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Epidemiologic Notes and Reports 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 14developed trichinosis*; 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 persons, 34 (87%) ate uncooked pork sausage, compared with four (31%) of the 13 well persons (p less than 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 Southeast 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* 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. Reported by: V Phabmixay, LA Wintermeyer, MD, State Epidemiologist, Iowa Dept of Public Health. D Kiser, T Overby, MD, Rockingham Memorial Hospital; S Landry, MS, C Caplen, MD, Central Shenandoah Health District; B Mays, Lord Fairfax Health District; L Branch, Office of Epidemiology; GB Miller, Jr, MD, State Epidemiologist, Virginia Dept of Health. Parasitic Diseases Br, Div of Parasitic Diseases, Center for Infectious Diseases, CDC.

Editorial Note

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 occurred 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 produced 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 pork are killed by freezing at 5 F (-15 C) for 21 days (or longer if meat is greater than 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. 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 University 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: Kim 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.

    • 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.

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