In late October 2007, a hunter in Northern California shot a black bear and brought the carcass home for a community feast the next day. At least 38 people ate a variety of dishes, some of which included bear meat which was not fully cooked. Within a week, people who had attended the event started getting sick with fever, chills, and muscle aches. Over the next few weeks, 30 people became ill. Based on the clinical symptoms and the history of a common meal of bear meat, a local physician suspected Trichinella infection was causing these illnesses and notified the county health officials.
Trichinella is a type of parasitic roundworm that causes infection when we eat undercooked or uncooked meat containing the roundworm larva. [See interesting life cycle here.] The human illness, trichinellosis, was once very common in the United States and was typically caused by eating undercooked pork. Thankfully, this scenario is much less common now – at least for those living in the U.S. – due to improved pork production practices. Today, most Trichinella infections in the United States are associated with consumption of some types of wild game meat, including bear. In mild infections, most symptoms get better within a few months. However, fatigue, weakness, and diarrhea can last longer.
In the outbreak that occurred in California in 2008, the common meal of bear meat was a good clue to what was causing the illnesses. The California Department of Public Health, working with the county, contacted the Division of Parasitic Diseases (DPD) for assistance on the outbreak. Within days, serum samples from the people who may have eaten the bear meat were on their way to CDC, and another hunt was on, this time to find any leftover bear meat to test for the presence of Trichinella. Only a single paw from the bear remained, which came to the CDC to be tested as well. The results of the serologic tests performed at the DPD were positive for Trichinella sp. In addition, encapsulated Trichnella sp. larvae were identified in the bear paw tissue by microscopic analysis of the material performed by Henry Bishop from DPD’s microscopy diagnostic lab. In order to determine which species of Trichinella was infecting the bear, the DPD’s molecular diagnostic laboratory used a technique developed by USDA that confirmed the presence of Trichinella murrelli. This is a species that has only been identified in North America and is considered to be the most common type circulating in wild animals in the United States.
There are twelve types of Trichinella known to infect humans. This outbreak will provide valuable information about the clinical course of human infection with T. murrelli. In addition to extensive clinical descriptions of the illnesses, the state and county public health departments collected information about how much of each meat dish was consumed and how long after the meal the illnesses began. This will help us understand the course of T. murrelli infection relative to the other Trichinella species, helping physicians caring for infected patients know what to expect from this disease and how to best manage patient care.
Proper cooking of meat dishes, especially dishes prepared with some types of game meats, will prevent trichinellosis. Meat products, including sausages or other prepared dishes, should be cooked to internal temperatures of at least 170oF or until juices run clear. Some species of Trichinella are resistant to freezing, so freezing may not be an effective prevention method.
Game hunting is a popular sport in the United States, as well as being an important part of some cultural traditions. This recent outbreak in California is not the first nor is it likely to be the last…