Trichinosis

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Basics

Description

  • Infection caused by ingestion of undercooked meat containing nematode (roundworm) larval cysts of the Trichinella genus
  • Clinical disease in humans characterized by an intestinal phase followed by a muscular phase
  • Extremely wide host range and geographic distribution

Epidemiology

  • Historically, most U.S. infections are due to Trichinella spiralis in commercial pork.
  • Currently, more U.S. infections are associated with wild game meat (especially bear) or through spillover to domestic animals.
  • Trichinella parasites found in animals from all continents except Antartica
  • Occasional grouped outbreaks (e.g., families and communities with common exposure)
  • Reservoir hosts include rodents, domesticated animals (e.g., dogs, cats), raccoons, opossums, and skunks.

Incidence

  • Estimated 10,000 cases per year worldwide, with a mortality rate of 0.2% in main 55 countries reporting
  • Incidence worldwide highly variable due to variations in reporting and cultural and religious practices
  • Between 2002 and 2007 in the United States, average of 11 cases annually
  • Decreasing reported case numbers attributed to decline in prevalence of Trichinella in commercial swine (1.41% in 1900, 0.125% in 1966, and 0.013% in 1995), federal regulation preventing uncooked meat consumption by commercial swine, and increased public awareness regarding proper meat handling and preparation
  • Likely underreported, particularly in developing countries with modest health controls

Prevalence

~4% of cadavers in 1970 study with evidence of previous infection (additional estimates range from 10% to 20% prevalence)

Risk Factors

  • Consumption of inadequately cooked meat, even in small quantities
  • Consumption of foreign meat (e.g., horse in France, dog in China) or wild game (e.g., bear, cougar, hyena, lion, panther, fox, horse, seal, walrus)
  • Exposure to adulterated food (e.g., pork mixed in beef product)
  • Traveling to underdeveloped countries
  • Compromised immune status of host

General Prevention

  • Consume only fully cooked meat, pork, and wild game; meat should reach >145°F internally, no pink color.
  • Freezing kills T. spiralis in pork (<6 inches thick) at −20°F for 6 days, −10°F for 10 days, and −5°F for 20 days.
  • Freezing may not kill other Trichinella species, particularly in wild game.
  • Curing, smoking, salting, and drying meat (including jerky) are not reliable sterilization methods.
  • Routinely clean meat processing equipment.
  • Irradiation may not kill Trichinella but should prevent replication.
  • Avoid feeding swine uncooked meat scraps.
  • Actively control reservoir hosts (e.g., rodents).

Pathophysiology

  • Trichinella are obligate intracellular parasites capable of infecting warm-blooded animals.
  • Currently 11 Trichinella species identified: T. spiralis (most common), Trichinella nelsoni, Trichinella patagoniensis, Trichinella britovi, Trichinella murrelli, Trichinella T6, Trichinella T8, Trichinella T9, (encapsulated species), Trichinella pseudospiralis, Trichinella papuae, and Trichinella zimbabwensis (nonencapsulated)
  • Life cycle of all species comprises two generations in the same host (broad range of >100 species—mammal, birds, and reptiles), but only humans become clinically affected.
  • Disease not transmissible person to person
  • Larvae in undercooked meat eaten by the patient are released after cyst wall digestion by gastric enzymes, pass to the small intestine, invade mucosa, and then develop into adult worms.
  • Incubation period is 1 to 2 weeks.
  • Fertilized females release larvae (~500) over 2 to 3 weeks; adult worms do not multiply in human host and are expelled in feces.
  • Newborn larvae travel the bloodstream to seed skeletal muscles, where they grow 10-fold, coil, encyst, and cause muscle fibers to enlarge and become edematous. Nonskeletal muscle may have granulomatous reactions, but larvae are found only in skeletal muscle.
  • Cysts (hyaline capsules) may calcify over several months to years.
  • Growing body of research on the ability of parasites to modulate the immune system and implications of this for immune-mediated diseases

Etiology

T. spiralis is the most common organism that causes trichinosis and is acquired by the consumption of raw or undercooked, infected meat.

Commonly Associated Conditions

  • Rheumatic syndromes: polyarteritis nodosa–like systemic necrotizing vasculitis, symmetric polyarteritis, glomerulonephritis
  • Immunocompromised hosts are at risk for more serious or prolonged infection.

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