A Herpesvirus Associated with Mass Mortality of Juvenile and Adult Koi, a Strain of Common Carp

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Abstract
A herpesvirus was isolated from adult koi, a strain of common carp Cyprinus carpio, suffering mass mortality in two outbreaks—one in the mid-Atlantic region of the United States and the second in Israel. The principal external signs of dying fish were pale and irregularly colored gills. There were few consistent internal signs in either outbreak. The most prominent microscopic lesions were in the gills, where hyperplasia and necrosis of the epithelium were severe. Other lesions included interstitial nephritis, splenitis, and enteritis. Affected cells often contained nuclei with marginated chromatin and faint intranuclear inclusions. Typical herpesvirus particles were present in branchial epithelial cells, hepatocytes, and among circulating leukocytes. Inoculations of the koi fin (KF-1) cell line with tissue extracts from the gill and kidney–spleen resulted in cytopathic effects characterized by severe vacuolation first detected after 7 d incubation at 20°C. Exposures of adult koi to the herpesvir...