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

Abstract

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.

Coinfection by Group C Streptococcus dysgalactiae subsp. equisimilis and Flavobacterium davisii in Nile Tilapia from the United States

“…equisimilis isolated from animals. The MLSA also supports the existence of fish-specific clades within the subspecies (Porcellato et al 2021;Hawke et al 2021). Further investigation, including incorporation of additional isolates into MLSA and virulence testing, complemented with whole genome phylogenetic analysis will better define these genetic groups and their importance in fish disease.…”

“…Each housekeeping gene fragment was aligned by MUSCLE with default settings in Geneious Prime 2022.0.2 to corresponding sequences from the fish-associated S. dysgalactiae strain STC3 (Heckman et al 2020; accession numbers MN955921.1, MN955970.1, MN956019.1, MN956068.1, MN956117.1, MN956166.1, MN956215.1, MN956264.1, and MN956313.1) and trimmed to the shared sequence length. A representative trimmed sequence of each housekeeping gene was used in BLAST searches of a localised database populated by the 44 S. dysgalactiae reference strains used previously for S. dysgalactiae MLSA by Hawke et al (2021). The silver carp (Hypophthalmichthys molitrix) strain LADL18-050A was also included in analysis (Hawke et al 2021).…”

“…A representative trimmed sequence of each housekeeping gene was used in BLAST searches of a localised database populated by the 44 S. dysgalactiae reference strains used previously for S. dysgalactiae MLSA by Hawke et al (2021). The silver carp (Hypophthalmichthys molitrix) strain LADL18-050A was also included in analysis (Hawke et al 2021). Sequences for each gene were concatenated alphabetically for each strain (arcC-glnA-groEL-gyrB-mutS-pheT-prkC-rpoB-tkt).…”

“…However, additional species of streptococci are becoming pathogens of concern in wild and cultured fish populations. While originally believed to be restricted to saltwater fish hosts (Nomoto et al 2006), Group C S. dysgalactiae is an emerging pathogen in both marine (e.g., Japanese yellowtail, Seriola quinqueradiata; amberjack, Seriola dumerili; pompano Trachinotus blochii) and freshwater (e.g., tilapia, O. niloticus; grass carp, Ctenopharyngodon idella; crucian carp, Carassius carassius) species (Costa et al 2013;Hawke et al 2021;Maekawa et al 2020;Netto, Leal, and Figueiredo 2011)). Previous studies on the genetic diversity of geographically distinct alpha hemolytic S. dysgalactiae isolates from tilapia and other fish demonstrate these fish-associated strains form discrete clades, indicating host specificity (Abdelsalam, Eissa, and Chen 2015;Hawke et al 2021).…”

Phylogenetic analysis and accessory genome diversity reveal insight into the evolutionary history of Streptococcus dysgalactiae

“…The subspecies SDSD has been considered strictly an animal pathogen, and it is commonly associated with bovine mastitis ( Rato et al, 2013 ) and infectious arthritis in sheep ( Smistad et al, 2021 ) and septicemia in vampire bats ( Mioni et al, 2018 ). SDSD has been considered an important pathogen in aquaculture systems worldwide ( Maekawa et al, 2020 ; Hawke et al, 2021 ). Phylogenetic studies based on the sodA gene revealed that isolates from human blood cultures are closely related to SDSD from the fish host, suggesting that SDSD represents an important potential causative agent of zoonoses ( Koh et al, 2009 , 2020 ).…”