An aquatic detective case report of suspected Spironucleosis in Gulf Coast walleye

Authors: Shelley, John; Lafrentz, Benjamin; Paulson, Matthew

Submitted to: Annual Eastern Fish Health Workshop
Publication Type: Abstract Only
Publication Acceptance Date: 3/15/2019
Publication Date: 4/1/2019
Citation: Shelley, J.P., Lafrentz, B.R., Paulson, M.D. 2019. An aquatic detective case report of suspected Spironucleosis in Gulf Coast walleye [abstract]. Annual Eastern Fish Health Workshop. p. 4.

Interpretive Summary:

Technical Abstract: The walleye (Sander vitreus) is a freshwater perciform that is a popular game fish throughout its native range of Canada and the United States. The walleye is commonly associated with northern locales; however, there is a genetically unique strain, the southern or Gulf Coast walleye, that is endemic to the upper portion of the Mobile River Basin in Alabama and Mississippi. The Gulf Coast walleye population is almost entirely sustained through hatchery stock enhancement programs due to the impoundment of rivers for navigation or hydroelectric power generation. In March of 2018 yolk-sac fry Gulf Coast walleye were transported to the USDA-ARS Aquatic Animal Health Research Unit in Auburn, AL from Private John Allen National Fish Hatchery in Tupelo, MS. The fish were acclimated to onsite environmental parameters and trained to accept commercial feed as the yolk-sac depleted. Three weeks post-arrival emaciated fish were noted to be prevalent. Failure to accept feed was ruled out because fish were observed to have food in their gastrointestinal tract and if the feed had been rejected a rapid increase in mortality would have been observed immediately after oil globule depletion. Additionally, the fish were negative for external parasites and pathogenic bacteria, though damaged fins were noted to be associated with tankmate aggression. After six weeks the fish were taken off flow-through conditions in the quarantine building and transferred to a recirculating aquaculture system. Over the following two months the fish were diagnosed with multiple cases of external protozoa and bacterial infections, but treatment and clearance of these pathogens did not eliminate the continued emaciation and mortality. In July 2018 a necropsy was performed on an emaciated moribund fish by the recently appointed onsite attending veterinarian which led to a suspected diagnosis of spironucleosis. The Gulf Coast walleye were treated with 6.6 ppm metronidazole baths once daily for 5 days. Subsequently the mortality dropped to near zero and the remaining fish showed improved body condition. After identification and clearance of the parasite it was discovered that Spironucleus spp. have never been reported in the literature from walleye. In October 2018 surveillance for Spironucleus spp. infestations resulted in only one positive fish out of fifty samples. Parasites and gastrointestinal tissues were preserved in 10% neutral buffered formalin and absolute ethanol. These samples are currently being used for morphological identification/histopathology and for molecular characterization to identify the Spironucleus spp. involved in this first report in walleye.