity at temperatures higher than 40°C and inactivity at 46°C. When the temperature drops, E. vermicularis again becomes less active. The optimal temperature of 33°C is close to the temperature of a human intestine. Movement seems to favor right angles, like most nematodes. This movement may be a leftover evolutionary trait from when its ancestors had to penetrate a host's skin. (Croll, 1970)
Nematodes in general have papillae, setae and amphids, which are the main sense organs. Setae detect motion (mechanoreceptors), while amphids detect chemicals (chemoreceptors). (Barnes, 1987; Brusca and Brusca, 2003)
Adult E. vermicularis live in the lumen of the transverse and descending colon. Humans are its only host. They feed on blood and tissue cells in the host's intestine. Food boluses move from the worm's mouth to its intestine through the esophagus, which consists of three esophageal regions. These regions create a type of peristalis, forcing the bolus downward. Extracellular digestion begins within the intestine, and the digestive cycle is completed intracellularly. Enterobius vermicularis, like all nematodes, feed more than is necessary and wastes much of what is brought in. (Barnes, 1987; Brusca and Brusca, 2003; Chitwood and Chitwood, 1950; Despommier, et al., 1994)
These parasites are usually not preyed on directly, but are ingested from host to host. Larval mortality is high as most of the parasites do not reach appropriate hosts.
Humans are this parasites only host.
Thirty-three percent of American children are infected with E. vermicularis and it may be one of the most common human parasitic infections in the world. Classic symptoms are perianal and vaginal itching caused by the migration of the female and the stickiness of her eggs. Scarring occasionally occurs and most individuals remain asymptomatic. Severe cases of heavy infestation have been known to cause sleeplessness, weight loss, hyperactivity, grinding of teeth, abdominal pain, and vomiting. E. vermicularis are not vectors of any know pathogens.
Occasionally, after a female oviposits on the perianal skin, she will migrate into a female host's vagina, instead of back into the anus. This produces irritative symptoms such as vulvovaginitis. In heavily infected females, migration into the vagina may cause a mucoid vaginal discharge. Enterobius vermicularis infestation of the uterus can cause bleeding from the vagina, which may be mistaken for menstruation. This infestation has caused postmenopausal women to believe they've began to menstruate again and prepubescent girls to believe they've begun to menstruate.
Enterobius vermicularis infestation is not necessarily one of poor hygiene -- most every human will have at least one infection in their lifetime. Total prevention has been described as neither realistic nor possible. The most effective deterrent to infection is to cut fingernails short, to wash hands thoroughly after using the bathroom and before preparing food, and to wash bed sheets and towels in hot water. (Al-Rufaie, et al., 1998; Bogitsh and Cheng, 1998; Despommier, et al., 1994; Garcia and Bruckner, 1997)
Doug Rett (author), University of Michigan-Ann Arbor, Teresa Friedrich (editor), University of Michigan-Ann Arbor.
lives on Antarctica, the southernmost continent which sits astride the southern pole.
Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.
living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.
living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.
living in the southern part of the New World. In other words, Central and South America.
living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.
living in landscapes dominated by human agriculture.
having body symmetry such that the animal can be divided in one plane into two mirror-image halves. Animals with bilateral symmetry have dorsal and ventral sides, as well as anterior and posterior ends. Synapomorphy of the Bilateria.
an animal that mainly eats meat
an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).
Found in coastal areas between 30 and 40 degrees latitude, in areas with a Mediterranean climate. Vegetation is dominated by stands of dense, spiny shrubs with tough (hard or waxy) evergreen leaves. May be maintained by periodic fire. In South America it includes the scrub ecotone between forest and paramo.
uses smells or other chemicals to communicate
having a worldwide distribution. Found on all continents (except maybe Antarctica) and in all biogeographic provinces; or in all the major oceans (Atlantic, Indian, and Pacific.
in deserts low (less than 30 cm per year) and unpredictable rainfall results in landscapes dominated by plants and animals adapted to aridity. Vegetation is typically sparse, though spectacular blooms may occur following rain. Deserts can be cold or warm and daily temperates typically fluctuate. In dune areas vegetation is also sparse and conditions are dry. This is because sand does not hold water well so little is available to plants. In dunes near seas and oceans this is compounded by the influence of salt in the air and soil. Salt limits the ability of plants to take up water through their roots.
animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature
union of egg and spermatozoan
forest biomes are dominated by trees, otherwise forest biomes can vary widely in amount of precipitation and seasonality.
having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.
fertilization takes place within the female's body
having the capacity to move from one place to another.
This terrestrial biome includes summits of high mountains, either without vegetation or covered by low, tundra-like vegetation.
active during the night
islands that are not part of continental shelf areas, they are not, and have never been, connected to a continental land mass, most typically these are volcanic islands.
found in the oriental region of the world. In other words, India and southeast Asia.
reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.
an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death
chemicals released into air or water that are detected by and responded to by other animals of the same species
the regions of the earth that surround the north and south poles, from the north pole to 60 degrees north and from the south pole to 60 degrees south.
rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.
scrub forests develop in areas that experience dry seasons.
remains in the same area
reproduction that includes combining the genetic contribution of two individuals, a male and a female
living in residential areas on the outskirts of large cities or towns.
uses touch to communicate
Coniferous or boreal forest, located in a band across northern North America, Europe, and Asia. This terrestrial biome also occurs at high elevations. Long, cold winters and short, wet summers. Few species of trees are present; these are primarily conifers that grow in dense stands with little undergrowth. Some deciduous trees also may be present.
that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).
Living on the ground.
the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.
A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.
A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.
A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.
A terrestrial biome with low, shrubby or mat-like vegetation found at extremely high latitudes or elevations, near the limit of plant growth. Soils usually subject to permafrost. Plant diversity is typically low and the growing season is short.
living in cities and large towns, landscapes dominated by human structures and activity.
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Chitwood, B., M. Chitwood. 1950. Introduction to Nematology. Baltimore: University Park Press.
Croll, N. 1970. The Behavior of Nematodes. London: Edward Arnold Ltd..
Despommier, D., R. Gwadz, P. Hotez. 1994. Parasitic Diseases. New York: Springer-Verlag.
Donowitz, L. 1999. Infection Control in the Child Care Center and Preschool. Philadelphia: Lippincott, Williams & Wilkins.
Garcia, L., D. Bruckner. 1997. Diagnostic Medical Parasitology. Washington, D.C.: ASM Press.
Ohio State University, 2001?. "Pinworms (Enterobius vermicularis, Oxyuris spp.)" (On-line). Parasites and Parasitological Resources. Accessed September 07, 2004 at http://www.biosci.ohio-state.edu/~parasite/enterobius.html.
University of Cambridge Dept. of Pathology Schistosomiasis Research Group, 1998. "Enterobius vermicularis" (On-line). Helminthology and General Parasitology Pages, Nematode Parasitic Infections. Accessed September 07, 2004 at http://www.path.cam.ac.uk/~schisto/Nematodes/Enterobius.html.