Trichinellosis in the United States: A Shift from Swine to Bears

Abstract & Commentary

By Brian G. Blackburn, MD, and Michele Barry, MD, FACP

Dr. Blackburn is Clinical Assistant Professor in the Division of Infectious Diseases and Geographic Medicine at Stanford University School of Medicine. Dr. Barry is Senior Associate Dean of Global Health at Stanford University School of Medicine.

Dr. Barry is a consultant for the Ford Foundation, and her program receives funding from the Johnson & Johnson Corporate Foundation. Dr. Blackburn report no financial relationships related to this field of study.

Synopsis: Fifty-four persons meeting the definition for a confirmed case of trichinellosis were reported to the CDC from 2002-07, a slight decline from the previous reporting period. Continuing the trend first reported in the 1997-2001 Trichinella Surveillance Summary, most cases in the United States now are acquired through ingestion of game meat, especially bear, rather than pork.

Source: Kennedy ED, Hall RL, Montgomery SP, et al; Centers for Disease Control and Prevention (CDC). Trichinellosis surveillance — United States, 2002-2007. MMWR Surveill Summ 2009;58:1-7.

Trichinellosis is a food-borne disease caused by multiple species of roundworms in the Trichinella genus. The parasite encysts in the muscles of carnivores and is acquired by the consumption of undercooked meat of animals containing Trichinella cysts. The parasite initially infiltrates the gastrointestinal tract, often resulting in diarrhea. A systemic phase of infection subsequently ensues as the parasite disseminates, with its particular tropism for striated muscle. Myositis, periorbital edema, conjunctivitis, fever, and eosinophilia can result; rarely, severely infected individuals may die of myocarditis or encephalitis.1

Transmission of Trichinella to humans traditionally has been associated with consumption of infected pork. However, changes in animal husbandry practices in the United States over the last several decades have led to a decrease in the prevalence of Trichinella in swine, and subsequently to a decrease in the overall number of reported human cases, mostly due to a decrease in pork-associated cases.2,3 In the previous Trichinella surveillance summary covering 1997-2001, the number of pork-associated cases was fewer than the number of non-pork-associated cases for the first time since reporting began more than 60 years ago.2,3

In the current surveillance summary, 66 cases of trichinellosis were reported to CDC from 2002-07, although only 54 were included for analysis (after excluding those that did not meet the definition for a confirmed case during the defined reporting period). Fifty-two (96%) of the 54 were Trichinella serology positive. Six small outbreaks accounted for 16 (30%) of the cases, mostly related to persons sharing the same infected meats. The overall mean incubation period was 18 days, and clinical manifestations included myalgias in 38 patients (70%), eosinophilia in 39 (72%), fever in 29 (54%), and periorbital edema in 15 (28%). Among those for whom the source of infection was known, trichinellosis was associated with consumption of pork in 10 cases (23%) (seven with commercial pork; two of these were acquired in Asia), and non-pork products in 27 cases (63%); bear meat was implicated in 21 (78%) of these latter cases.

Commentary

Trichinellosis is a potentially serious parasitic infection associated with the consumption of improperly cooked meat of infected animals. In the 1940s and 1950s, several hundred cases were reported annually in the United States, whereas in the years covered by the current report (2002-07), an average of fewer than 10 cases occurred annually. A slight decrease was seen even compared to the few cases observed in the most recent previous report (1997-2001).2 This decline is mostly attributable to fewer pork-associated cases, which has paralleled the declining Trichinella prevalence among pigs in the United States.2,3 Much progress has been made in improving animal husbandry practices during that time, many aimed specifically at decreasing risk factors for Trichinella infection in swine. These include the cessation of such behaviors as feeding swine potentially Trichinella-infected waste products, allowing the consumption of rodents or other wildlife infected with Trichinella, and cannibalism among pigs within an infected herd.1-3

The efforts geared at reducing Trichinella prevalence in domestic swine have been successfully implemented and have subsequently resulted in a dramatic decrease in the number of human cases in the United States. With the risk of trichinellosis associated with infected pork now so low, public health efforts should be focused on increased awareness in the community regarding the potential for game meat, especially bear meat, to transmit this parasite, and of the importance of proper cooking procedures for such meat. As highlighted in the surveillance summary, consumption of other game, such as cougar, walrus, seal, and wild boar also can result in transmission. In addition, despite the fact that Trichinella is transmitted via consumption of infected meat, some herbivores (such as cattle and deer) occasionally have been associated with human Trichinella cases, although their diet makes them more unusual hosts. International travelers should be aware of the potentially higher risk or trichinellosis with consumption of all meat products while abroad, including commercial pork, as regulations surrounding animal husbandry differ internationally.

References

  1. Gottstein B, Pozio E, Nockler K. Epidemiology, diagnosis, treatment, and control of trichinellosis. Clin Microbiol Rev 2009;22:127–45.
  2. Roy SL, Lopez AS, Schantz PM. Trichinellosis surveillance—United States, 1997–2001. MMWR Surveill Summ 2003;52 (No. SS-6).
  3. Gamble HR, Bush E. Seroprevalence of Trichinella infection in domestic swine based on the National Animal Health Monitoring System's 1990 and 1995 swine surveys. Vet Parasitol 1999;80:303–10.