Streptococcus dysgalactiae: A Pathogen of Feral Populations of Silver Carp from a Fish Kill Event

In August 2018, a series of large fish kills involving only Silver Carp Hypophthalmichthys molitrix occurred on the Mississippi River in northern Louisiana. Clinical signs observed in moribund animals included erratic swimming behavior, such as spiraling and spinning at the surface. A moribund specimen was captured by dip net near the surface at Lake Providence Landing in East Carroll Parish, northern Louisiana, and was submitted for analysis. An aseptic necropsy was performed, and diagnostic procedures, including bacteriology, parasitology, histopathology, virology, and electron microscopy, revealed that a gram-positive coccus was the primary pathogen. Pure cultures of the organism were obtained from the brain, and it was the predominant colony type isolated from the spleen, kidney, and liver. Bacterial sepsis caused by the gram-positive coccus and involving multiple organ systems was diagnosed histologically. Bacterial colonization and necrotic lesions were seen in the spleen, liver, kidney, heart, eye, and brain. Numerous cocci were observed dividing intracellularly in phagocytic cells of the kidney and brain by transmission electron microscopy. The organism was identified as Streptococcus dysgalactiae ssp. dysgalactiae by conventional biochemical methods and subsequently by the API 20 Strep system. The identity of the pathogen was later confirmed by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and sequencing of the 16S ribosomal RNA gene. Multilocus sequence analysis clustered this isolate along with two other S. dysgalactiae isolates from fish in a divergent phyletic group that was separate from other S. dysgalactiae ssp. dysgalactiae isolates from terrestrial animals, implying a possible novel clade that is pathogenic for fish.