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Trichinosis (cont.)

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What causes trichinosis?

Trichinosis is caused by Trichinella species (parasitic nematodes, intestinal worms, and roundworms) that initially enter the body when meat containing the Trichinella cysts (roundworm larvae) is eaten. For humans, undercooked or raw pork and pork products, such as pork sausage, has been the meat most commonly responsible for transmitting the Trichinella parasites. It is a food-borne infection and not contagious from one human to another unless infected human muscle is eaten. However, almost any carnivore (meat eater) or omnivore (eats meat and plants for food) can both become infected and, if eaten, can transmit the disease to other carnivores and omnivores. For example, undercooked or raw bear meat can contain livable Trichinella cysts. Therefore, if humans, dogs, pigs, rats, or mice eat the meat, they can become infected. In rare instances, larvae that inadvertently reaches cattle feed can infect cattle. There are six species that are known to infect humans:

  • T. spiralis found in many carnivorous and omnivorous animals worldwide.
  • T. britovi found in carnivorous animals in Europe and Asia.
  • T. pseudospiralis found in mammals and birds worldwide.
  • T. nativa found in arctic mammals (for example, bears, foxes).
  • T. nelsoni found in African mammals (for example, lions, hyenas).
  • T. murrelli found in wild animals in the U.S.

Two other species, T. papuae (found in pigs in New Guinea) and T. zimbabwensis (found in crocodiles in Tanzania) have not been reported to infect humans to date. There are other strains (antigenic variants related to named species) that are unnamed and can infect humans.

What is the history and life cycle of trichinosis?

The life cycle of this particular parasite is key to the way disease develops in humans. In 1835, J. Paget and R. Owen first discovered larvae of Trichinella in a piece of human muscle during an autopsy. That same year, R. Virchow and F. Zenker discovered the complicated life cycle of Trichinella by feeding a dog Trichinella-infected meat.

In humans, the larvae are ingested, released from an encapsulated (encysted) cyst by stomach acid (except for the nonencapsulated T. pseudospiralis), and then develop into adult male and female worms in the gastrointestinal tract. The adult worms mate and then shed larvae that penetrate the gastrointestinal tract and reach the bloodstream and lymph drainage system. The larvae are then distributed to body cells (mainly skeletal muscle cells). Some skeletal muscle cells develop into nurse cells that support and protect the larvae from the host's immune system. The life cycle that requires at least two hosts, is illustrated below. Humans are usually an incidental host.

Trichinellosis in humans is acquired by ingesting meat containing cysts (encysted larvae) of Trichinella. After exposure to gastric acid and pepsin, the larvae are released from the cysts and invade the small bowel mucosa where they develop into adult worms (female 2.2 mm in length, males 1.2 mm; life span in the small bowel: four weeks). After one week, the females release larvae that migrate to the striated muscles where they encyst. Trichinella pseudospiralis, however, does not encyst. Encystment is completed in four to five weeks, and the encysted larvae may remain viable for several years. 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 cooked meat of these carnivorous animals (or eating food contaminated with such meat).

Medically Reviewed by a Doctor on 10/25/2013

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Source: MedicineNet.com
http://www.medicinenet.com/trichinosis/article.htm

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