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.

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.