MICROBIOLOGY

• Trichinosis occurs with ingesting undercooked meat contaminated with infective larvae of Trichinella spp.
  
  • Seven species of roundworms from the genus Trichinella cause human disease:
    • T. spiralis (most common) is found worldwide in various carnivorous and omnivorous hosts.
    • T. nativa is found in arctic regions (bears, foxes, walruses).
    • T. nelsoni is found in sub-Saharan Africa (felines, hyenas, bush pigs).
    • T. britovi is found in Europe, Asia, and Africa (various carnivores).
    • T. pseudospiralis is found worldwide (in wild mammals and birds).
    • T. murelli is found in the United States and Japan (wild mammals).
    • T. papuae is found in Papua New Guinea and Southeast Asia (domestic and feral pigs, saltwater crocodiles, and turtles).
  • T. zimbabwensis in Tanzania (crocodiles) and T. patagoniensis in South America (cougars) are not known to cause human disease.
• Epidemiology: worldwide.
  • From 1986-2009, there were 65,818 reported cases across 41 countries (with 42 deaths).
    • Now an estimated 10,000 cases/year globally.
    • In the U.S., ~16 cases/years were reported (2011-2016).
      • Decreased cases ascribed to improved pig-raising practices in the pork industry, commercial and home freezing of pork, and public awareness of the danger of eating raw or undercooked meat products.
      • Cases associated with raw or undercooked wild game meats have remained relatively constant.
• Life cycle:
  • Undercooked meat containing encysted larvae is eaten.
  • Larvae are released from the cysts with exposure to gastric acid and pepsin.
  • Larvae invade the small bowel mucosa, where they mature into adults.
  • Adult females release larvae that migrate to striated muscle where they encyst and may remain infective for years.
• Adult worms are 1.5 x 0.05mm (male) and 3.5 x 0.06mm (female).
• Carnivorous animals keep the life cycle going by feeding on infected rodents or meat from other animals.
• Common hosts:
  • Pigs are the most common source of human infection worldwide, but most U.S. swine are fed grains and are, therefore, uninfected.
  • Bears are the most common source in the U.S.