US Fish & Wildlife Service


Date of this Version



Published in Journal of Parasitology, Vol. 47, No. 3 (Jun. 1961), pp. 399-400.


It is well known that migrating helminth larvae do a great amount of damage to the host. Often, however, the host is relatively large and the damage done by few larvae is not serious. When the host is very small the mechanical damage, at least, must be proportionately greater for each larva present. An unusual case of mortality of small brook trout (Salvelinus fontinalis) roughly 2 to 4 inches long was brought to our attention. Examination of the mortalities, as well as survivors, revealed relatively large diphyllobothrid larvae (fig. 1) in the chambers of the heart, liver, pericardial sac, and body cavity. Death of the fish was attributed to hemorrhaging caused by the migrating larvae. Some larvae appeared to be quiescent and were surrounded with sparse connective tissue and very little white cell infiltration. There was some tissue necrosis adjacent to some of the worms, and the livers were "fatty" with foam fat and sparse fat vacuoles. We do not believe that either of these conditions contributed to the mortalities. No other disease entities could be found. Proteocephalid larvae were present in the intestine, but there was no evidence that they contributed to the mortalities.