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HELMINTHS

NEMATODES

The nematodes, or roundworms, that are of medical significance can be broadly classified as either tissue or intestinal parasites.

Tissue Nematode Infections

With the exception of trichinellosis, these infections are due to invasive larval stages that do not reach maturity in humans.

Trichinellosis
Microbiology and Epidemiology

Eight species of Trichinella cause human infection; two—T. spiralis and T. pseudospiralis—are found worldwide.

  • Infection results when humans ingest meat (usually pork) that contains encysted Trichinella larvae.

    • – The larvae invade the small-bowel mucosa.

    • – After 1 week, female worms release new larvae that migrate to striated muscle via the circulation and encyst.

  • The host immune response has little effect on muscle-dwelling larvae.

  • About 12 cases of trichinellosis are reported annually in the United States.

Clinical Manifestations

Most light infections (<10 larvae per gram of muscle) are asymptomatic. A burden of >50 larvae per gram can cause fatal disease.

  • In the first week of infection, large numbers of parasites invading the gut usually cause diarrhea, abdominal pain, constipation, nausea, and/or vomiting.

  • In the second week of infection, pts develop symptoms related to larval migration and muscle invasion: hypersensitivity reactions with fever and hypereosinophilia; periorbital and facial edema; and hemorrhages in conjunctivae, retina, and nail beds. Deaths are usually due to myocarditis with arrhythmias or heart failure.

  • 2–3 weeks after infection, larval encystment in muscle causes myositis, myalgias, muscle edema, and weakness (especially in extraocular muscles; the biceps; and muscles of the jaw, neck, lower back, and diaphragm).

  • Symptoms peak at 3 weeks; convalescence is prolonged.

Diagnosis

Eosinophilia develops in >90% of pts, peaking at a level of >50% at 2–4 weeks after infection.

  • An increase in parasite-specific antibody titers after the third week of infection confirms the diagnosis.

  • Detection of larvae by microscopic examination of ≥1 g of fresh muscle tissue (i.e., not routine histopathologic sections) also confirms the diagnosis. Yields are highest near tendon insertions.

TREATMENT Trichinellosis

  • Mebendazole (200–400 mg tid for 3 days; then 400 mg tid for 8–14 days) or albendazole (400 mg bid for 8–14 days) is active against enteric-stage parasites; the efficacy of these drugs against encysted larvae is inconclusive.

  • Glucocorticoids (e.g., prednisone at 1 mg/kg daily for 5 days) may reduce severe myositis and myocarditis.

Prevention

Cooking pork until it is no longer pink or freezing it at –15°C for 3 weeks kills larvae and prevents infection by most Trichinella species.

Visceral and Ocular Larva Migrans
Microbiology and Epidemiology

Humans are an incidental host for nematodes that cause visceral larva migrans. Most cases are caused by the canine ascarid Toxocara canis. Infection results when humans—most ...

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