Trichinosis pathophysiology

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: Danitza Lukac

Overview

Trichinella spp. is usually transmitted to the human host by eating undercooked meat containing cysts of Trichinella spp. Following ingestion, exposure to gastric acid and pepsin release the larvae from the cysts and invade the small bowel mucosa.When a person eats meat from an infected animal, Trichinella cysts break open in the intestines and grow into adult roundworms. Females are 2.2 mm in length; males 1.2 mm. The life span in the small intestine is about four weeks. After 1 week, the females release more larvae that migrate to voluntarily controlled muscles where they encyst. The roundworms produce other worms that move through the gut wall and into the bloodstream. These organisms tend to invade muscle tissues, including the heart and diaphragm. They can also affect the lungs and brain. Characteristic findings on microscopic histopathological analysis of the muscle, the nurse cell has a collagen capsule with larvae inside and is surrounded by cellular infiltrates.

Pathogenesis

Trichinella life cycle.

• Trichinellosis is acquired by ingesting meat containing cysts (encysted larvae) (1) of Trichinella.
• During the enteral phase:
  • After exposure to gastric acid and pepsin, the larvae are released (2) from the cysts and invade the small bowel mucosa
  • Thirty to 34 hours after the cysts were originally ingested, they develop into adult worms (3) within (female 2.2 mm in length, males 1.2 mm; life span in the small bowel: 4 weeks).
  • The adults worms mate, and within five days produce larvae.
  • The worms can only reproduce for a limited time because the immune system will eventually expel them from the small intestine.
  • After 1 week, the females release larvae (4)
    • Trichinella pseudospiralis, however, does not encyst.
  • The newborn larvae then use their piercing mouthpart, called the "stylet", to pass through the intestinal mucosa
• During the parental phase:
  • The newborn larvae enter the lymphatic vessels, and then enter the bloodstream.
  • The larvae travel by capillaries to various organs, such as the retina, myocardium, or lymph nodes; however, only larvae that migrate to skeletal muscle cells survive and encyst.
  • Skeletal muscle cells act as nurse cells where larvae can develop and after weeks, months or years calcify.
  • Encystment is completed in 4 to 5 weeks and the encysted larvae may remain viable for several years. (5)
• Ingestion of the encysted larvae perpetuates the cycle.
• Rats and rodents are primarily responsible for maintaining the endemicity of this infection.
• Carnivorous/omnivorous animals, such as pigs or bears, feed on infected rodents or meat from other animals.
• Different animal hosts are implicated in the life cycle of the different species of Trichinella.
• Humans are accidentally infected when eating improperly processed meat of these carnivorous animals (or eating food contaminated with such meat).

Microscopic Pathology

• Males T. spiralis:
  • Measure between 1.4 and 1.6 mm long and are more flat anteriorly than posteriorly.
  • The anus can be found in the terminal (side) and they have a large copulatory pseudobursa on each side.
• Females T. spiralis:
  • Are about twice the size of the males
  • Have an anus found terminally.
  • The vulva is located near the esophagus.
  • The single uterus of the female is filled with developing eggs in the posterior portion, while the anterior portion contained the fully developed juveniles.
• Muscle biopsy (hematoxylin-eosin staining):
  • Nurse cell has a collagen capsule (thickness depends on Trichinella species) with larvae inside and surrounded by cellular infiltrates.