Trichinella spiralis

Food Safety October 28, 2015 Print Friendly and PDF

History and Incidence

Historically, trichinellosis has been associated with eating Trichinella-infected pork from domesticated sources. However, wild game meat has now become the most common source of infection. During 1997-2001, 72 cases of trichinellosis were reported to the Centers for Disease Control and Prevention (CDC). Of these, 31 (43 percent) cases were associated with eating wild game—29 with bear meat, one with cougar meat, and one with wild boar meat. In comparison, only 12 (17 percent) cases were associated with eating commercial pork products, including four cases traced to a foreign source. Nine (13 percent) cases were associated with eating noncommercial pork from home-raised or direct-from-farm swine where U.S. commercial pork production industry standards and regulations do not apply.

 

End of Trichinella worm showing copulatory apparatus.

Statistics have been kept on trichinellosis in the United States since 1947 and incidence of this disease has been steadily declining. Between 1947 and 1951, a median of 393 cases (range, 327-487) was reported annually, including 57 trichinellosis-related deaths. During 1997-2001, the incidence decreased to a median of 12 cases annually (range, 11-23) and no reported deaths.

According to the CDC, most of the decline in reported trichinellosis cases is a result of improvements in animal husbandry practices in the U.S. commercial pork industry, which has reduced Trichinella prevalence among swine (see below).

Year: Estimated trichinellosis prevalence in swine

  • 1900: 1.41%
  • 1966-1970: 0.125%
  • 1995: 0.013%

Reasons for Decline

Trichinellosis prevalence among swine has decreased for several reasons. Historically, the major sources of Trichinella-infected pork were swine fed garbage that contained animal waste products. Garbage cooking laws were passed in 1953-54 and 1962 to control specific swine diseases, and, in 1980, Congress prohibited feeding potentially contaminated garbage to swine.

In 1994, the United States Department of Agriculture, the National Pork Producers Council, and the pork processing industry developed the Trichinae Herd Certification Program. This is a voluntary pre-harvest pork safety program that provides documentation of swine management practices to minimize Trichinella exposure. The goal of the program is to establish a system under which pork production facilities that follow good production practices might be certified as Trichinella-safe.

In addition to reducing Trichinella prevalence in commercial pork, processing methods have also contributed to the dramatic decline in human trichinellosis associated with pork products. USDA has identified specific cooking temperatures and times, freezing temperatures and times and curing methods that kill Trichinella larvae in processed pork products.

Preventing Infection

Here the parasitic disease Trichinosis is manifested by splinter hemorrhages under the finger nails.


USDA recommends that consumers of fresh pork cook the meat to an internal temperature of 160 degrees F. Trichinella spiralis larvae in pork are killed at lower temperatures (for example, 140 degrees F for 2 minutes or 131 degrees F for 6 minutes). However, USDA has recommended a higher temperature to allow for different cooking methods that might result in uneven temperature distributions throughout the meat (for example, microwave cooking).

Freezing kills Trichinella spiralis larvae in pork. Pork less than 6 inches thick can be made safe if frozen to minus 20 degrees F for 6 days, -10 degrees F for 10 days, or 5 degrees F for 20 days. However, freezing might not kill other species and types of Trichinella found in wild game, such as bear, wolverine or fox.

Advice for prevention of trichiellosis:

  • Cook meat products to an internal temperature of 160 degrees F.
  • Freeze pork less than 6 inches thick for 20 days at 5 degrees F to kill any worms.
  • Cook all meat fed to pigs.
  • Do not allow hogs to eat uncooked carcasses of other animals, including rats, which may be infected with trichinellosis.
  • Clean meat grinders thoroughly if you prepare your own ground meats.

Curing (salting), drying, smoking, or microwaving meat does not consistently kill infective worms.

Sources

National Institute for Animal Agriculture, Swine Health Report, Fall 2003., page 3,

 

Roy, S. L., Lopez, A.S., & Schantz, P.M. Trichinellosis Surveillance— United States, 1997-2001, Morbidity Mortality Weekly Report, Surveillance Summaries, July 25, 2003/52(SS06); 1-8, Trichinellosis Fact Sheet

Photo by B Nummers/ CC BY http://creativecommons.org/licenses/by-nc-sa/3.0/

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.