Acute mortality in laboratory medaka (Oryzias latipes)

A laboratory fish facility experienced an acute increase in mortality in 1-year-old medaka (Oryzias latipes) over a 2-month period.The facility housed approximately 400 medaka in 60 2-L tanks.Up to 10 fish/d were found dead or dying, and often a whole tank of fish was affected.No increase in morbidity or mortality was noted in younger generations of medaka.Fish had been spawned and reared for several generations in the room.The last importation of medaka was 6 months earlier, in November 2021.Medaka were housed on a recirculating aquaculture system of conditioned reverse osmosis water with carbon filters, UV sterilizers delivering 130,000 µWsec/cm 2 , and 10% system water changed daily.No aberrations in system parameters were detected (pH, 6.6 to 7.0; conductivity, 600 to 1000 µS; ammonia, 0 ppm; nitrite, 0 ppm; nitrate, 10 to 40 ppm).The room contained 2 aquatic housing systems-1 for medaka and 1 for zebrafish (Danio rerio).Approximately 200 zebrafish were housed in 25 2-L tanks.Both medaka and zebrafish were maintained on the same recirculating water and fed the same dry diet plus brine shrimp.Equipment and support staff were also shared.No morbidity or mortality was noted in the zebrafish.Ten medaka that appeared to be dying (lying on the bottom of the tank but still respiring) were euthanized, fixed in Dietrich fixative, and processed for histopathology. Clinical and Gross FindingsMedaka showed no discernible clinical signs before being found dead or dying.Possible ventral redness was noted upon inspection of the carcasses, but it was unclear whether this was a pre-or postmortem lesion.Formulate differential diagnoses, then continue reading.