Pacific oyster nocardiosis: Isolation of the bacterium and induction of laboratory infections

A recently isolated bacterium is associated with and augments mortalities among adult Pacific oysters, Crassostrea gigas, reared in warm, eutrophic embayments in Washington state and British Columbia, Canada. The bacterium was isolated from both hemolymph and adductor muscle pustules from infected animals. The actinomycete-like bacterium stained Gram-positive, acid-fast, and periodic acid-Schiff base-positive. The presence of catalase, a trilaminar cell wall, and common antigens with Nocardia spp. indicates that the bacterium belongs in this genus. Experimentally induced infections of oysters had gross and microscopic signs and mortality levels similar to those observed in natural infections with the bacterium. Concentrations of 1.44 × 10⁵ cfu/ml injected into the heart of adult Pacific oysters maintained in full-strength sea water at 21°C resulted in a 47% mortality within 34 days.