INTENSE INFECTIONS WITH A VARIANT OF MYXOBOLUS-PROCERUS (MYXOSPOREA) IN MUSCLE OF TROUT-PERCH (PERCOPSIS-OMISCOMAYCUS) IN DULUTH HARBOR, LAKE-SUPERIOR

Intense infections of a variant of Myxobolus procerus (Kudo, 1934) are described from trout-perch (Percopsis omiscomaycus (Walbaum)) collect ed in Duluth Harbor, Lake Superior, USA. This particular population of parasites has spores that are identical in shape (narrow pyriform) to those described for M. procerus except that they are significantly sm aller (13-14.5 mu m long versus 15-17 mu m long). In contrast to what was originally described for M. procerus, the plasmodia develop primar ily within red and white striated muscle fibres and only rarely among the subdermal connective tissue. Most plasmodia were at or near the sa me stage of development. Typical development involves growth within th e fibre. The parasite eventually replaces the entire content of the ho st cell and appears to halt development before rupturing the outer cel l membrane. The only obvious host response was an occasional cyst bein g invaded by a localized cellular infiltrate. Infected fish appeared o f normal health and no grossly evident myoliquefaction was seen. The i nfections involved several hundred plasmodia per fish and the question of why such unusually high levels of infection would develop in hosts inhabiting a polluted habitat is raised. It is suggested that prolife ration of a pollution tolerant oligochaete (the suspected alternate ho st) in the harbour and/or a compromised host immune system may have in creased the probability of successful transmission and development in trout-perch living in the harbour.