Trichinosis

INTRODUCTION

Human trichinosis, an infection with worldwide distribution, is caused by tissue-dwelling roundworms of the species Trichinella spiralis. The organism is acquired by eating raw or inadequately cooked meat products containing encysted larvae. Abdominal pain and diarrhea, the first symptoms of trichinosis (the gastrointestinal phase), correspond to the maturation of the adult worms in the first week after ingestion. The classic symptoms of periorbital edema, fever, and myalgia occur when the larvae are encysting in the muscle, approximately 2-3 weeks after the infected meat is eaten. Pork products continue to be the major source of infection in the United States, although game animals are also a source of contaminated meat products.

From 1982 to 1986, an average of only 57 cases were reported each year with three associated deaths for the period, in contrast to an average of 402 cases each year and a total of 57 deaths reported from 1947 to 1951 [1,2]. This decline is primary attributed to legislation prohibiting the feeding of raw garbage to swine, widespread freezing of pork, and increased public awareness of the dangers associated with eating inadequately cooked pork products [3]. No apparent change in the surveillance system can account for this trend.

METHODS

State health departments report new cases of trichinosis by week to the National Morbidity Reporting Service. Supplemental epidemiologic information is submitted by the reporting state on Surveillance Case Report Forms (CDC Form 54.7, Rev 2-90) to the Division of Parasitic Diseases (DPD), National Center for Infectious Diseases, CDC. Additional cases are identified through reported results of trichinosis serologic tests performed by the Paratisic Diseases Branch, DPD, and through investigations conducted by DPD.

The CDC case definition for trichinosis is as follows:

1. Trichinella-positive muscle biopsy or a positive serologic test for trichinosis in a patient with one or more clinical symptoms compatible with trichinosis, such as eosinophilia, fever, myalgia, and periorbital edema. OR

2. In an outbreak, at least one person must meet criterion #1. Associated cases are defined by either a positive serologic test for trichinosis or one or more clinical symptoms compatible with trichinosis (such as eosinophilia, fever, myalgia, and periorbital edema) among persons who shared the epidemiologically implicated meal or ate the implicated meat product.

As in the past, cases reported by the states are not included in this report if they are not accompanied by written surveillance reports or do not fit the case definition. For the purposes of data analysis, commercial pork products are defined as pork purchased at supermarkets, butcher shops, wholesale meat suppliers, or public eating places. Noncommercial pork products are specifically identified according to source, such as hunting or trapping.

RESULTS

From 1987 through 1990, 206 cases of trichinosis from 22 states, including 14 multiple-case outbreaks, were reported to CDC (Figure 1). One hundred fifty-four (75%) of these cases occurred in Iowa (78 cases), Alaska (38), Virginia (16), California (11), and New Jersey (11) (Table 1). The 14 common-source outbreaks accounted for 171 (83%) of the 206 cases (Table 2). One hundred one (49%) cases occurred among males, 103 (50%) occurred among females, and gender was not reported in two cases. The mean age of patients was 35.4 years, ranging from 1 to 81 years (Figure 2).

In many previous years, trichinosis in the United States had exhibited a seasonal pattern, with a sepak in December and January related to eating homemade pork sausage during the Christmas holidays. However, from 1987 through 1990, only 9% of cases occurred in December and January, in contrast to 49% of cases reported in these months in 1986. One hundred eleven (60%) of the 184 cases for which the month of onset was reported occurred from June to August. Ninety of these 111 cases resulted from one outbreak in Iowa associated with a single meal in July 1990.

One hundred seventy-six (85%) patients reported at least one common symptom or sign of trichinosis: 159 (77%) had myalgia, 156 (76%) had fever, 132 (64%) had periorbital edema, and 97 (47%) had eosinophilia. One death was associated with acute trichinosis in 1987. One case of acute trichinosis involved a woman who was 16 weeks pregnant. The pregnancy was completed without complication, and the baby's physical examination at birth was normal.

The mean incubation period for the 152 cases for whcih the dates of consumption of the incriminated meat and the onset of symptoms were known was 20.5 days, with a range of 2 to 53 days. Trichinella serology was found to be positive in 110 (80%) of the 137 persons for whom the results were reported. Muscle biopsies were performed for 19 (9%) persons, and 18 of the biopsies were positive.

In the 192 instances in which a suspect food item was identified, pork was implicated in 144 (75%) cases, walrus meat in 34 (18%), and bear meat in 14 (7%) (Figure 3). Sausage, the most frequently implicated pork product, was associated with 128 of the 139 cases for which a form of ingested pork was specified (Table 3). The meat was examined in 55 cases and found positive for Trichinella larvae in 51 cases.

DISCUSSION

Although the incidence of trichinosis has decreased substantially since national reporting was initiated in 1947, a dramatic increase in 1990, resulting from two large outbreaks, emphasizes the continuing need for public education about the dangers of eating inadequately cooked pork. Despite this large increase in total cases reported in 1990, the mean number of cases per year for the period 1987-1990 was lower than the mean number of cases for the period 1982-1986 (51.5 vs. 57). If the 1990 Iowa outbreak had not occurred, the mean number of cases per year in 1987-1990 would have been 29 cases. Before 1990, the proportion of cases of trichinosis attributable to eating commercial pork had declined steadily (Figure 4). This decline was probably due to a combination of factors, including laws prohibiting the feeding of garbage to hogs, the increased use of home freezers, and the practice of thoroughly cooking pork. Many outbreaks in recent years have been associated with eating wild game, including bear, wild boar, and walrus [2,4].

The outbreak in Iowa was the fourth such event among Southeast Asian refugees in the United States in the past 15 years [4,5]. The three previous outbreaks were related to eating undercooked pork that was not obtained from a commercial producer [4,5]. Previous reports have emphasized that Southeast Asians are at particular risk for trichinosis becase of their dietary habits, and the outbreak in 1990 supports this observation [5].

The outbreak in Virginia did not occur among persons of one distinct ethnic group but involved unrelated persons who obtained pork from the same supplier and routinely ate uncooked sausage [4]. This outbreak emphasizes that not only Southeast Asians but any persons who eat undercooked pork are at risk for trichinosis.

The prevalence of Trichinella infection in commercial pork, as measured by examination of hogs at slaughterhouses, ranges from 0% to 0.7% [6,7]. Any pork product sold as "ready to eat," which accounts for approximately 40% of the pork produced in the United States each year, must be made with trichina-free pork, or pork that has been adequately cooked or treated to kill trichina larvae. Trichinella larvae in pork are killed by freezing at -15 C for 21 days (longer if meat is >15 cm thick). However, because Trichinella larvae in wild game are often relatively resistant to freezing [8], cooking is the most reliable method of destroying Trichinella in any type of meat. A temperature of 170 F (77 C) is well above the thermal death point and is usually achieved if the meat is cooked until it is no longer pink [9].

Sporadic cases and outbreaks associated with eating commercial pork continue to occur. Health officials in areas with large populations of Southeast Asians should consider education programs directed at preventing trichinosis. Physicians need to be aware of the continued presence of Trichinella spiralis in commercial pork in the United States and should consider the diagnosis in any patient with an illness compatible with trichinosis whose dietary preferences put him or her at risk for acquiring this infection.

Recent analysis of cases reported in the United States from 1975 to 1989 revealed 26 cases acquired in association with foreign travel [10]. This finding suggests that physicians must include trichnosis in their differential diagnosis of eosinophilia among persons returning from abroad and should also include trichinosis prevention measures in their pretravel counseling.

Public education will continue to play a prominent role in preventing illness among persons who eat wild animal meat. Even more important to the complete elimination of trichinosis in the United States are sustained efforts by state and local governments to enforce federal regulations regarding hog management, eliminate infected herds, and increase educational measures directed at both pork producers and the public.

References

[1] Schantz PM. Trichinosis in the United States, 1947-1981. Food Technol 1983; (March):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(1):5-11.

[3] CDC. Trichinosis surveillance, United States, 1986. In: CDC Surveillance Summaries, December 1988. MMWR 1988;37(No. SS-5):1-8.

[4] CDC. Trichinella spiralis infection - United States, 1990. MMWR 1991;40:57-60.

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

[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 or trichinosis. Albany, New York: State University of New York Press, 1985:224-8.

[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, 1985:251-5.

[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, 1981:151-5.

[9] Leigthy JC. Control 1 - public-health aspects (with special reference to the United States). In: Campbell WC, ed. Trichinella and Trichinosis. New York: Plenum Press, 1983:501-13.

[10] McAuley JB, Michelson MK, Schantz PM. Travel-associated Trichinella. J Infect Dis 1991; 164:1013-6.

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