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Ascaridida

An order of nematodes in which the oral opening is generally surrounded by three or six labia; in some taxa labia are absent, but the cephalic sensilla are always evident. Usually there are eight cephalic or labial sensilla; the submedians may be fused and then only four sensilla are seen. The stoma varies from being completely reduced to spacious or globose. The esophagus varies from club shaped to nearly cylindrical, never rhabditoid. There may be posterior esophageal or anterior intestinal ceca. The collecting tubules of the excretory system may extend posteriorly and anteriorly. Males generally have two spicules; however, in some taxa there may be none or only one. The gubernaculum may also be present or absent. Though females generally have two ovaries, multiple ovaries do occur. The number of uteri is also variable: two, three, four, or six. Phasmids are sometimes large and pocketlike. Reportedly, the larvae lack a stomatal hook or barb.

The order probably comprises seven superfamilies: Ascaridoidea, Seuratoidea, Camallanoidea, Dracunculoidea, Subuluroidea, Dioctophymatoidea, and Muspiceoidea (incertae sedis).

The Ascaridoidea include about 65 genera which comprise large parasitic roundworms whose adult stages usually occur in the stomach or small intestine of terrestrial and aquatic mammals, birds, reptiles, and fishes; the parasitic larval stages of many species occur, either temporarily or indefinitely, in other parts of the host's body.

Many species have a direct life cycle. Others, mainly species with marine mammals, birds, and fishes as definitive hosts, require an intermediate host, such as a fish, amphibian, insect, crustacean, or small mammal. Infestation is typically characerized by pulmonary damage and distress initially, and digestive disturbances later. Damage may also occur during larval migration to other parts of the body, including the liver and brain.

Dracunculoidea, the superfamily of parasitic nematodes, comprise obligate tissue parasites of fishes, reptiles, and mammals. All known species require an intermediate host in order to complete their life cycle, and that host is always a water flea (Cyclops). The most widely known example is Dracunculus medinensis, the guinea worm.

Ingestion of water containing infective Cyclops is the only known source of infection. The encysted nematode larvae are released from Cyclops by the digestive juices of the duodenum. Then the larvae burrow through the intestinal wall, and upon reaching the loose connective tissue, they develop to adulthood in 8 months to 1 year.

The gravid females, 28–48 in. (70–120 cm) long, migrate from the site of development to the surface of the skin, and a papule is formed, then a blister, usually on the lower extremities. When the blister comes in contact with fresh water, the uterus bursts through the anterior part of the nematode's body, and the worm also bursts, releasing cloudlike swarms of motile larvae. These larvae are then filtered from the water by Cyclops and subsequently ingested.

The formation of the blister and subsequent rupturing of the female produce a profound allergic reaction. This reaction results from the release of large amounts of toxic by-products from the worm. Upon discharge of larvae in fresh water, much of the allergic reaction abates. The reactions and systemic prodromes include erythema, urticarial rash, pruritus, vomiting, diarrhea, and giddiness. Septicemia, suppurating cysts, and chronic abscesses are not uncommonly associated with these infections. The worms can be removed surgically, or in the native manner of winding upon a stick. Chemotherapy is also available.

Control in endemic areas includes keeping infected persons from wading or bathing in water used for drinking purposes, and the education to avoid drinking suspect water. Nemata

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From McGraw-Hill Concise Encyclopedia of Environmental Science. The Content is a copyrighted work of McGraw-Hill and McGraw-Hill reserves all rights in and to the Content. The Work is © 2008 by The McGraw-Hill Companies, Inc.
 
 
 
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