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Ascaris lumbricoideshuman intestinal roundworm

By Karen Guy

Geographic Range

Ascaris lumbricoides infections have been reported in more than 150 countries across the globe, particularly in tropic, subtropic and temperate regions. Approximately 1.4 billion people worldwide are infected, 4 million of whom live in the United States. As an obligate internal parasite of humans, Ascaris lumbricoides can theoretically be found wherever humans are present. The highly durable eggs can remain dormant in the soil for up to 10 years and are resistant to many adverse conditions. (Chong, 2003; Dora-Laskey, et al., 2009; Khuroo, 1996)

Habitat

The roundworm, Ascaris lumbricoides is an obligate internal parasite and adults usually reside in the small intestine of humans, specifically the jejunum. The worm produces a pepsin inhibitor to prevent host enzymes from digesting it and uses muscular activity to avoid being excreted. The life cycle involves no free-living stages or intermediate hosts, although fertilized eggs require up to 3 weeks of embryonation in soil before becoming infective and can survive for up to 10 years in soil under warm, moist conditions. As part of the life cycle, larva briefly migrate via the circulatory and lymphatic systems through the liver, heart and lungs. Rarely the worms may migrate to other regions of the body including the appendix, pancreas, kidneys or brain. Temporary infections can be induced in other mammals (rodents), but following migration through the liver and lungs the larvae are expelled from the intestine. (Baron, et al., 1996; Bethony, et al., 2006; Crompton, 1988; Dora-Laskey, et al., 2009; Khuroo, 1996; Sprent, 1952)

Physical Description

Adults: The roundworm Ascaris lumbricoides is the largest intestinal nematode infecting humans, with females averaging 30 cm in length (ranging from 20-49 cm) and measuring 3-6 mm in diameter. Males are smaller, ranging from 15-30 cm in length and 2-4 mm in diameter. Both sexes have an elongated, cylindrical body which tapers at both ends; in males the tail curves ventrally. In addition to size, sexes may be differentiated by the vulval opening in females, located ventrally at a point of constriction approximately one third of the body length from the anterior end, and by the papillae in males, grouped pre- and post-anally. Both sexes are cream-colored, sometimes with a pink tinge. The integument of the worm is a chitinous layer of nonnucleated cuticula with circular striations. A. lumbricoides lacks circular muscles, the only muscle bands being longitudinal, and the worm uses muscular activity to remain in the intestinal lumen of the host. This roundworm also lacks a circulatory system and its digestive, excretory, nervous and reproductive systems are all suspended within the pseudocoelom.

There are three forms of eggs: fertilized, decorticate and unfertilized. Fertilized eggs are golden brown in color and ovoid in shape, measuring 30-40 μm by 50-60 μm. The egg is termed decorticate if the thick, external mamillated layer is absent. Unfertilized eggs are larger (reaching 90 μm in length) and more elongated in shape, have a thinner shell and are poorly organized internally, being a mass of variably sized granules. (Chong, 2003; Khuroo, 1996)

  • Sexual Dimorphism
  • female larger
  • sexes shaped differently
  • Range length
    20 to 49 cm
    7.87 to 19.29 in

Development

Ascaris lumbricoides develops from egg to adult through four larval stages, each followed by a molt in which the cuticle is shed. Noninfective eggs are expelled from the host intestine and into the soil where they embryonate in approximately 3 weeks, given warm, moist conditions. At this stage they are infective, and once ingested, the infective eggs hatch in the duodenum. The larva then penetrate through the intestinal mucosa and enter the lymphatic and circulatory systems, migrating through the liver to the heart and lungs. From the lungs they migrate up the trachea, upon which the host coughs the larva up into the mouth and then swallows them, returning them to the small intestine. The entire migration takes several days, during which time the larva molts. The adult stage is reached 2-3 weeks post-infection and 8-12 weeks after infection the worms reach sexual maturity. (Baron, et al., 1996; Chong, 2003; Crompton, 1988)

Reproduction

Male nematodes use chemotaxis to locate females. They have no visual abilities, and instead are attracted to specific sex pheromones which females release. Once the male has located a mate, it uses copulatory accessories such as papillae, spicules and its curved tail to direct sperm and stabilize the female during mating. There is no evidence of post-copulatory behaviors such as mate-guarding, although males of other species of nematode have been observed to secrete copulatory plugs into the vulva to prevent other males from fertilizing the same female. However, no information was found regarding the specific mating systems of Ascaris lumbricoides. (Gaugler, et al., 2004)

Ascaris lumbricoides is dioecious and copulation between individuals of opposite sexes is necessary for fertilization, and some evidence suggests pheromones play a role in mating. Males possess two testes and a curved posterior end with spicules for copulation. Females possess ovaries which are continuous with an oviduct and a tubular uterus; the uteri join to form a vagina which opens into the vulva. Sperm is transferred into the vulva of the female, enters the ovum and forms a zygote. The zygote then secretes a fertilization membrane which thickens to form the chitinous shell that protects the egg when it is expelled from the host. Females have been shown to lay as many as 234,000 eggs per day, and this daily egg output implies year-round mating with no specific breeding season. The eggs can survive in the soil for some time, and the larvae require 8-12 weeks after ingestion to reach reproductive maturity. (Baron, et al., 1996; Brown and Cort, 1927)

  • Range number of offspring
    234,000 eggs laid daily (high)
  • Average number of offspring
    200,000 eggs laid daily
  • Range age at sexual or reproductive maturity (female)
    8 to 12 weeks
  • Range age at sexual or reproductive maturity (male)
    8 to 12 weeks

Ascaris lumbricoides exhibits no parental care of offspring. The large fecundity of females (they produce about 200,000 eggs daily), the unusual resistance of the eggs themselves and the lack of free living stages ensures that some eggs will be ingested by a host and survive to reach reproductive maturity. (Bethony, et al., 2006; Brown and Cort, 1927)

  • Parental Investment
  • no parental involvement

Lifespan/Longevity

The adult stage is reached 8-12 weeks after being ingested by a human host, and adults live for approximately a year. (Baron, et al., 1996; Khuroo, 1996)

  • Range lifespan
    Status: wild
    6 to 18 months

Behavior

Ascaris lumbricoides lacks circular muscles and as a result moves in a serpentine pattern as it alternates contracting longitudinal muscles on either side of its body. No other information was found regarding the behavior of this species. (Khuroo, 1996)

Communication and Perception

Nematodes have limited visual abilities, instead relying on chemosensory interactions to find mates and food and to orient themselves inside the host. Specifically, the females release sex pheromones to attract males. Roundworms also possess papillae, used for tactile sensation and particularly employed in copulation. However, no specific information is known regarding communication and perception in Ascaris lumbricoides. (Gaugler, et al., 2004)

Food Habits

Ascaris lumbricoides uses feeds in the human intestine liquid contents flow past. (Khuroo, 1996)

  • Animal Foods
  • body fluids

Predation

Ascaris lumbricoides has no known predators while inside the host, nor outside it as there are no free-living stages. The eggs are eaten by the host but are usually ingested accidentally (the only exception being in the case of laboratory experiments in which doses are purposefully administered to subjects). (Khuroo, 1996)

Ecosystem Roles

Ascaris lumbricoides is the most common intestinal parasite of humans. It has no known predators or other hosts, although the closely related species A. suum is a parasite of pigs. (Chong, 2003; Khuroo, 1996)

Species Used as Host

Economic Importance for Humans: Positive

No information was found regarding benefits to humans provided by Ascaris lumbricoides, but as an intestinal parasite it is unlikely that there are any.

Economic Importance for Humans: Negative

Ascaris lumbricoides infects approximately 1.4 billion people in over 150 countries worldwide, and is estimated to infect anywhere from one fourth to one third of the global population. In 2001, an estimate of disability-adjusted life years due to Ascaris lumbricoides was 1-2 years. This roundworm contributes significantly to the burden of abdominal surgical emergencies, as the rate of complications from infection can be as high as 67%, primarily due to intestinal and biliary tract obstruction. Infection is rarely fatal, but because of its high prevalence it is still responsible for 8000-100,000 deaths annually. Infection with Ascaris lumbricoides is also an important cause of malnutrition, particularly in children, causing protein energy loss and vitamin A and C deficiencies. Overall, it can cause stunting of linear growth, leading to both physical and mental deficits. (Bethony, et al., 2006; Crompton, 1988; Dora-Laskey, et al., 2009; Khuroo, 1996)

Conservation Status

Ascaris lumbricoides is the most common multicellular intestinal parasite of humans, found in over 150 countries worldwide. (Chong, 2003)

Contributors

Karen Guy (author), University of Michigan-Ann Arbor, Heidi Liere (editor), University of Michigan-Ann Arbor, John Marino (editor), University of Michigan-Ann Arbor, Barry OConnor (editor), University of Michigan-Ann Arbor, Renee Mulcrone (editor), Special Projects.

Glossary

Australian

Living in Australia, New Zealand, Tasmania, New Guinea and associated islands.

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Ethiopian

living in sub-Saharan Africa (south of 30 degrees north) and Madagascar.

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Nearctic

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.

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Neotropical

living in the southern part of the New World. In other words, Central and South America.

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Palearctic

living in the northern part of the Old World. In otherwords, Europe and Asia and northern Africa.

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agricultural

living in landscapes dominated by human agriculture.

bilateral symmetry

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.

carnivore

an animal that mainly eats meat

causes disease in humans

an animal which directly causes disease in humans. For example, diseases caused by infection of filarial nematodes (elephantiasis and river blindness).

chemical

uses smells or other chemicals to communicate

cosmopolitan

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.

ectothermic

animals which must use heat acquired from the environment and behavioral adaptations to regulate body temperature

fertilization

union of egg and spermatozoan

heterothermic

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.

internal fertilization

fertilization takes place within the female's body

iteroparous

offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).

motile

having the capacity to move from one place to another.

oceanic islands

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.

oriental

found in the oriental region of the world. In other words, India and southeast Asia.

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oviparous

reproduction in which eggs are released by the female; development of offspring occurs outside the mother's body.

parasite

an organism that obtains nutrients from other organisms in a harmful way that doesn't cause immediate death

pheromones

chemicals released into air or water that are detected by and responded to by other animals of the same species

sexual

reproduction that includes combining the genetic contribution of two individuals, a male and a female

suburban

living in residential areas on the outskirts of large cities or towns.

tactile

uses touch to communicate

temperate

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).

terrestrial

Living on the ground.

tropical

the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

urban

living in cities and large towns, landscapes dominated by human structures and activity.

year-round breeding

breeding takes place throughout the year

References

Baron, S., G. Castro, D. Wakelin, J. Cross. 1996. Medical Microbiology. Galveston, Texas: The University of Texas Medical Branch of Galveston. Accessed March 19, 2011 at http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mmed&part=Section4.bxml.

Bethony, J., S. Brooker, M. Albonico, S. Geiger, A. Loukas, D. Diemert, P. Hotez. 2006. Soil-transmitted helminth infections: ascariasis, trichuriasis, and hookworm. The Lancet, 367 (9521): 1521-1532. Accessed March 19, 2011 at http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(06)68653-4/fulltext.

Brown, H., W. Cort. 1927. The Egg Production of Ascaris lumbricoides. The Journal of Parasitology, Volume 14 Issue 2: 88-90.

Chong, Y. 2003. "Ascaris lumbricoides" (On-line). Web Atlas of Medical Parasitology. Accessed March 19, 2011 at http://www.atlas.or.kr/atlas/alphabet_view.php?my_codeName=Ascaris%20lumbricoides.

Crompton, D. 1988. The prevalence of ascariasis. Parasitology Today, Volume 4 Issue 6: 162-169.

Dora-Laskey, A., U. Ezenkwele, E. Weiss. 2009. "Ascaris lumbricoides" (On-line). eMedicine - Medscape. Accessed March 19, 2011 at http://emedicine.medscape.com/article/788398-overview.

Gaugler, R., A. Bilgrami, R. Huettel. 2004. Nematode Behavior. Trowbridge, UK: CABI. Accessed April 06, 2011 at http://books.google.com/books?hl=en&lr=&id=gr10r8veZ9gC&oi=fnd&pg=PA127&dq=Ascaris+lumbricoides+reproductive+behavior&ots=5f1xK5jhAe&sig=R1VA6-5wjwsEr6NxzOdaGnPAlv0#v=onepage&q=&f=false.

Khuroo, M. 1996. Ascariasis. Gastroenterology Clinics, 25 (3): 553-577.

Sprent, J. 1952. On the migratory behavior of the larvae of various Ascaris species in white mice: I. Distribution in tissues. The Journal of Infectious Diseases, 90 (2): 165-176.

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