Wildlife, including large mammals like bears, naturally harbors a variety of parasites. Many people wonder about the presence of these organisms in wild animals and the potential implications for human health. Understanding the types of parasites bears carry, how they acquire them, and the resulting health effects for both bears and humans is important for safe interaction with wildlife and proper food handling.
Types of Parasites in Bears
Bears can host several types of internal parasites, commonly referred to as “worms.” These include nematodes, cestodes, and trematodes. Nematodes, or roundworms, are cylindrical and can be found in various tissues; examples include Trichinella species and Baylisascaris species. Trichinella larvae encyst in muscle tissue, while Baylisascaris transfuga are large roundworms, 10 to 25 centimeters long, residing in the large intestines of bears. Hookworms are another type of intestinal nematode detected in bears.
Cestodes, known as tapeworms, are flat, segmented parasites that attach to the intestines. Alaskan bears, particularly, are noted for harboring tapeworms, which can sometimes reach lengths of 30 feet or more, especially in populations that consume salmon. Trematodes, or flukes, are also reported as parasites in bears.
How Bears Acquire Parasites
Bears acquire parasites primarily through their feeding habits. Consuming infected prey is a common route; for example, bears can contract Trichinella by eating the meat of other infected carnivores or omnivores. Tapeworms are frequently acquired by bears eating infected fish, such as salmon.
Bears can also ingest parasite eggs from the environment, such as through contaminated soil, which is a known transmission route for roundworms like Baylisascaris. Additionally, certain parasites, like filarial worms, are transmitted through insect vectors, such as black flies, during a blood meal.
Impact on Bear Health
For many wild bear populations, parasitic infections often cause mild or no noticeable symptoms. Bears and their parasites have co-evolved, as it is not beneficial for a parasite to kill its host. Despite this, some bears may experience discomfort, such as those with long tapeworms.
More severe health impacts, though less common, can occur. Heavy parasitic loads can lead to weight loss, lethargy, or anemia, particularly observed with tapeworm infestations. While Baylisascaris transfuga infections are typically asymptomatic, very heavy infections could potentially cause illness or even death. Young, old, or immunocompromised bears may exhibit more pronounced symptoms.
Risks to Humans from Bear Parasites
Parasites in bears pose zoonotic risks, meaning they can be transmitted to humans. The most significant concern is Trichinella, a roundworm that causes trichinellosis. People contract trichinellosis by consuming raw or undercooked meat from infected animals, with bear meat being a frequent source in North America.
Symptoms vary but can include nausea, vomiting, diarrhea, and abdominal pain within days of consumption. As larvae migrate and encyst in muscles, later symptoms may develop, like muscle cramps, headaches, fever, and facial swelling. In severe cases, the heart, lungs, or brain can be affected, potentially leading to serious complications or death. While cases are rare in the United States, bear meat is the suspected source in most reported outbreaks.
Preventing Human Exposure
Preventing human exposure to bear parasites involves proper handling and cooking of bear meat. Cook bear meat thoroughly to an internal temperature of at least 160°F (71°C), or preferably 165°F (74°C), and hold that temperature for at least three minutes. Some sources recommend cooking until the meat reaches 170°F or even 185°F to ensure no pink remains.
Freezing bear meat is not a reliable method to kill all strains of Trichinella, especially freeze-resistant ones found in Arctic and subarctic regions. Additionally, smoking, drying, or microwaving meat does not consistently eliminate the parasites. Practicing good hygiene during meat preparation, such as washing hands and utensils after handling raw meat and preventing cross-contamination, further minimizes risk.