Great Basin Naturalist


Diphyllobothrium cordiceps (Leidy 1871) plerocercoids are present as enzootic parasites in the viscera and skeletal muscle of cutthroat trout (Salmo clarki) from Yellowstone National Park, Wyoming. Eight cutthroat trout from the Yellowstone River were examined by histological technique and scanning electron microscopy to determine the response of host tissue to the presence of diphyllobothriid larvae. All fish sampled contained encysted plerocercoids on the serosa of the pyloric caeca and intestine. In addition to infection of the exterior of the alimentary tract, muscle tissue was infected in two of the fish sampled, whereas one fish had infection of the liver, spleen, and testis. Intact plerocercoids were encapsulated with connective tissue that was infiltrated with lymphocytes and macrophages. Granulomatous tissue that was fibrotic was also present. Pancreatic tissue was displaced in infections associated with the alimentary tract. The liver showed general necrosis with edema, and the spleen demonstrated a reduction in cellularity and increased connective tissue. Testicular tissue compressed by an adjacent plerocercoid appeared to be in an otherwise normal stage of development. Necrotic myofibrils near encapsulated parasites were separated by edema and fatty infiltration. A brief morphologic description of D. cordiceps is presented. In general, D. cordiceps did not appear to produce a serious debilitation of cutthroat trout.