Trichinosis

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

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 hemorrhages, 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, and the amount of meat consumed. 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 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.

Screening for compounds active against Trichinella

(1) An in vitro screening test for compounds active against the parenteral stages of Trichinella spiralis.

Tropenmed Parasitol. 1981 Mar; 32(1): 31-4. (Jenkins DC, Carrington TS.) A new in vitro screening test for compounds showing activity against the tissue stages of Trichinella spiralis is described Freshly decapsulated larvae of the parasite are exposed to low concentrations of experimental compound in a medium capable of supporting the partial development of the worms. The screen detects the activity of those compounds known to be effective against the parenteral stages of the parasite.

(2) Trichinella pseudospiralis as a model for the "in vitro" screening of anthelmintics. Wiad Parazytol. 1986; 32(3): 303-11. (Gomez-Barrio A, Bolas-Fernandez F, Martinez-Fernandez AR.)

Risk factors

Eating raw or undercooked meats, particularly pork, bear, wild feline (such as a cougar), fox, dog, giraffe, 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. 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 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 ^*.

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 ^*.

It has been suggested that trichinosis may be one of several factors that lead to religious prohibitions against eating pork products, such as in the kashrut dietary laws. The medieval Jewish philosopher Maimonides advocated such a theory in his Guide for the Perplexed. The matter is controversial, see Kashrut#Reasons for the Biblical dietary laws.

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 −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.
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.
Public control and destruction of pork meat with trichins, ie control each pigs diaphragma before allowing it to be sold to the public.

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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