Interactions between a gull tapeworm Diphyllobothrium dendriticum (Cestoda) and trout (Salmo trutta L.)

Abstract

The interaction between Diphyllobothrium dendriticum (Nitzsch 1824) (Cestoda) and trout was studied using the following materials: 1) brown trout Salmo trutta m. lacustris (L.) and sea trout Salmo trutta m. trutta (L.) from the Muonio Fish Farm; 2) brown trout from Lake Inari from 1994 and 1995; and 3) data from four laboratory experiments with D. dendriticum and brown trout. D. dendriticum was found to invade the heart atrium of fish in varying prevalences at the Muonio Fish Farm, Lake Inari and in experiments. In experimental studies the size and migration activity of D. dendriticum in brown trout increased along with water temperature and D. dendriticum infection had an increasing impact on the blood lymphocyte and neutrophil counts. The same dose of intubated infective procercoids caused overdispersed distributions of plerocercoids in brown trout, which was obviously due to individual differences in the susceptibility of fish to D. dendriticum infection. Negative effects on feed intake and growth rate were not observed when fed ad libitum. Mortality induced by a few D. dendriticum was observed in brown trout and sea trout at the Muonio Fish Farm in the early 1990s when plerocercoids penetrated the heart of the fish. Direct or indirect evidence of D. dendriticum induced mortality of the stocked brown trout could not be found in Lake Inari. Moreover, D. dendriticum did not cause a provable mortality in brown trout aged 0+ - 1+ in experimental studies. It is concluded that brown trout normally respond to the harmful effects of D. dendriticum successfully. The observed mortality at Muonio Fish Farm shows, however, that the balance between D. dendriticum and fish may collapse under certain circumstances. These studies indicate that a small proportion (maybe 5-10%) of the trout population will be lost annually to D. dendriticum heart infection, at least in lakes with strong D. dendriticum infection.