Tokyo Food Safety Information Center » Good things to know »Food parasites »Trichinella spp., genus of roundworm

Trichinella spp., genus of roundworm


Size Male: 1.4 to 1.6 mm.  Width of 0.04 to 0.05 mm.
Female: 2 to 4 mm.  Width of 0.06 to 0.07 mm.

Sources of infections:

In the United States, trichinella infections are caused by oral ingestion of encysted larvae from muscle tissue contained in insufficiently cooked pork, sausages, etc.  In Eastern Europe and Central Asia, the sources of infections are game meats such as horse and deer meat.  In Japan, infections caused by oral ingestion of raw meat (sashimi) from Japanese black bears and Hokkaido brown bears have been found.

Effects on humans:

Initial symptoms include fever, muscle aches, and swelling around the eye sockets.
Three stages are recognized with regard to the growth of trichinella larvae:

1.Imago invasion stage

In the period one week or two after infection, imagoes (mature stage) intrude into mucous membranes of the small intestine and produce larvae.  Irritation due to production of larvae causes symptoms including stomachaches, fever, and increase in eosinophils.

2.Larval dissemination stage

In this stage, during the second week to the sixth week after infection, the larvae produced within mucous membranes migrate to muscles throughout the whole body.  Symptoms include: eyelid edemata (swelling), muscle aches, and in some cases, difficulty of breathing.  Encephalitis, myocarditis, and so forth may occur, resulting in a very serious condition.

3.Larval encystation stage

After migrating into muscles, the larvae encyst there.  In case of a human, in the sixth week or so after infection, eyelid edema (swellings) appear much more apparent and in serious cases, such symptoms as anasarca, anemia, pneumonia, and cardiac insufficiency may occur, resulting in death, in some cases.

Main host animals:

Bears, horses, and pigs

Methods for prevention:

  • Do not eat raw bear meat.
  • Sufficiently heat/cook foods made from pork.  The trichinella larvae within game muscles are very resistant against low temperatures.  An outbreak has been reported from bear meat stored at -30℃ for four months.

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