28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 Pathology Associated With A Severe Infestation Of Tetrahymena sp. In Cultured Hybrid Striped Bass, Morone saxatilis X M. chrysops Stephen A. Smith, Kathleen P. Hughes and David. J. Pasnik Aquatic Medicine Laboratory, Department of Biomedical Sciences and Pathobiology, Virginia-Maryland Regional College of Veterinary Medicine, Virginia Polytechnic Institute and State University, Blacksburg, VA, 24061 A population of commercially cultured juvenile hybrid striped bass, Morone saxatilis x M. chrysops, experienced a gradual onset of morbidity and mortality.  Clinical signs included excessive skin mucus production, fin and tail erosion, petechial hemorrhage and focal ulcerations of the skin.  Biopsies from areas of skin appearing normal revealed a moderate infestation of a small pyriform protozoan with uniform ciliation identified as belonging to the genus Tetrahymena.  Skin scrapes taken directly from the ulcerated skin lesions revealed a heavy infestation of the ciliated protozoan, as well as, numerous bacterial and fungal elements.  Bacterial cultures from the ulcerated skin lesions grew Aeromonas hydrophila, A. sobria and Shewanella putrefaciens, while cultures from the posterior kidney were negative for any bacterial growth.  Histopathologic evaluation revealed moderate to severe epithelial hyperplasia and a severe localized cellular necrosis of the skin.  Large numbers of the parasite were found invading the epithelium of the skin, beneath scales, within the dermis, and migrating down facial sheaths surrounding muscle bundles.  A moderate to severe infiltration of inflammatory cells and occasionally an accumulation of bacteria accompanied these parasites.  The parasite had also penetrated the tissues of the nasal cavity and the retrobulbar fat surrounding the optic nerve of the eye.  Additionally, the parasite had successfully invaded neural sheaths of nerve fibers, the meninges of the brain, and in one instance, completely encircled the spinal cord.  Further examination revealed that the ciliated protozoan had also penetrated the visceral organs including the spleen, liver, gonads and vessels surrounding the heart of the fish.  A few of the parasites were also observed free in the peritoneal cavity accompanied by a significant peritonitis. Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page