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ACC MLAB 1331 - Nematode

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Class Nematoda - The Roundworms A. Introduction - nematodes comprise the group of organisms containing the largest number of helminth parasites of humans. They are unsegmented, bilaterally symetrical, and exhibit great variation in their life cycles. Generally, they are long-lived (1 - 30+ years). 1. Includes both free-living and parasitic forms. Some can be both free-living and parasitic at times, i.e. Strongyloides stercoralis. 2. Vary greatly in size from a few millimeters to over a meter. 3. The male worm frequently has a curved or coiled posterior end with copulatory spicules and in some species a bursa is present. 4. The adult anterior may have oral hooks, teeth or plates in the buccal cavity for purpose of attachment. 5. Body is fairly complex. 6. The outer body surface is called a cuticle, there are muscle layers underneath. 7. Internal organs include a complex nerve cord, a well-developed digestive system and complete reproductive organs. a. The male has testes, vas deferens, seminal vesicle and an ejaculatory duct. b. The female has ovaries, oviduct, seminal receptacle, uterus and vagina. 8. Reproductive capacity (fecundity) is proportional to complexity of life cycle. 9. Humans are definitive hosts for many roundworms of medical importance. 10. The adult female produces fertilized eggs, or larvae which may be infective to new host in three ways: a. Eggs are immediately infective after ingestion by humans. b. Eggs or larvae require a period of development in the environment to become infective. c. Eggs or larvae are transmitted to a new host by an insect. 11. Developing larvae go through a series of 4 molts with the third stage, the filariform larva, being most often the infective stage. 12. Infection with roundworms can be by ingestion of infective eggs or larvae or by larval penetration of skin or by transmission of larvae through insect bite. 13. About one-half of the nematodes parasitic for man are intestinal, the others are found in various tissues. 14. Pathogenicity of intestinal nematodes may be due to larval migration through body tissues, piercing of intestinal wall, bloodsucking activities of worms or allergic reactions to secretions, worms or larvae. Nematode.doc 1 of 18B. Overview 1. Terms a. Filariform larvae - the infective stage; long, thread-like; often “designed” for penetration. b. Rhabditiform larvae - characterized by the presence of a muscular esophagus and bulbular pharynx. The first “molt “ worms after leaving the egg are termed “rhabditiform”. 2. Life cycle stages a. Egg - characteristic of the Genus. Size & shape are relatively consistent. b. Larvae - undergo several molts (third stage generally the infective one) c. Adult - varies in size from Genus to Genus; range from less than 1 mm to over one meter. 3. General Life Cycle of Intestinal nematodes a. Humans ingest infective eggs. Hookworm and Strongyloides stercoralis are exceptions, in these filariform larvae penetrate the skin to gain entry. b. Larvae hatch in intestine c. Male and female adults develop in the intestine. With Ascaris lumbricoides, Hookworms, and Strongyloides stercoralis larvae penetrate the intestinal mucosa and initiate a heart – lung cycle enroute to the intestinal tract to mature to adults. d. Fertilized eggs are produced e. Diagnostic stage - eggs or larvae in feces. f. Larvae develop within the egg in warm, moist soil (except for Strongyloides stercoralis, whose eggs hatch in the intestine, with larvae passing in the feces) C. Intestinal nematodes 1. Enterobius vermicularis - The Pinworm Anterior end of adult worm Pinworm eggs Pinworm egg, enlarged a. Distribution (1) Most common helminth infection in the U.S.A. Transmission is Nematode.doc 2 of 18direct, person-to-person; egg is infective immediately or within hours of being shed by the female. (2) Common worldwide but more prevalent in temperate climates. (3) Higher prevalence in Caucasians than in Negroes. (4) It is a group infection especially common among children. Very often associated with low sanitation and hygiene. (5) Humans are the only known host. Dogs and cats are not infected. b. Life cycle (1) Eggs are ingested, hatch in intestine, larvae mature, adults live in the colon. (2) Gravid females migrate to the perianal area at night to lay eggs. Each female produces up to 15,000 eggs. (3) Eggs develop to the infective stage within 4-6 hours. (4) Eggs are resistant to drying, and can survive for extended periods in cool, moist environment. Viable eggs can be found on bed linens, towels, furniture, windowsills, door jams and in dust. Cleaning eggs from the environment and treating all persons in the household is important in order to break life cycle. (5) Eggs are found only rarely in fecal samples because release is most often external to the intestines. On occasion, a worm will die, releasing her eggs in the bowel. This is rare, however. c. Morphology (1) Adults - female: creamy white, ~ 8-13 mm long, with sharply pointed tails, and wing-like flaps (cervical alae) at the head end; male: small (2-5 mm) with strongly curved posterior. (2) Eggs - 50 to 60 x 20 to 32 microns, broadly oval, and flattened on one side. Somewhat compressed laterally; normally are embryonated (contain a larva). d. Diagnosis (1) Recovery and identification of eggs or adults from the perianal region utilizing the cellophane tape preparation. (2) Specimens must be collected the first thing in the morning upon waking, especially before bathing or bowel movements. (3) Eggs are rarely found in fecal samples because release is usually external to the intestines. e. Major pathology and symptoms Nematode.doc 3 of 18(1) One third of all cases are asymptomatic (2) Infections rarely cause serious lesions. (3) Other symptoms may be associated with the migration of the female out of the anus to lay her eggs and include: severe peri-anal itching due to hypersensitivity reaction (eggs can get on hands and re-infect), mild nausea or vomiting, loss of sleep, irritability, slight irritation of the intestinal mucosa, and vulval irritation in girls from migrating worms (after egg laying, worm may migrate into the vagina instead of the anus). 2. Trichuris trichiura - The Whipworm T. trichiura female T. trichiura male T. trichiura egg a.


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