Salmon Tapeworm: Diphyllobothrium dendriticum

Volume 4, Issue 1
January 2002

Diphyllobothrium dendriticum

Figure 2. Diphyllobothrium dendriticum cysts on the stomach & intestine of an Atlantic salmon. Cysts contain immature cestode larvae (plerocercoids).

Wild Atlantic salmon Salmo salar caught by anglers in Maine are sometimes infested by the salmon tapeworm.

This cestode parasite lives in the pyloric caeca and small intestine of adult salmon. The number of these segmented worms found in an individual fish can vary from a few to hundreds. This parasite can be fatal to salmon and brook trout if infestations overwhelm the fish. The adult D. dendriticum release eggs in the gut of the fish which are excreted into the water with the fish’s feces. The eggs hatch and are eaten by zooplankton which are in turn eaten by small fish. The small fish are eaten by gulls. When gulls defecate into the water they spread the salmon infective life stage of the parasite.

Unlike many fish parasites, D. dendriticum can infect other animals including dogs, cats, guinea pigs, fox, rat, squirrel, terns, other salmonids, and humans. Most animals are paratenic hosts, which means that the parasite lives in the
animal’s body, but cannot complete it’s lifecycle.

Adult Cestode

Figure1. Diphyllobothrium dendriticum adult Cestode from the intestine of an Atlantic salmon.

D. dendriticum is also known as Diphyllobothrium sebago after it was described by Ward 1910 in Atlantic salmon from Lake Sebago, Maine.

Cestode's scolex (head)

Fig. 3. Closeup of Cestode’s scolex (head).

While it may be impossible to completely remove the parasite from the environment, the population of parasites in salmon can be decreased by reducing the number of gulls living in an area.

Figure 2 contains images of the immature plerocercoids encysted in the intestinal serosa of an Atlantic salmon. The cysts are usually small
1-3 mm, and yellow-cream. Anglers may see these yellow cysts when cleaning their catch.

Special points of interest:

If eaten uncooked fish with D. dendriticum can transmit the parasite to people.

D. dendriticum is also called D. sebago.

D. dendriticum infects many species of fish, birds, and mammals.

For more information read: Hoffman 1999. Parasites of NA Freshwater Fishes.

Images were made possible by a grant from the Maine Outdoor Heritage fund. Specimens donated by Vermont Fish and Game.