28th ANNUAL EASTERN FISH HEALTH WORKSHOP April 21-25, 2003 A Bad Case Of Zebras Jeffrey C. Wolf Registry of Tumors in Lower Animals, Experimental Pathology Laboratories, 22866 Shaw Rd., Sterling, VA  20166 USA A broad spectrum of fishes are used in toxicological research.  At our facility, we routinely evaluate small aquarium fish from laboratory experiments, food or game fish from hatcheries, and feral fish harvested during surveys.   Whereas the wild-caught fishes are typically “parasite factories”, internal parasitism is rare or non-existent among the aquarium fishes that we examine most often (Japanese medaka Oryzias latipes and fathead minnow Pimephales promelas).  Therefore, I was surprised to find the number and variety of parasites in histological sections of several zebra danios (zebrafish) Danio rerio that were used in an active research project. This project was conducted by the research arm of a large private company that does not specialize in the use of fish models.  A set of glass microscope slides was received for each of four fish.  Parasites evident among the examined sections from all four fish included: microsporidia (consistent with Pseudoloma neuophilia) in the spinal cord, brainstem and/or paraspinal ganglia; nematodes (consistent with Pseudocapillaria tomentosa) in the intestinal mucosa and intestinal lumen; and nematodes (possibly also P. tomentosa) in non-enteric tissues.  Single encysted helminth larvae (presumptive digeneans) were observed in three of the four fish.  The intramural enteric nematodes were associated with hyperplasia of the intestinal mucosa, and mononuclear cell and/or granulomatous enteritis.  Malignant neoplasia was surprisingly present in three of the zebrafish, which consisted of adenocarcinomas of the intestine (two fish) and a bile duct carcinoma (cholangiocarcinoma) of the liver (one fish).  Because these fish had been purchased at a pet shop, I surmised that the advanced degree of internal parasitism was consistent with their origin in farm ponds of Florida or Southeast Asia.  This assumption was incorrect.  Infestations of P. tomentosa and P. neurophilia are also endemic problems in some of the zebrafish facilities that supply stock intended for research.  Other workers have postulated a potential relationship between the presence of P. tomentosa and intestinal tumors in zebrafish. Glass slides from these zebrafish have been accessioned into the Registry of Tumors in Lower Animals collection (RTLA Accession Nos. 7480, 7481, and 7482).  Return to 28th Annual Eastern Fish Health Workshop Return to Leetown Science Center Home Page