Megalocytivirus infection in cultured Nile tilapia Oreochromis niloticus.

Megalocytiviruses, such as infectious spleen and kidney necrosis virus (ISKNV), induce lethal systemic diseases in both ornamental and food fish species. In this study, we investigated an epizootic affecting Nile tilapia Oreochromis niloticus cultured in the US Midwest. Diseased fish displayed lethargy, gill pallor, and distension of the coelomic cavity due to ascites. Histopathological examination revealed a severe systemic abundance of intravascular megalocytes that were especially prominent in the gills, kidney, spleen, liver, and intestinal submucosa. Transmission electron microscopic examination revealed abundant intracytoplasmic polygonal virions consistent with iridovirus infection. Comparison of the full-length major capsid protein nucleotide sequences from a recent outbreak with a remarkably similar case that occurred at the same facility many years earlier revealed that both epizootics were caused by ISKNV. A comparison of this case with previous reports suggests that ISKNV may represent a greater threat to tilapia aquaculture than previously realized.

Polycystic lesions in the liver of the white sturgeon.

Polycystic lesions have been reported from the kidney and liver of many different fish species and are attributed to pollution and genetic anomalies. In June 2002, eggs and milt were collected from two female and five male white sturgeon Acipenser transmontanus below Bonneville Dam on the Columbia River. Fertilized eggs were hatched at the Abernathy Fish Technology Center in Longview, Washington, and 20,000 juveniles were raised for approximately 1 year before release into the wild. Just before release, 12,000 juveniles were scute-marked, during which it was noticed that about 30 fish had distended abdomens. Gross necropsy revealed a large, jellylike mass in the peritoneal cavity. Histopathological examination of four fish showed that this growth was a polycystic lesion of liver tissue. This is the first report of this type of lesion in white sturgeon and is presumed to be the product of a genetic anomaly.