Bear Meat, Frozen 45 Days, Still Carried Trichinella: A 2022 Outbreak

By Shama Cash-Goldwasser and colleagues from the Minnesota, South Dakota and Arizona health departments and CDC. Morbidity and Mortality Weekly Report (MMWR).

In brief

• Known: trichinellosis is rare in the U.S. and usually comes from eating wild game.
• New: of eight people who shared a meal of black bear meat, frozen for 45 days, six got sick — including two who ate only the vegetables cooked with it. Live, freeze-resistant Trichinella nativa larvae were found in leftover meat frozen for more than 15 weeks.
• So what: only cooking meat to 165°F (74°C) reliably kills the parasite, and infected meat can contaminate other food.

The patient

In July 2022, the Minnesota Department of Health heard about a 29-year-old man hospitalized with fever, severe muscle pain, swelling around the eyes, high eosinophils and other abnormal lab results. He had sought care four times and been hospitalized twice over 17 days before doctors learned he had eaten bear meat and started albendazole. A Trichinella antibody test confirmed it.

The meal

Six days before he fell ill, he and eight relatives from Arizona, Minnesota and South Dakota gathered in South Dakota and ate kabobs of black bear that one of them had shot in northern Saskatchewan in May 2022. The outfitter had advised freezing the meat to kill parasites; it spent 45 days in a home freezer. Grilled with vegetables, it was first served rare — the dark meat made doneness hard to judge — and recooked once people noticed.

Trichinella larvae from the bear meat. Images: CDC.

The investigation

• Who got sick: of eight people interviewed, five ate the meat and all eight ate the vegetables. Six became ill — four who ate meat and vegetables, two who ate only vegetables — aged 12 to 62, in Arizona (1), Minnesota (4) and South Dakota (1).
• Severity: three were hospitalized and treated with albendazole; two had sought care at least twice first. All six recovered.
• Tests: only two of six blood samples showed antibodies — early testing misses many cases — but a cell-free DNA sequencing test found Trichinella DNA in both plasma samples tried.
• The meat: after 110 days in the freezer, it held more than 800 live larvae per gram. Tests identified Trichinella nativa, 100% matching by genome sequencing.

The hunter was told to discard the rest, and CDC notified the Public Health Agency of Canada.

Why it matters

• Wild game is the source now: changes in pork production mean most U.S. cases come from game. From January 2016 to December 2022, seven U.S. outbreaks (35 cases) were reported, mostly from bear. Between 1% and 24% of black bears in Canada and Alaska carry the parasite, and even more polar bears, wolverines and cougars.
• Hard to diagnose: symptoms are vague; eye swelling, eosinophilia and high creatine kinase are clues. Broad DNA-based tests can help when standard workups come up empty.
• Freezing isn't enough: freezing kills the species common in pork, but freeze-resistant ones like T. nativa and the T6 genotype dominate in Arctic and sub-Arctic regions — the larvae here survived nearly 4 months.

Advice for hunters and cooks

• Cook wild game to at least 165°F (74°C), checked with a meat thermometer — color is not a reliable guide.
• Keep raw meat separate from other foods, including its juices.
• Hunting organizations should teach hunters about these risks.

Sources

Based on "Outbreak of Human Trichinellosis — Arizona, Minnesota, and South Dakota, 2022," Morbidity and Mortality Weekly Report, Centers for Disease Control and Prevention; MMWR material is in the public domain.