Location: Warmwater Aquaculture Research UnitTitle: Lactococcosis in silver carp Hypophthalmichthys molitrix Author
Submitted to: Journal of Aquatic Animal Health
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/17/2013
Publication Date: 3/30/2014
Citation: Khoo, L., Austin, F., Quiniou, S., Gaunt, P., Riecke, D., Jacobs, A., Meals, K., Dunn, A., Griffin, M. 2014. Lactococcosis in silver carp Hypophthalmichthys molitrix. Journal of Aquatic Animal Health. 26:(1)1-8. Interpretive Summary: Several fisk kill events occurred along the Mississippi river and its associated waterways early September 2011. One asian carp collected during those events was analyzed for diagnostic. Necropsy examination, bacterial culture, biochemical and molecular analysis of the pathogen revealed the infectious agent to be Lactococcus lactis. This is believed to be the first report of Lactococcus lactis infection in asian carp.
Technical Abstract: An adult silver carp (Hypophthalmichthys molitrix) was submitted to the Aquatic Research & Diagnostic Laboratory (ARDL) in Stoneville, MS, as part of an investigation of a fish kill event in September 2011, at the Tunica Cutoff, Tunica county, Mississippi. Gross lesions included a focally extensive skin lesion (~6 cm in diameter) with scale loss and gelatinous underlying muscles near the caudal peduncle on the left side and mild iridial hemorrhage in the left eye. Touch impressions of the musculoskeletal lesion revealed small cocci (~1µm in diameter) in pairs or chains within an inflammatory milieu. A pure Gram-positive cocci isolate was obtained from the brain while cultures of the kidney and muscle yielded multiple bacterial colony types including the Gram-positive cocci seen in the brain. This bacterium was characterized biochemically and identified as Lactococcus spp. Analysis of the near complete 16S small-subunit ribosomal DNA (SSU) (100% match, 1372/1372 bp) and DNA gyrase subunit B (gyrB) (99.7% match, 1779/1785 bp) gene sequences revealed the bacterium to be Lactococcus lactis subspecies lactis (SSU – 100% identity, 1372/1372 bp; gyrB – 99.7% identity, 1779/1785 bp). Comparatively, at the gyrB locus the case isolate shared less than 90% similarity to L. lactis subspecies cremoris (1599/1781 bp) and <80% similarity to L. garveiae (1409/1775 bp). Histopathological examination confirmed a severe meningoencephalitis, a moderate mononuclear myositis and a mild interstitial nephritis. We believe this represents the first report of a natural infection by this organism in silver carp.