Alaskan brown bears in Katmai National Park

Alaskan bears have been found to often have long tapeworms trailing from their rear ends. These tapeworms can reach several meters in length and emerge from the bear’s intestine through the rectum. Researchers were surprised to discover how common it is for Alaskan bears to be infected with these large parasites.

What are Tapeworms

Tapeworms are a type of flatworm parasite that lives in the intestines of animals. They have long, flat bodies made up of many small segments. Tapeworms attach themselves to the walls of the intestine using hook-like mouthparts or suckers on their head segment. As adult tapeworms grow inside a host, new segments called proglottids form at the back end of the worm. These proglottids are full of eggs and will eventually break off from the main body of the worm and exit the host through the feces. This allows the tapeworm eggs to spread to new hosts.

Tapeworms in Bears

Bears become infected with tapeworms when they swallow tapeworm eggs or larva. This might happen when bears eat infected fish, rodents, or other prey animals. The eggs then hatch inside the bear’s intestine, where the tapeworm anchors itself and grows into an adult.


Over time, the tapeworm matures and proglottids packed with eggs break off and are passed out in the feces. These egg-filled sections exiting the rectum are what gives the appearance of long worms trailing out of infected bears. Researchers think the tapeworms don’t typically cause illness in the bears, but can become an annoyance.

Research on Tapeworms in Alaskan Bears

Recently, researchers carried out a study focusing specifically on tapeworm infection rates in coastal brown bears from Alaska’s Katmai National Park. They collected and analyzed fecal samples from the park’s bears between 1997 and 2013. Shockingly, they found that 61% of sampled bears had tapeworm eggs in their feces. This indicates the bears had an active tapeworm infection with mature proglottids releasing eggs inside them.

In some cases, the bears passed entire intact tapeworms up to 4–5 meters long. The researchers said it was surprising to see how commonly the bears were infected with tapeworms. However, they noted the parasites likely don’t cause significant health issues. The team thinks the coastal habitat may make bears more prone to tapeworms since they feed on infected salmon. But more research is needed to understand the tapeworm life cycle in Katmai’s brown bears.

Tapeworms in Alaskan Bears

Effects of Tapeworms on Bears

Tapeworm infection may have minimal effects on bears in most cases. However, large tapeworm burdens could potentially contribute to intestinal blockages or nutritional deficits if enough nutrients are robbed from the bear. Heavily infected cubs or elderly bears with compromised health may suffer more severe impacts. But the study authors note there’s no evidence Katmai’s tapeworm-trailing bears experience any signs of illness.

In fact, the park’s coastal brown bears are well-known for being exceptionally large and healthy. They grow big on a diet of protein-rich salmon. So it seems these bears tolerate tapeworm infections without health consequences thanks to their nutrient-rich diet and prime habitat. Still, scientists need a better understanding of how parasitic infection affects bear health over the long term.

Introduction to Tapeworms in Alaskan Bears

In conclusion, Alaskan bears are now known to commonly suffer from surprisingly heavy tapeworm infections. But the beloved coastal brown bears of Katmai National Park appear healthy despite the parasite burdens. Researchers were amazed to find most sampled bears positive for tapeworms. The next steps are learning more about tapeworm life cycles in bears and continuing to monitor the long-term health impacts on infected bear populations. For now, tapeworm trailing from bear behinds seems normal in Alaska’s wilderness.