Lactococcus garvieae infection in the giant freshwater prawn Macrobranchium rosenbergii confirmed by polymerase chain reaction and 16S rDNA sequencing

Abstract

An epizootic bacterial infection in the giant freshwater prawn Macrobranchium rosenbergii occurred in Taiwan from May to June 1999. The cumulative mortality was approximately 30 to 75%. The diseased prawns showed opaque and whitish muscles and were approximately 2 mo old with total lengths from 5 to 6 cm. Histopathologically, they showed marked edema and necrotic lesions with inflammation in the muscles and hepatopancreas. Bacteria isolated using brain heart infusion medium or tryptic soy agar were Gram-positive and ovoid. Three isolates from diseased prawns at different farms were tested using the API 20 Strepsystem and conventional tests and identified as Lactococcus garvieae. Experimental infections with these isolates gave gross signs and histopathological changes similar to those seen in the naturally infected prawns. The LD50 value of isolate MR1 was 6.6 x 10(5) colony forming units/prawn. Identification of MR1 was confirmed by a PCR assay for L. garvieae that gave the expected amplicon of 1100 bp. In addition, its 16S rDNA sequence (GenBank accession number AF283499) gave 99% sequence identity to Enterococcus seriolicida (synonym L. garvieae; GenBank accession number AF061005). This is the first report of confirmed L. garvieae infection in prawn aquaculture.