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

Departments of Veterinary Pathology (SM,¹ LMJ, FAL, ES)and of Biology (DPC), University of Saskatchewan Saskatoon, SK, Canada Canadian Cooperative Wildlife Health Center, Headquarters Office, Saskatoon, SK, Canada (FAL), Québec Regional Center, Département de Pathologie et Microbiologie, Université de Montréal, St-Hyacinthe, QC, Canada (ADD, CU), Ministère des Ressources Naturelles et de la Faune du Québec, Québec, QC, Canada (MM), Aquatic Diagnostic Services, Atlantic Veterinary College, University of Prince Edward Island, Charlottetown, PE, Canada (DG), Direction des Laboratoires d'Expertises et d'Analyses Alimentaires du Ministère de l'Agriculture, des Pêcheries et de l'Alimentation du Québec, Sainte-Foy, QC, Canada (JPR, GP), Institute of Aquaculture, University of Stirling, Stirling, Scotland, UK (HWF)

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 <0.22 µm) inocula prepared from a homogenate of tissues from carp affected by the natural outbreak. Although significant clinical disease or mortality was not induced by experimental challenge, lymphocytic encephalitis similar to the one observed in naturally affected carp was induced in four of seven (57%) fish administered crude inoculum and four of seven (57%) fish administered filtered inoculum. None of the control fish inoculated with sterile phosphate-buffered saline (n = 6) were affected by encephalitis. The cause of the encephalitis observed in carp from the natural outbreak and in experimentally inoculated fish could not be determined by use of virus isolation and transmission electron microscopy.

Key words: Aeromonas hydrophila; carp; Cyprinus carpio; encephalitis; Flavobacterium columnare; massive mortality; septicemia.

Request reprints from Dr. S Monette, Department of Pathology, The Animal Medical Center, 510 East 62nd Street, New York, NY, 10021-8314 (USA). E-mail: sebastien.monette{at}amcny.org