Trichinosis

Basics

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

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.

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

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

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

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).

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

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

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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