NCBI Bookshelf. Baron S, editor. Medical Microbiology.
Symptoms correlate with worm load:light lo are asymptomatic; heavier lo cause abdominal symptoms, diarrhea, and sometimes malnutrition. A bolus of worms may obstruct the intestine. Migrating larvae can cause pneumonitis and eosinophilia. Ascaris lumbricoides is the largest intestinal nematode of humans.
Females are up to 30 cm long; males are smaller. Three types of eggs may appear in feces: fertilized, unfertilized, and decorticated.
Adults in the small intestine produce eggs that pass in feces, embryonate in soil, are ingested, and hatch. The larvae migrate from the intestine to the lung and back to the intestine, where they mature. Migrating larvae cause eosinophilia and sometimes allergic reactions. Erratic adult worms may invade other organs. Heavy infections can impair nutrition. Egg viability is supported by warm, moist soil. Transmission is favored by unsanitary disposal of feces. Prevalence is highest in children. Diagnosis is made most often by identifying eggs in stool; occasionally, erratic adults emerge from body orifices.
Pneumonitis, cough, dyspnea and hemoptysis may mark the migration of larvae through the lungs. Depending on the adult worm load, intestinal infection can cause anorexia, fever, diarrhea, weight loss, and anemia. Two species of hookworms infect humans: Ancylostoma duodenale and Necator americanus.
They are distinguished by the morphology of the mouth parts and male bursa. Females are larger. Eggs are oval, thin-shelled, and transparent.
Eggs hatch to release rhabditiform larvae, which mature into filariform infective stage larvae. Adults attach to the mucosa of the small intestine. Eggs passed in feces embryonate and hatch in soil; mature larvae penetrate the skin and migrate first to the lungs, and then to the intestine, where they mature into the adult stage. Larvae entering skin often cause an erythematous reaction.
Larvae in the lung may cause small hemorrhages, eosinophilic infiltration, and pneumonitis.
Blood loss from sites of intestinal attachment may cause iron-deficiency anemia. Spontaneous self-cure may represent a hypersensitivity reaction. Infection induces high levels of IgE. Diagnosis is by detection of eggs and sometimes larvae in stool. Low levels of hemoglobin are suggestive. Ground itch may occur where larvae penetrate the skin.
Pneumonitis, epigastric pain, mucous diarrhea, and eosinophilia may occur. In immunocompromised individuals, worms may disseminate to other organs. Males are free-living; females may be free-living or parasitic. Eggs develop into rhabditiform and then filariform infectious larvae. Parasitic females parthenogenetically produce embryonated eggs, which hatch in the intestine. Rhabitiform larvae pass in the feces, mature to the infective filariform stage in soil, penetrate the skin, and migrate to the lungs and other organs, then the intestine. Autoinfection also occurs. Free-living worms reproduce sexually in soil.
Worms cause inflammation and ulceration of the intestines.
Migrating larvae cause cutaneous pruritus and pneumonitis. Hyperinfection causes sloughing of mucosa, and disseminated infection occasionally le to pulmonary hemorrhage, pneumonia, or meningitis and death. Immunity is not well understood.
Infection induces elevated IgE and eosinophilia. Impairment of cell-mediated immunity favors disseminated disease and autoinfection. Prevalence is usually low; the infection is more common in tropical countries with poor sanitation, especially Southeast Asia and parts of Africa. Dogs occasionally serve as a reservoir.
Epigastric pain, eosinophilia, and mucous diarrhea are suggestive; diagnosis is confirmed by detecting rhabditiform larvae in feces, duodenal aspirates, or sputum. Fecal cultures and serology may be helpful. Diarrhea, anemia, weight loss, abdominal pain, nausea, vomiting, eosinophilia, tenesmus, rectal prolapse, stunted growth and finger clubbing may occur. Adults are whip-shaped, slender anteriorly and broader posteriorly. Males are shorter than females and have a coiled posterior. The unembryonated eggs are barrel-shaped with bipolar plugs.
Adults in the large intestine lay eggs which pass in feces and embryonate in soil. Eggs that are ingested hatch and larvae mature to adults in the gut. Adults prefer the cecum but will also colonize the large intestine. Worms cause mucosal inflammation, eosinophilic infiltration, and minor blood loss; heavy infections may lead to anemia and nutrititional deficiency.
Enterobiasis is most common in children, who usually manifest pruritus ani and sometimes insomnia, abdominal pain, anorexia, and pallor. Genitourinary infection may occur in females. Worms are white and spindle-shaped with a large, bulbar esophagus.
Males are smaller and have a curved posterior. Eggs are ovoid, thin-shelled, and flat on one side. Females usually migrate out the anus at night and depositeggs on the perianal skin. The eggs embryonate quickly and, if ingested, hatch and mature in the intestines.
Enterobius vermicularis is the most common helminth in the United States. Household and institutional epidemics occur, usually in children. Transmission is usually by hand to mouth transfer of infective eggs. Control is by anthelmintic treatment and by improved personal hygiene, including washing the perianal region and changing nightclothes.
Enteric nematodes are among the most common and widely distributed animal parasites of humans. Sincethe world population has doubled and, by all indications, enteric nematode infections of humans have kept pace. The most common intestinal roundworms are those transmitted through contact with the soil for example Ascaris lumbricoidesTrichuris trichiura, the hookworms, and Strongyloides stercoralis.
In Stoll's estimate, these worms, with Enterobius vermicularis, ed for three-quarters of all helminthic infections. Most enteric nematodes have established a well-balanced host-parasite relationship with the human host; humans tolerate these parasites well. Little disease is associated with light infection, but when the worm load increases, a corresponding increase in disease usually occurs.
The worms may irritate the intestinal mucosa, causing inflammation and ulceration. The larger worms may become entangled and block the intestinal tract. Larval worms that migrate through the tissue to complete their life cycle may lose their way, end up in the wrong organ, and cause severe disease. Nutritional problems occasionally are associated with the intestinal parasitosis, and persons with deficient diets often suffer from polyparasitism.
Diagnosis usually is based on microscopic examination of feces for eggs and larvae, except in the case of pinworm infections, which are diagnosed by examining samples taken with perianal swab. Many antihelmintics are available to treat patients with these infections.
Control depends largely on proper disposal of human feces and on personal hygiene. The enteric nematodes discussed in this chapter are A lumbricoides ; the hookworms N americanus and A duodenale; S stercoralis; T trichiura; and E vermicularis. Adult A lumbricoides infections involving only a few worms are usually asymptomatic, but as the worm load increases, symptoms of abdominal discomfort, nausea, vomiting, weight loss, fever, and diarrhea develop.
Allergic manifestations in hypersensitized persons lead to pneumonitis, cough, low-grade fever, and eosinophilia. Large s of worms may form a bolus and cause intestinal obstruction. Stimulation causes adult worms to become erratic and invade the appendix and bililary and pancreatic ducts.
Worms may enter and block small orifices. Migrating adults have been vomited and passed from the nose and mouth, anus, umbilicus, and lacrimal glands. They can perforate the intestines and enter the peritoneal cavity, the respiratory tract, urethra, and vagina, and even the placenta and fetus.
Excessive worm lo, especially among the malnourished, can lead to nutritional impairment because the worms interfere with the absorption of proteins, fats, and carbohydrates. Ascaris lumbricoides is the largest and most common intestinal nematode of humans. Females are approximately 30 cm long; sexually mature males are smaller.
The diameter varies from 2 to 6 mm. The mammillated coat of unfertilized eggs is irregular and the contents are granular and disorganized. Some eggs are passed without the outer mammililated coat decorticated eggs and can be confused with eggs from hookworms or other worms. Ascaris lumbricoides is found in the small intestine, particularly the jejunum.