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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 is common where raw or undercooked pork, such as ham or sausage, is regularly consumed as part of the diet. more...

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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. 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.


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.


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Trichinella spiralis infection - United States, 1990 - trichinosis outbreaks in Des Moines, Iowa, and Virginia
From Morbidity and Mortality Weekly Report, 2/1/91

Trichinella spiralis Infection -- United States, 1990

Since 1947, when the Public Health Service began to record statistics on trichinosis, the number of cases reported by state health departments each year has declined: in the late 1940s, health departments reported an average of 400 cases and 10-15 deaths each year from 1982 through 1986, the number declined to an average of 57 per year (Figure 1) and a total of three deaths (1,2). Although this trend reflects a decline in the number of cases related to commercially purchased pork, recent outbreaks of trichinosis in Iowa and Virginia emphasize the continuing need for education about the dangers of eating inadequately cooked pork.

Des Moines, Iowa. From July 21 through September 3, 1990, 90 (36%) of 250 persons who attended or ate food taken from a wedding in Des Moines on July 14 developed trichinosis (*1) most (approximately 95%) of the 250 persons had immigrated to the United States since 1975 from Southeast Asian countries. Of those who became ill, 52 (58%) were treated by physicians one of the 52 was hospitalized.

Detailed case histories were obtained from 39 ill and 13 well persons who attended the wedding. Of the 39 ill well persons, 34 (87%) ate uncooked prok sausage, compared with four (31%) of the 13 well persons (p 0.01, Mantel-Haenszel test) no other foods were associated with illness. The sausage had been prepared from 120 lbs of commercially purchased pork and was served uncooked, as is customary for that food item in Southest Asian culture. No pork was available for analysis at the time of investigation.

The meat could not be traced back to the source farm because the meat-packing company that supplied the pork slaughters 14,000-15,000 hogs a day from hundreds of farms, and the exact date the hogs were slaughtered was unknown.

Only four (4%) of 107 persons who attended the wedding and were interviewed knew about trichinosis or about the potential hazards of eating undercooked pork. The Iowa Refugee Health Program, Iowa Department of Health, prepared a brief information sheet describing trichinosis and ways to avoid infection and translated this information into the three major languages (Laotian, Cambodian, and Vietnamese) of the Southeast Asian community in Iowa the information sheet is being distributed by the Bureau of Refugee Programs.

Staunton, Virginia. In November and December 1990, 15 cases of trichinosis (*1) were reported by eight local physicians in Augusta, Page, Rockingham, and Shenandoah counties to the Central Shenandoah Health District, Virginia Department of Health. Six cases were confirmed by muscle biopsy, five had positive serology by bentonite flocculation, and four were epidemiologically linked. Nine of these persons required hospitalization. All patients had fever, myalgia, and periorbital edema all nine patients for whom eosinophil counts were available had elevated levels.

Detailed case histories were available for all ill persons. Fourteen (93%) persons reported eating pork sausage 4-21 days before onset of symptoms 10 (67%) ate the sausage uncooked. One person who denied eating undercooked sausage was employed as a meat handler in the plant that processed the implicated sausage.

The investigation was limited to those who were ill no controls were interviewed. The 14 persons who had consumed sausage had purchased bulk pork sausage from several local retail grocery stores the stores had purchased this sausage from a local processing plant. No pork was available for analysis at the time of investigation. During the 6 weeks before the outbreak, the plant purchased hogs from two brokers who had obtained hogs from multiple producers in Virginia and surrounding states. The plant produces 1500 lbs of sausage per week, which is distributed throughout eight counties in the Shenandoah Valley.

The health department issued an areawide alert to physicians and hospitals and a news release to all area newspapers that included information on proper cooking and handling of raw pork.

Editorial Note: Since 1975, the proportion of trichinosis cases associated with consumption of contaminated commercial pork has declined in the United States. This decline probably reflects a combination of factors, including laws prohibiting the feeding of offal to hogs, the increased use of home freezers, and the practice of thoroughly cooking pork. In recent years the relative importance of consumption of wild game (including bear, wild boar, and walrus) [2,3! as a cause of trichinosis has increased. Consumption of meat from any carnivorous animal that has fed on trichina-infested flesh poses a risk (Figure 2). In addition to the two multiple-case outbreaks in this report, 15 other cases were reported in 1990. At least three cases were sporadic information on the remaining 12 is unavailable.

The outbreak in Iowa is the fourth since 1975 that occured among the 900,000 Southeast Asian refugees who have immigrated to the United States [4,5!. The three previous outbreaks were related to consumption of undercooked pork that was not obtained from a commercial producer [4!. This outbreak is consistent with previous reports indicating that recent immigrants from Southeast Asia are at particular risk for developing trichinosis because of their dietary habits [4!.

Based on serologic examination of hogs at abattoirs, the prevalence of Trichinella infection in commercial pork ranges from 0 to 0.7% [6,7!. Approximately 80 million hogs are slaughtered commercially each year in the United States. About 40% of the pork produces is sold as "ready to eat" pork products such products must be made with trichina-free pork or pork adequately cooked or treated to kill trichina larvae. Trichinella larvae in prok are killed by freezing at 5 F (-15 C) for 21 days (or longer if meat is 15 cm thick) however, Trichinella larvae present in wild game are often relatively resistant to freezing [8!. Cooking is one of the most common methods of assuring that Trichinella are destroyed a temperature of 170 F (77 C) substantially exceeds the thermal death point and is usually achieved if the meat is cooked until it is no longer pink [9!.

Public health officials in areas with large populations of immigrants from Southeast Asia should consider education programs directed at the prevention of trichinosis. Physicians need to be aware of the continued presence of T. spiralis in commercial pork in the United States and should consider the diagnosis in any patient with an illness compatible with trichinosis and whose dietary preferences put them at risk for infection.

(*1) The CDC case definition for trichinosis is 1) a Trichinella-positive muscle biopsy or positive serologic test for trichinosis in a patient with eosinophilia, fever myalgia, and/or periorbital edema or 2) in an outbreak, at least one person must meet the first criterion associated cases are defined by either a positive serologic test for trichinosis or eosinophilia, fever, myalgia, and/or periorbital edema in persons who have shared the epidemiologically implicated meal or consumed the implicated meat product.


[1.! Schantz PM. Trichinosis in the United States, 1947-1981. Food Technol 1983 37:83-6.

[2.! Bailey TM, Schantz PM. Trends in the incidence and transmission patterns of human trichinosis in the United States, 1982-1986. Rev Infect Dis 1990 12:5-11.

[3.! CDC. Trichinosis surveillance, United States, 1986. MMWR 1988 37(no. SS-5):1-8.

[4.! Stehr-Green JK, Schantz PM. Trichinosis in Southeast Asian refugees in the United States. Am J Public Health 1986 76:1238-9.

[5.! US Department of Health and Human Services. Report to the Congress: Refugee Resettlement Program. Washington DC: US Department of Health and Human Services, January 31, 1990.

[6.! Duffy CH, Schad GA, Leiby DA, et al. Slaughterhouse survey for swine trichinosis in Northeast United States. In: Kim CW, ed. Trichinellosis, proceedings of the Sixth International Conference on Trichinosis. Albany, New York: State Unversity of New York Press, 1985.

[7.! Hill RO, Spencer PL, Doby KD, et al. Illinois swine trichinosis epidemiology project. In: Kim CW, ed. Trichinellosis, proceedings of the Sixth International Conference on Trichinosis. Albany, New York: State University of New York Press, 1985.

[8.! Dick TA, Chadee K. Biological characterization of some North American isolates of Trichinella spiralis. In: CW, Ruitenberg EJ, Teppema TS, eds. Trichinellosis, proceedings of the Fifth International Conference on Trichinosis. Surrey, England: Reedbooks, 1981.

[9.! Leighty JC. Control I public-health aspects (with special reference to the United States). In: Campbell WC, ed. Trichinella and trichinosis. New York: Plenum Press, 1983.

COPYRIGHT 1991 U.S. Government Printing Office
COPYRIGHT 2004 Gale Group

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