Trichinosis

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

Trichinosis, also called trichinellosis, or trichiniasis, is a parasitic disease caused by eating raw or undercooked pork and wild game products infected with the larvae of a species of roundworm Trichinella spiralis, commonly called the trichina worm. The few cases in the United States are mostly the result of eating undercooked game or home reared pigs. It is most common in the developing world and where pigs are commonly fed raw garbage.

Signs and symptoms

Trichinosis initially involves the intestines. Within 1-2 days of contagion, manifestations such as nausea, heartburn, dyspepsia, and diarrhea; the severity of symptoms depends on the number of worms ingested. Later on, as the worms encyst in different parts of the human body, other manifestations may occur, such as headache, fever, chills, cough, eye swelling, joint pain and muscle pain, petechiae, and itching.

Most symptoms subside within a few months. The most dangerous case is worms entering the central nervous system. They cannot survive there, but they may cause enough damage to produce serious neurological deficits (such as ataxia or respiratory paralysis), and even death. Infestation of the heart may also lead to death.

Life cycle

The worm can infect any species of mammal that consumes its encysted larval stages. When an animal eats meat that contains infective Trichinella cysts, the acid in the stomach dissolves the hard covering of the cyst and releases the worms. The worms pass into the small intestine and, in 1–2 days, become mature. After mating, adult females produce larvae, which break through the intestinal wall and travel through the lymphatic system to the circulatory system to find a suitable cell. Larvae can penetrate any cell, but can only survive in skeletal muscle. Within a muscle cell, the worms curl up and direct the cell functioning much as a virus does. The cell is now called a nurse cell. Soon, a net of blood vessels surround the nurse cell, providing added nutrition for the larva inside.

Diagnosis

A blood test or muscle biopsy can identify trichinosis. Stool studies can identify adult worms, with females being about 3 mm long and males about half that size.

Treatment

Symptoms can be treated with aspirin and corticosteroids. Thiabendazole can kill adult worms in the intestine; however, there is no treatment that kills the larvae.

Epidemiology

Trichinosis was known as early as 1835 to have been caused by a parasite, but the mechanism of infection was unclear at the time. It was not until a decade later that American scientist Joseph Leidy pinpointed undercooked meat as the primary vector for the parasite, and not until two decades afterwards that this hypothesis was fully accepted by the scientific community [2].

Infection was once very common, but is now quite rare in the developed world. From 1991 to 1996, an annual average of 12 cases per year were reported in the United States. The number of cases has decreased because of legislation prohibiting the feeding of raw meat garbage to hogs, increased commercial and home freezing of pork, and the public awareness of the danger of eating raw or undercooked pork products. Today, one of the primary causes of trichinosis in America is the consumption of raw or undercooked wild game meats.

In the developing world, most infections are associated with undercooked pork. For example, in Thailand, between 200 and 600 cases are reported annually around the Thai New Year. In parts of Eastern Europe, the WHO reports that some swine herds have trichinosis infection rates above 50%, and there are correspondingly large numbers of human infections [3].

It has been suggested that trichinosis may be one of several factors that led to religious prohibitions in Islam, Judaism, etc. against eating pork products, such as in the kashrut and dhabiĥa halal dietary laws. The medieval Jewish philosopher Maimonides advocated such a theory in his Guide for the Perplexed.

International Commission on trichinellosis

The International Commission on trichinellosis (ICT) was created in 1958 in Budapest and is aiming to exchange information on the biology, the physiopathology, the epidemiology, the immunology, and the clinical aspects of trichinellosis in humans and animals. Prevention is a primary goal. Since the creation of the ICT, its members (more than 110 from 46 countries) have regularly gathered and worked together during meetings held every 4 years : the International Conference on Trichinellosis.

Prevention

• Cooking meat products until the juices run clear or to an internal temperature of 170 °F (77 °C).
• Freezing pork] less than 6 inches thick for 20 days at 5 °F (−15 °C) or three days at −4 °F (−20 °C) kills larval worms.
• Cooking wild game meat thoroughly. Freezing wild game meats, unlike freezing pork products, even for long periods of time, may not effectively kill all worms. This is because the species of trichinella that typically infects wild game is more resistant to freezing than the species that infects pigs.
• Cooking all meat fed to pigs or other wild animals.
• Keeping pigs in clean pens with floors that can be washed (such as concrete). This is standard in Germany, where raw pork is a common delicacy and trichinosis is rarer than in the U.S.
• Not allowing hogs to eat uncooked carcasses of other animals, including rats, which may be infected with trichinosis.
• Cleaning meat grinders thoroughly when preparing ground meats.
• Control and destruction of meat containing trichinae, e.g., removal and proper disposal of porcine diaphragms prior to public sale of meat.

The Centers for Disease Control and Prevention makes the following recommendation: "Curing (salting), drying, smoking, or microwaving meat does not consistently kill infective worms." However, under controlled commercial food processing conditions some of these methods are considered effective by the United States Department of Agriculture.

Additional Resources

• The text of the original version of this article was taken from the public domain resource at http://www.cdc.gov/ncidod/dpd/parasites/trichinosis/factsht_trichinosis.htm
• Centers for Disease Control and Prevention. Trichinosis Surveillance, United States, 1987-1990, MMWR 1991;40:(SS-3)35-42.
• Moorhead A, Grunenwald PE, Dietz VJ, Schantz PM. Trichinellosis in the United States, 1991-1996: Declining but not gone. Am J Trop Med Hyg 1999; 60:66-69.
• US FDA regulations - Title 9 - Chapter 3 - Part 318 - includes "Prescribed treatment of pork and products containing pork to destroy trichinae."

Web pages

International Commission on trichinellosis web pages