Massive Mortality of Common Carp (Cyprinus carpio carpio) in the St. Lawrence River in 2001: Diagnostic Investigation and Experimental Induction of Lymphocytic Encephalitis

A massive fish kill affecting exclusively common carp (Cyprinus carpio carpio) in the St. Lawrence River, Québec, Canada, during the summer of 2001 was investigated by use of laboratory diagnostic methods and by an attempt to experimentally induce the disease. The ultimate causes of mortality were opportunistic bacterial infections with Aeromonas hydrophila and Flavobacterium sp. secondary to immunosuppression induced by physiologic (i.e., spawning) and environmental (i.e., high temperatures and low water levels) stressors, and possibly enhanced by an infection causing lymphocytic encephalitis observed in 9 of 18 (50%) fish examined. Experimental induction of disease was attempted in captured wild carp by administration of crude and filtered (particulate