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Trichinosis Surveillance, United States, 1986

Theodore M. Bailey, M.D., M.P.H. Peter M. Schantz, V.M.D., Ph.D. Parasitic Diseases Branch Division of Parasitic Diseases Center for Infectious Diseases

Trichinella spiralis is a parasite of carnivorous animals that causes the disease trichinosis. In the United States, people become infected by eating poorly cooked pork products or wild animal meat that is infected with the parasite. Although fewer than 100 cases per year are reported to CDC, trichinosis continues to persist as a public health problem in this country. Public health officials believe that the reported cases represent only a fraction of the total number of cases, since many of the mild or asymptomatic infections are undetected or are misdiagnosed unless they are related to more severe cases.

In 1986, 51 cases of trichinosis were reported to CDC from 12 states and the District of Columbia. Thirty-six (71%) of these cases occurred in New Hampshire, Hawaii, Massachusetts, and Pennsylvania. Pennsylvania reported the largest number of cases, 15, or 29% of the total.

In 1986, commercial pork products accounted for only three isolated cases of trichinosis. The other cases of trichinosis caused by pork included wild boar or pork purchased directly from a farm.

Among those cases in which the food item was known or suspected, pork was incriminated in 26 (61%) cases, bear meat in 14 (33%), and other meat in three (7%) cases.

Trichinosis is a preventable disease. The U.S. Department of Agriculture requires that ready-to-eat pork products be heat treated or frozen to kill the parasite before the products are sold to consumers. Fresh pork products, unless they are specifically labeled as "trichinae tested" or "trichinae free," must be cooked to 77C (170F), a temperature that will kill the parasite. All wild animal meat must be assumed to be infected with trichinae and also cooked to 77C (170F) to safeguard against this disease. INTRODUCTION

Trichinosis is an infection caused by eating undercooked meat containing larvae of the parasitic worm Trichinella spiralis. The larvae are found in various meats, including pork products, wild boar, bear, walrus, and other carnivorous game (1). The number of reported cases per year in the United States has declined since CDC first began collecting these statistics in 1947 (2) (Figure 1). In the period 1977-1981, the average number of reported cases per year was 137. In 1982-1986, this figure dropped to 57 cases. No apparent change in the surveillance system can account for this downward trend. Although infected pork continues to be the main source of human infection (Figure 2), the decline in the annual incidence of trichinosis over the last 12 years appears to be due to a decrease in cases traceable to commercial pork products (Figure 3). The absolute number of cases attributed annually to wild animal sources, such as wild boar, bear, and other carnivorous game, has remained relatively constant since 1975 (Figure 2); therefore, although commercial pork is declining as a source of infection, the percentage of annual cases due to wild animal meat has increased. MATERIALS AND 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 54.7-Rev 7-81) to the Division of Parasitic Diseases (DPD), Center for Infectious Diseases, CDC. Additional cases are identified through reported results of trichinosis serologic tests performed by the Parasitic Diseases Branch, DPD, and through investigations conducted by the DPD staff.

The CDC case definition for trichinosis is as follows:

  1. Trichinella-positive muscle biopsy or 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) in persons who have shared the epidemiologically implicated meal or have consumed the implicated meat product. As in the past, cases reported by the states but that are not accompanied by written surveillance reports or that do not fit the case definition are not included in this report. For data analysis, commercial pork products are defined as pork purchased at supermarkets, butcher shops, or public eating places. Noncommercial pork products are defined as pork purchased directly from a farm. Other pork products are specifically identified according to source, such as hunting or trapping. RESULTS

In 1986, 51 cases of trichinosis from 12 states and the District of Columbia were reported to CDC. Thirty-six (71%) of these cases occurred in Pennsylvania, New Hampshire, Hawaii, and Massachusetts. Pennsylvania reported the most cases, 15 (29%). The Middle Atlantic and New England states accounted for 62.7% of the reported cases. In 1986, states with the highest trichinosis incidence were New Hampshire (9.0 cases/million population) and Hawaii (6.6). Several states reported 0.1-1.0 case/million population (Table 1).

Of the 51 cases reported in 1986, 64.7% occurred in males and 35.3% in females. The mean age of patients was 36.7 years, ranging from 7 to 81 years (Figure 4).

In previous years, there has been a consistent seasonal pattern for trichinosis in the United States, with a peak in December and January related to the consumption of homemade pork sausage during the Christmas holidays. In 1986, 25 (49%) cases were reported in January and December (Figure 5). Of the 11 cases reported for January, four were traced to a common-source outbreak associated with homemade pork sausage in Massachusetts. The pork used to prepare this sausage came directly from a farm. Five additional cases were traced to a common-source outbreak associated with wild boar in New Hampshire. Of the 14 cases reported for December, 12 were traced to a common-source outbreak associated with bear meat in Pennsylvania.

All but one of the patients reported at least one common sign or symptom of trichinosis: 34 (70.8%) of 48 had fever, 28 (59.6%) of 47 had periorbital edema, 43 (87.8%) of 49 had myalgia, and 31 (91.2%) of 34 had eosinophilia. The patient reporting none of the common symptoms was an 8-year-old boy whose illness was diagnosed by serology during the investigation of an outbreak. One death in 1986 was attributed to trichinosis because myocarditis was found at autopsy.

The mean incubation period for the 35 cases for which the dates of consumption of incriminated meat and the onset of symptoms were available was 19.7 days (range = 1-76 days). A diagnosis of trichinosis was confirmed by serologic tests for 34 persons (81% of those who had serologic tests). Muscle biopsy was performed on nine patients, and seven were positive.

The suspected meat was examined for Trichinella larvae in three outbreaks and in one isolated case. Two of these outbreaks involved wild boar meat (Hawaii, seven cases; New Hampshire, seven cases), and the third involved bear meat (Pennsylvania, 14 cases). The consumption of Trichinella-positive cougar meat caused an isolated case in Oregon.

In instances in which the food item was known or suspected (43 cases), pork was incriminated in 26 (60.5%) cases, bear meat in 14 (32.6%), and other meat in three (6.9%). Wild boar was the form of pork most frequently implicated, and it accounted for 14 (32.6%) cases (Table 2). In the other 12 cases due to pork, six patients purchased their pork directly from a farm; three, from a supermarket; and in three cases the origin of the pork was unknown (Table 3).

The method of meat preparation was identified in 42 cases: in 21 (50%), it was roasted over an open fire; in nine (21.4%), the meat was eaten raw; in six (14.3%), it was fried; in five (11.9%), it was microwaved; and in one case, it was boiled.

The source of the meat was identified in 39 cases: in 29 (74.4% of cases for which a source was reported), the implicated meat was wild game; in six (15.4%), it came directly from the farm; and in four (10.2%), the meat came from a supermarket. Of the 29 cases in which the meat was wild game, bear accounted for 14 cases, wild boar for 14, and cougar for one case.

Five common-source outbreaks accounted for 34 (66.7%) of the 51 cases reported to CDC in 1986. Two of the outbreaks involved homemade pork sausage (six cases). In both of these outbreaks, the pork used to prepare the homemade sausage came directly from a farm. The other three outbreaks involved wild animal meat (28 cases). A report on one of these outbreaks, which occurred in Hawaii, has been published (3). In that outbreak, signs and symptoms of trichinosis developed in seven of 21 persons who had eaten wild boar meat. The implicated meat was positive for Trichinella larvae. Four of the 21 persons ate the meat after it had been microwaved, and the other 17 ate the meat fried. All four who ate microwaved meat became ill; the other three persons had eaten the meat fried.

The largest outbreak, in Pennsylvania, involved 14 persons who ate bear meat at a hunting-season cookout. The meat was served as hamburger or fried steak; examination of remaining meat revealed Trichinella larvae. The final outbreak, in New Hampshire, involved seven persons who ate ground sausage made from wild boar meat. The sausage was roasted over an open fire at an ice-fishing party. DISCUSSION

The continuing decrease in the annual incidence of trichinosis in the United States is related to the decline of commercial pork products as a cause of trichinosis (Figure 3). In 1986, commercial pork products were implicated in only three isolated cases of trichinosis.

The exact causes of this decrease are unclear. Since 1980, however, several activities at the national and state level have improved prospects for the control and prevention of trichinosis in commercial pork products. The Federal Swine Health Protection Act (Public Law 96-468) of October 17, 1980, prohibits the feeding of garbage to swine unless the garbage is heat treated to kill disease-causing organisms, including T. spiralis. Enforcement of this act is delegated to the individual states contingent on their having adequate laws and regulations and a program for enforcement (4). Otherwise, enforcement of the act is the responsibility of the U.S. Department of Agriculture's (USDA's) Animal and Plant Health Inspection Service (APHIS), as is the case in 15 states (unpublished data, D. Galbreath, APHIS). Since 1982, the National Pork Producers Council (NPPC), in cooperation with USDA, has promoted research to develop rapid enzyme-linked immunosorbent assay (ELISA) tests to detect hogs with trichinosis before slaughter, and these tests are being evaluated in the field. Both NPPC and USDA actively promote hog-management practices that avoid factors epidemiologically linked to the transmission of trichinosis; such practices include the immediate removal of dead hogs from feed lots, effective rat control, and other methods that eliminate the interaction of commercial hogs and wild animals.

Some states have implemented programs to eliminate trichinae from their hog populations. An example is Illinois, where feeding garbage to swine is prohibited by state law and where there is an active enforcement program. In addition, the 1986 Illinois Trichinosis Control Act allows the state's Department of Agriculture to quarantine infected herds. It also requires the testing of herds thought to be infected and the depopulation of trichinae-infected herds. Education about proper hog-management practices is also an integral part of the program. Since the passage of this act, the Illinois Department of Agriculture has eliminated trichinae from all but two foci in the state.

The best estimate of the overall prevalence of trichinosis in marketed hogs in the United States is 0.1%. This estimate is based on a study of farm-raised butcher hogs for 1966-1970 (5). Recent studies have shown marked regional differences. In a slaughterhouse-based survey of 1,223 hogs in Louisiana, only one (less than 0.1%) infected animal was detected (6). No infection was found among 3,245 hogs from slaughterhouses in Minnesota, Wisconsin, Iowa, South Dakota, and North Dakota examined from 1983 to 1985. The animals examined were from small family farms where pigs were raised for home consumption and from large commercial operations (7).

In comparison, 0.7% of 5,315 slaughter hogs from the New England states were infected. Infected hogs were found in five of six states (8). In a similar study, 33,482 slaughter hogs from the Mid-Atlantic states were examined. In those states, the sources of hogs varied from small slaughterhouses (fewer than 50 hogs per day) to large commercial operations (more than 4,000 hogs per day). The overall prevalence was 0.6%, and all the infected hogs were found in the smaller slaughterhouses (fewer than 1,000 hogs per day). In this study, most of the positive samples were obtained from one slaughterhouse, and analysis of the geographic origin of the hogs indicated that most of the infected hogs were from New Jersey and Pennsylvania. Infected hogs could not be traced back to the farm of origin, because there was no identification system for marketed hogs (9).

The total elimination of trichinosis from the commercial pork supply ultimately depends on the efforts of individual states to 1) establish active programs to enforce the Swine Health Protection Act, 2) educate hog producers on proper hog-management practices and see that these practices are enforced, and 3) provide mechanisms at the state level to identify and eliminate trichinosis from infected herds. The role of public education concerning the proper cooking of pork to eliminate the risk of trichinosis cannot be overstated. Whereas commercial pork products are declining as a cause of trichinosis, noncommercial sources of pork--such as small farms not using modern hog-management practices--and wild animal meats are emerging as important sources of human trichinosis in the United States. Persons who buy pork from small farms or who eat wild animal meats should be educated as to how to eliminate their risk of acquiring trichinosis.


  1. Pawlowski ZS. Clinical aspects in man. In: Campbell WC, ed. Trichinella and trichinosis. New York: Plenum Press, 1983:367-401.

  2. Schantz PM. Trichinosis in the United States--1947-1981. Food Tech March 1983:83-6.

  3. Centers for Disease Control. Trichinosis--Hawaii. MMWR 1987;36:14-6.

  4. Leighty JC. Control of trichinosis, public-health aspects (with special reference to the United States). In: Campbell WC, ed. Trichinella and trichinosis. New York: Plenum Press, 1983:501-13.

  5. Zimmermann WJ, Zinter DE. The prevalence of trichiniasis in swine in the United States, 1966-70. HSMHA Health Rep 1971;86:937-45.

  6. Hugh-Jones ME, Stewart TB, Raby C, Morrison JE, Isenstein RS, Klei TR. Prevalence of trichinosis in southern Louisiana swine. Am J Vet Res 1985;46:463-5.

  7. Stromberg BE, Prouty SM. Prevalence of trichinellosis in the North-Central United States. Proc Helminthol Soc Wash 1987;54:231-2.

  8. Schad GA, Leiby DA, Duffy CH, Murrell KD. Swine trichinosis in New England slaughterhouses. Am J Vet Res 1985;46:2008-10.

  9. Schad GA, Kelly M, Leiby DA, Blumrick K, Duffy C, Murrell KD. Swine trichinosis in mid-Atlantic slaughterhouses: possible relationship to hog marketing systems. Prev Vet Med 1985;3:391-9.

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