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 few deleterious effects 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.
• Approximately 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.

Prevention

Cooking pork until it is no longer pink or freezing it at -15°C (5°F) for 3 weeks kills larvae and prevents infection by most Trichinella spp.

Visceral and Ocular Larva Migrans

Microbiology and Epidemiology Humans are incidental hosts for nematodes that cause visceral larva migrans. Most cases are caused by the canine ascarid Toxocara canis. Infection results when humans—most often preschool children—ingest soil contaminated by puppy feces that contain infective T. canis eggs. Larvae penetrate the intestinal mucosa and disseminate hematogenously to a wide variety of organs (e.g., liver, lungs, CNS), provoking intense eosinophilic granulomatous responses.

Clinical Manifestations Symptomatic infections result in fever, malaise, anorexia, weight loss, cough, wheezing, rashes, hepatosplenomegaly, and occasional profound eosinophilia (up to 90%). Ocular disease usually develops in older children or young adults and includes an eosinophilic mass that mimics retinoblastoma, endophthalmitis, uveitis, and/or chorioretinitis.

Diagnosis The clinical diagnosis can be confirmed by an ELISA for toxocaral antibodies. Stool examination for eggs is ineffective because larvae do not develop into adult worms in humans.

Cutaneous Larva Migrans This disease is caused by larvae of animal hookworms, usually the dog and cat hookworm Ancylostoma braziliense. Larvae in contaminated soil penetrate human skin; intensely pruritic, erythematous lesions form along the tracks of larval migration and advance several centimeters each day. Ivermectin (a single dose of 200 µg/kg) or albendazole (200 mg bid for 3 days) can relieve the symptoms of this self-limited infestation.