Ultrastructure and sequential development of infectious pancreatic necrosis virus.

The morphology and sequential development of infectious pancreatic necrosis (IPN) virus, a pathogen of trouts, were studied by electron microscopy. Mature virions were seen in the cytoplasm of infected cells incubated at 24 C as early as 6 hr after infection. These virions were hexagonal in profile and approximately 55 nm in diameter. Generally between 8 to 10 hr after infection, virus crystals of various sizes were occasionally observed. Although virus replication did not appear to be confined to a particular cytoplasmic locus, mature virions were sometimes seen in association with unidentified tubular structures approximately 45 nm in outside diameter. Negative stains of virus revealed unenveloped icosahedra approximately 65 nm in diameter with probably 92 capsomeres. Contrary to a previous communication which reported IPN virus to have picornavirus-like morphology, we found it to morphologically resemble members of the reovirus group.