Epizootic mortality in the pilchard Sardinops sagax neopilchardus in Australia and New Zealand in 1995 .2. Identification of a herpesvirus within the gill epithelium

Mass mortalities among pilchards Sardinops sagax neopilchardus occurred around Australia and New Zealand from March to September 1995. The mortalities spread rapidly both with and against currents and usually affected fish longer than 11 cm. Examination showed mortalities to be associated with the presence of replicating herpesvirus in the gill epithelium; herpesviruses were not observed within equivalent cells of unaffected fish collected ahead of the mortality front. By negative contrast electron microscopy the virus was demonstrated to possess a 96 nm icosahedral capsid containing 162 capsomers. Many virions were enveloped and possessed surface projections. The nuclei of infected epithelial cells contained nucleoids, capsids and nucleocapsids which were released from the nucleus into the surrounding cytoplasm following degeneration of the nuclear membrane. Within the cytoplasm, the capsids and nucleocapsids acquired a tegument and subsequently were enveloped by passing into smooth-surfaced vesicles or by budding from the plasma membrane. Release of viruses from the cells was, in the main, associated with lysis of infected gill epithelial cells. The morphology of the virus and ultrastructure of infected cells suggest that this virus belongs to the family Herpesviridae. The presence of a replicating herpesvirus within the gill epithelium is discussed in context with the epizootic.