Ascaris lumbricoides
human intestinal roundworm

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)

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)

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)

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)

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)

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)

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)

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)

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)

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

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)

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)

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

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)

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