Nematodes are cylindrical, unsegmented, elongated white worms. Their mode of entry into the human host varies from ingestion of eggs (Ascaris and Enterobius) to penetration of the skin (Necator, Ancylostoma, and Strongyloides) to inoculation by insect bite ( Wuchereria).
ASCARIS Ascaris lumbricoides has a worldwide distribution, and its life span if left treated is 2 to 7 years. Larval invasion follows the ingestion of Ascaris eggs, and during this stage the parasite migrates through the lungs. Clinical disease is due to pulmonary hypersensitivity or intestinal complications. Patients may have fever, cough, dyspnea, hemoptysis, and eosinophilia. Obstruction of the common bile duct and the intestine have been described. 9 The chest x-ray may reveal eosinophilic pneumonitis (Loeffler syndrome). The diagnosis is made by finding eggs or, occasionally, an adult worm in the stool. Serologic tests, including bentonite flocculation, ELISA, and indirect hemagglutination, may be helpful. Treatment is with mebendazole, albendazole, or pyrantel pamoate. Intestinal obstruction may necessitate surgery, especially in children.
ENTEROBIUS (PINWORM) Adult Enterobius (pinworm) resides in the cecum, appendix, ileum, and ascending colon after its eggs are ingested. The gravid female migrates to the anus, especially at night, where it causes intense pruritus. Autoinfection with hand-to-mouth transmission is possible after scratching. A host of problems from vaginitis to enuresis have been attributed to Enterobius infection without good evidence. It is most prevalent in temperate climates during the winter and fall. The diagnosis is confirmed with a cellophane tape swab of the anus. All family members should be examined. Treatment is with pyrantel pamoate, albendazole, or mebendazole and should be repeated after 2 weeks.
NECATOR (HOOKWORM) Necator americanus prevails in the southern United States and is often seen in immigrants from warmer climates. Infection is associated with the use of human fertilizer and the lack of shoes and latrines. Because each worm can withdraw 0.03 to 0.2 mL of blood a day, infection often leads to chronic anemia. Pica and geophagy are often seen in infected children. Patients may have cough, low-grade fever, abdominal pain, diarrhea, weakness, weight loss, heme-positive stools, and eosinophilia. The diagnosis is made by finding ova in the stool. In mild infections, multiple stool specimens or concentration techniques may be necessary. The parasite burden may be estimated using the Beaver stool or Kato slide smear method. Infections with less than 2100 eggs per gram of feces (<50 adult worms) are usually not hematologically important, whereas infections with over 11,000 eggs per gram result in significant anemia. Hookworm is best treated with mebendazole, albendazole, or pyrantel pamoate.
STRONGYLOIDES (THREADWORMS) Adult threadworms reside in the mucosa of the small intestine. Because entry of the parasite is through the skin, penetration can lead to allergic manifestations, pruritus, and an erythematous rash. Migration throughout the lungs can produce cough, dyspnea, and pneumonia. The intestinal phase is manifested by abdominal pain, diarrhea with mucus and blood, and eosinophilia. Autoinfection can occur due to internal production of infective larvae. Larval migration in the skin produces larva currens. Fatalities may occur due to hyperinfection in elderly and immunocompromised patients (e.g., patients with leprosy, nephrotic syndrome, hepatic disease, or lymphoproliferative disorders and those on steroids). The diagnosis is confirmed by finding larvae in the stool. Occasionally, use of a formalin ether concentration method or duodenal aspiration may be necessary. Various stages of the parasite may be found in the sputum. An ELISA test is also available. An upper gastrointestinal series may reveal a deformed duodenal bulb, and Strongyloides may be confused with ulcer disease. Treatment is with thiabendazole or ivermectin.
TRICHURIS TRICHIURA (WHIPWORM) Like Ascaris, Trichuris trichiura is found in rural communities in the southern United States. The infection is most often acquired in childhood because the ova are deposited in the soil where children play and defecate freely. The adult worm resides in the cecum. Patients complain of anorexia, insomnia, abdominal pain (including pain in the right upper quadrant), fever, flatulence, bloody diarrhea, weight loss, and pruritus and may have eosinophilia and microcytic hypochromic anemia. Trichuris can result in colitis or rectal prolapse in children. The diagnosis is made with the finding of ova in the stool. Mebendazole or albendazole is the treatment of choice.
TRICHINELLA SPIRALIS Trichinosis is common in Mexico and the United States and results from the consumption of infected pork and, less commonly, bear and walrus meat. In the early stages of infection with Trichinella spiralis, the patient may present with acute myocarditis, nonsuppurative meningitis, bronchopneumonia, or catarrhal enteritis. The primary lesions are in striated muscle. Clinical symptoms depend on the site of invasion. Patients may present with nausea and vomiting, diarrhea, fever, urticaria, periorbital edema, (pathognomonic) splinter hemorrhages, myalgia, muscle spasm, stiff neck, headache, and psychiatric disturbances. Laboratory manifestations of trichinosis include leukocytosis, eosinophilia, elevated creatine phosphokinase and electrocardiographic changes. The diagnosis can be confirmed with latex agglutination, skin test, and a bentonite flocculation test. Biopsy of tender muscle may be helpful after the fourth week. Since T. spiralis encysts in striated muscle, stool examination is not helpful after the initial gastrointestinal phase in making the diagnosis. The differential diagnosis includes staphylococcal and salmonella food poisoning, shigellosis, and amebiasis. Mebendazole is indicated for treatment of the intestinal phase but may be ineffective after encystment. Steroids are indicated for severe infections, such as CNS disease and myocarditis, but are not advocated routinely because their use can increase the number of circulating larvae. Most cases are mild and never come to medical attention.
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