Intestinal nematodes infect >1 billion persons worldwide, most commonly in regions with poor sanitation and particularly in developing countries in the tropics or subtropics. Because most helminthic parasites do not self-replicate, clinical disease (as opposed to asymptomatic infection) generally develops only with prolonged residence in an endemic area and is typically related to infection intensity.

Ascariasis

Microbiology Ascariasis is caused by Ascaris lumbricoides, the largest intestinal nematode, which reaches lengths up to 40 cm.

• Humans—primarily younger children—are infected by ingestion of fecally contaminated soil that contains ascarid eggs.
• Larvae hatch in the intestine, invade the mucosa, migrate to the lungs, break into the alveoli, ascend the bronchial tree, are swallowed, mature in the small intestine, and produce up to 240,000 eggs per day that pass in the feces.

Clinical Manifestations Most infections have a low worm burden and are asymptomatic. During lung migration of the parasite (~9-12 days after egg ingestion), pts may develop a cough and substernal discomfort, occasionally with dyspnea or blood-tinged sputum, fever, and eosinophilia.

• Eosinophilic pneumonitis (Löffler's syndrome) may be evident.
• Heavy infections with numerous entangled worms can occasionally cause pain, small-bowel obstruction, perforation, volvulus, biliary obstruction and colic, or pancreatitis.

Laboratory Findings Ascaris eggs (65 by 45 µm) can be found in fecal samples. Adult worms can pass in the stool or, much less commonly, through the mouth or nose.

Hookworm

Microbiology Two hookworm species, Ancylostoma duodenale and Necator americanus, cause human infections. Infectious larvae present in soil penetrate the skin, reach the lungs via the bloodstream, invade the alveoli, ascend the airways, are swallowed, reach the small intestine, mature into adult worms, attach to the mucosa, and suck blood (0.2 mL/d per Ancylostoma adult) and interstitial fluid.

Clinical Manifestations Most infections are asymptomatic. Chronic infection causes iron deficiency and—in marginally nourished persons—progressive anemia and hypoproteinemia, weakness, and shortness of breath. Larvae may cause pruritic rash (“ground itch”) at the site of skin penetration as well as serpiginous tracks of SC migration (similar to those of cutaneous larva migrans).

Laboratory Findings Hookworm eggs (40 by 60 µm) can be found in the feces. Stool concentration may be needed for the diagnosis of light infections.

Strongyloidiasis

Microbiology and Epidemiology Unlike other helminths, Strongyloides stercoralis can replicate in the human host, permitting ongoing cycles of autoinfection from endogenously produced larvae.

• Infection results when filariform larvae in fecally contaminated soil penetrate the skin or mucous membranes.
  • - Larvae travel through the bloodstream to the lungs, break through into alveolar spaces, ascend the bronchial tree, are swallowed, reach the small intestine, mature into adult worms, and penetrate the mucosa of the proximal small bowel; eggs hatch in the intestinal mucosa.
  • - Rhabditiform larvae can pass with the feces into the soil or can develop into filariform larvae that penetrate the colonic wall or perianal skin and enter the circulation to establish ongoing autoinfection.
• Autoinfection is constrained by unknown factors of the host immune system, disruption of which (e.g., by glucocorticoid therapy) can lead to hyperinfection.

Clinical Features Uncomplicated disease is associated with mild cutaneous and/or abdominal manifestations such as recurrent urticaria, larva currens (a pathognomonic serpiginous, pruritic, erythematous eruption along the course of larval migration that may advance up to 10 cm/h), abdominal pain, nausea, diarrhea, bleeding, and weight loss.

• Disseminated disease involves tissues outside the GI tract and lungs, including the CNS, peritoneum, liver, and kidney.
  • - Gram-negative sepsis, pneumonia, or meningitis can complicate or dominate the clinical course.
  • - Disease can be fatal in pts given glucocorticoids; disseminated infection is uncommon among pts with HIV-1 infection.
• Fluctuating eosinophilia is common in uncomplicated disease but is uncommon in disseminated disease.

Diagnosis A single stool examination detects rhabditiform larvae (~250 µm long) in about one-third of uncomplicated infections. Duodenojejunal contents can be sampled if stool examinations are repeatedly negative.

• Antibodies can be detected by ELISA.
• In disseminated infection, filariform larvae can be found in stool or at sites of larval migration (e.g., sputum, bronchoalveolar lavage fluid, surgical drainage fluid).

Enterobiasis

Microbiology and Epidemiology Enterobiasis (pinworm) is caused by Enterobius vermicularis and affects ~40 million people in the United States (primarily children).

• Gravid female worms migrate nocturnally from the cecum to the perianal region, each releasing up to 2000 immature eggs that become infective within hours.
• Autoinfection and person-to-person transmission result from perianal scratching and transport of infective eggs to the mouth.

Clinical Manifestations Perianal pruritus is the cardinal symptom and is often worst at night. Eosinophilia is uncommon.

Diagnosis Eggs (55 by 25 µm and flattened on one side) are detected by microscopic examination of cellulose acetate tape applied to the perianal region in the morning.