Granulomatosis of cultured three-line grunt Parapristipoma trilineatum caused by an intracellular bacterium

Abstract

Mortalities with an unknown etiology occurred in three-line grunt Parapristipoma trilineatum, which had been imported from China or produced from parental fish imported from China, in fish aquaculture facilities around the Bungo Channel, Japan. The affected fish characteristically showed small white spots (0.2-1 mm in diameter) in the kidney and spleen. No known pathogen for three-line grunt was detected by conventional histological, microbiological and immunological diagnoses. The observations of histological specimens and tissue imprints suggested that the spots were granulomas caused by the intracellular infection with a coccoid gram-negative bacterium (0.5-1.0 μm in diameter). When healthy three-line grunt were injected intraperitoneally with the spleen homogenate of affected fish, they died from 5th day post-inoculation showing granulomas with the intracellular bacterium similar to that in naturally affected fish. These results indicate that the disease was caused by the intracellular bacterium.