Isolation and some properties of an iridovirus-like agent from white sturgeon Acipenser transmontanus

Abstract

ABSTRACT: An iridovirus-like agent (WSIV) associated with fatal infections of the integument of juve- nile white sturgeon Acipenser transrnontanus was isolated from fish that exhibited gross and micro- scopic signs of the disease. The virus induced cell enlargement and slowly progressive degeneration of a recently established cell line from white sturgeon spleen (WSS-2). Virus replication occurred at 10, 15 and 20 OC but not at 5 or 25 OC. The most rapid growth occurred initially at 20 OC but the greatest con- centrations (106 TCIDSO ml-') of cell-free virus were detected at 10 and 15 "C. Numerous virions were observed in the cytoplasn~ of infected WSS-2 cells and ca 70 % of the infectious virus remains cell- associated. Virions possessed an external capsid with an envelope of 260 to 280 nm in diameter that surrounded an inner capsid with a dense nucleoid. Residual viral infectivity (ca 1 %) was found follow- ing incubation of the virus at a temperature of 56 "C for 30 min. The WSIV genome is presumably DNA since 50 pg ml-' 5-bromo-2'-deoxyuridine completely inhibited viral replication in the WSS-2 line. Virulence for juvenile sturgeon of virus grown in WSS-2 cells was demonstrated by induction of fatal infections (80 % cumulative mortality) following bath exposures to the virus at concentrations of ca 103 TCIDSO g-' fish for 30 min. There were no mortalities among lake sturgeon Acipenser fulvescens, striped bass Morone saxatilis or channel catfish Ictalurus punctatus following bath exposures to the same concentrations of vir.us (TCID50 g-'1 but the agent was recovered from lake sturgeon examined at 1 and 2 wk post-exposure. Microscopic signs of experimentally-induced infections included: presence of large amphophilic to deeply basophilic cells; epithelia1 hyperplasia and degeneration; necrosis in the integument and gill epithelium of white surgeon. Lake sturgeon examined at 1 and 2 wk postinfection demonstrated a similar response, although the cellular hypertrophy was not prom~nent compared to that observed in white sturgeon. INTRODUCTION