Trichinosis

Trichinosis, also called trichinellosis, 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. Infection occurs worldwide, but is most common in areas where raw or undercooked pork, such as ham or sausage, is eaten.

Signs and symptoms

Symptoms can be divided into two types: symptoms caused by worms in the intestine, and symptoms caused by worms elsewhere.

In the intestine, infection can cause:

Nausea, diarrhea, vomiting, fatigue, fever, and abdominal discomfort
Later, as the worms encyst in different parts of the body, other symptoms occur such as:

Headaches, fevers, chills, cough, eye swelling, aching joints and muscle pains, pinpoint hemmorhages, itchy skin, and heightened numbers of white blood cells.

If worms penetrate nervous tissue, they cannot survive, but patients may experience difficulty coordinating movements, and respiratory paralysis. In severe cases, death may occur. Heart infection can also cause death.

For mild to moderate infections, most symptoms subside within a few months. Fatigue, weakness, and diarrhea may last for months.

Incubation time

Abdominal symptoms can occur 1-2 days after infection. Further symptoms usually start 2-8 weeks after eating contaminated meat. Symptoms may range from very mild to severe and relate to the number of infectious worms consumed in meat. Often, mild cases of trichinosis are never specifically diagnosed and are assumed to be the flu or other common illnesses.

Life cycle

The worm can infect any species of mammal (including humans) 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 cells 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.

Risk factors

Eating raw or undercooked meats, particularly pork, bear, wild feline (such as a cougar), fox, dog, wolf, horse, seal, or walrus puts one at risk for trichinosis. This is the only way that infection can occur. It is not transmitted from one person to another, except through cannibalism. Even ingesting infected feces will not cause trichinosis because adults and unencysted larvae cannot survive in the stomach.

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

Epidemiology

Infection was once very common; however, infection is now relatively rare. From 1991-1996, an annual average of 38 cases per year were reported in the USA. 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. Cases are less commonly associated with pork products and more often associated with eating raw or undercooked wild game meats.

Prevention

Cooking meat products until the juices run clear or to an internal temperature of 144°F (62°C).

Freezing pork less than 6 inches thick for 20 days at 5°F (-15°C) or three days at (-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.

Cooking all meat fed to pigs or other wild animals.

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.

Curing (salting), drying, smoking, or microwaving meat does not consistently kill infective worms.

References

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. 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." (revised January 1, 2003)

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