Trichinellosis (TRICK-a-NELL-o-sis), also known as trichinosis or trichiniasis, is an infection caused by a roundworm of the genus Trichinella, usually the species Trichinella spiralis. The infection is contracted by eating meat (usually pork, but also the meat of wild game) that contains live helminth (parasitic worm) cysts. These cysts are the larvae or young of the worm, which are curled up inside tiny protective capsules. The cysts hatch in the small intestine and breed a new generation of larval worms, which then infect various body tissues. Thorough cooking destroys the larvae and renders infected meat safe to eat. Trichinellosis is rare in most of the world, but is fairly common in Eastern Europe and is increasing in frequency in other areas. Eating undercooked pork is the most common path of trichinellosis infection worldwide; in North American, eating wild game is the most common path of infection. Death from the infection is rare.
British physician James Paget (1814–1899) discovered the worm that causes trichinellosis in 1835 while still a medical student. However, he did not know how humans became infected with the worm. In 1846, American parasitologist Joseph Leidy (1823–1891) discovered the parasite in pork. His early results were misreported in 1851 in Europe, the other region where trichinellosis is common, as being descriptions of Trichinella affinis, which does not infect humans. In 1853 and 1856 Leidy again published accounts of finding Trichinella spiralis in pork and reported that thorough cooking destroyed the parasite and made the meat safe to eat. Despite these later publications, European application of his results was delayed for decades by the original error.
Today, some experts consider that trichinellosis should be categorized as a reemerging disease because it is increasingly being reported in previously unaffected areas.
Characteristics and Transmission
When meat containing encysted larvae is eaten, the larvae are liberated by the digestive process. They develop into adults in the small intestine, then mate and produce offspring. These adult worms are eventually excreted. The new larvae drill through the wall of the intestine and enter the bloodstream, which conveys them to destinations throughout the body, including the muscles, eyes, lungs, and brain. The larvae encyst themselves in muscle and become dormant. Since humans with the disease are usually not eaten by other animals or people, that is usually the end of the disease cycle in human beings. If the encysted larvae are in the muscle of any animal that might be eaten by human beings or other carnivores, the life cycle can continue.
Abdominal symptoms appear a day or two after infection and include nausea, diarrhea, vomiting, and abdominal pain. Other symptoms may appear two to four weeks after infection and include headaches, fevers and chills, muscle and joint pain, itching, diarrhea, rash, and swelling of the eyes. The later-stage symptoms are caused by the larvae encysting in the muscles and the body's immune response to their presence. Not all cases of infection, even in humans, produce noticeable symptoms.
Trichinellosis occurs worldwide today but is found mostly in North America, Europe, and China. From 1991 to 1992, there were more than 20,000 cases in Europe. In Eastern Europe and parts of China, some swine herds have a trichinellosis prevalence of about 50%.
WORDS TO KNOW
DORMANT: Inactive, but still alive. A resting non-active state.
ENCYSTED LARVAE: Encysted larvae are larvae that are not actively growing and dividing, and which are more resistant to environmental conditions.
HELMINTH: A representative of various phyla of worm-like animals.
IN CONTEXT: PERSONAL RESPONSIBILITY AND PROTECTION
- Cook meat products until the juices run clear or to an internal temperature of 170°F (76.6°C).
- Freeze pork less than 6 inches thick for 20 days at 5°F (15°C) to kill any worms.
- Cook wild game meat thoroughly. Freezing wild game meats, unlike freezing pork products, even for long periods of time, may not effectively kill all worms.
- Cook all meat fed to pigs or other wild animals.
- Do not allow hogs to eat uncooked carcasses of other animals, including rats, which may be infected with trichinellosis.
- Clean meat grinders thoroughly if you prepare your own ground meats.
In North America, the primary source of trichinellosis infection is wild game, with only about 12 cases per year being reported in the United States. Outbreaks of trichinellosis have occurred among Eskimos eating undercooked walrus. Almost all mammals can be infected by one or more species of Trichinella, but humans are more likely than most other species to develop symptoms.
Pork and bear meat are primary sources of Trichinella spiralis infection in humans. Beaver, opossums, rats, walruses, and whales can also carry the parasite. Infected animals remain asymptomatic; however, symptoms in humans—which can begin as soon as five or a late as 45 days after exposure—can range from asymptomatic to, rarely, death. Severity depends upon the number of parasites ingested. Although trichinosis is found in some grain-fed pigs, swine fed on garbage containing infected meat scraps are the primary source of human trichinosis.
Diagnosis is confirmed by observing the adult worms in a stool sample or through finding larvae in a muscle biopsy (a small piece of muscle tissue removed for laboratory testing). Treatment is supportive, except during the intestinal phase of the infection when several drugs can be given to kill the worms in the intestine. These anti-helminthic drugs include mebendazole and thiabendazole. No drug exists that can kill the encysted larvae, which may persist alive in the tissue—though inactive— for many years.
Trichinellosis can be prevented by eating only thoroughly cooked meats. Laws have been passed in both the United States and Europe forbidding feeding garbage-containing raw meat to hogs. To prevent trichinellosis, the U.S. Centers for Disease Control (CDC) recommends cooking pork to a temperature of 160 degrees Fahrenheit (71°C) before eating or freezing pork less than six inches thick for 20 days at 5°F (–15°C). Micro-waving does not reliably kill larvae in meat.
The economic impact of trichinellosis is high, because the measures taken to reduce its presence in the food supply can be so expensive. In the European Union, the domestic pig control program designed to minimize trichinellosis costs $500 million per year. In China, large herds of infected pigs are occasionally destroyed, which can be a severe hardship for uninsured farmers.
In recent years, an increase in trichinellosis cases related to travel prompted many countries to adopt stricter bans on the importation of pork and game products by travelers to some regions. Many popular tourist destinations, such as Argentina, Croatia, Mexico, Romania, Serbia, and Laos, have endemic problems with trichinellosis. In 2005, nearly two-thirds of the reported cases of trichinellosis in the United Kingdom and France were in people who had contracted the infection while traveling abroad or who had consumed infected products— such as sausages—that had been imported by travelers. Many nations now include trichinellosis in traveler health warnings.
IN CONTEXT: EFFECTIVE RULES AND REGULATIONS
The Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Parasitic Diseases, states that trichinellosis “infection is now relatively rare. During 1997–2001, an average of 12 cases per year were reported.” The CDC further asserts that “the number of cases has decreased because of legislation prohibiting the feeding of raw-meat garbage to hogs, 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.”
SOURCE: Centers for Disease Control and Prevention, National Center for Infectious Diseases, Division of Parasitic Diseases
Despommier, Dickson D., et al. Parasitic Diseases, 5th ed. New York: Apple Trees Productions, 2005.
Bruschi, F., and K.D. Murrel. “New Aspects of Human Trichinellosis: The Impact of New Trichinella Species.” Postgraduate Medical Journal. 78 (2002): 15–23.
Moorhead, Andrew. “Trichinellosis in the United States, 1991–1996: Declining but not Gone.” Journal of the American Medical Association. 281 (1999): 1472.
Centers for Disease Control (U.S. Government). “Trichinellosis Fact Sheet.” February 6, 2007. <http://www.cdc.gov/ncidod/dpd/parasites/trichinosis/factsht_trichinosis.htm> (accessed April 12, 2007).