Erysipelothrix Causes Disease in Ornamental Fish

Erysipelothrix rhusiopathiae is a facultative anaerobe that causes disease in swine, poultry, and marine mammals. The zoonotic bacterium is also the etiologic agent of the human diseases “whale finger,” “seal finger,” and “pork finger.”

Erysipelothrix is persistent in aquatic environments and has been isolated from mucosal surfaces of several fish, reptile, mammal, and invertebrate species.

Researchers recently associated Erysipelothrix with multiple disease outbreaks in freshwater ornamental fish cultured in the southeastern United States. Their findings were presented in the Journal of Fish Diseases.

Disease Outbreaks

From 2012 to 2016, disease outbreaks at aquaculture facilities occurred in 8 tetra (family Characidae) and 2 barb (family Cyprinidae) species. Affected fish were lethargic and displayed ulcerative stomatitis and dermatitis. Low to moderate mortality was observed primarily at water temperatures ranging from 24 to 30ºC.

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Diagnostic workup of affected fish included necropsy, histology, and, in some cases, bacterial culture and antibiotic susceptibility testing of the brain and posterior kidney. Histology revealed gram-positive bacteria associated with necrotizing dermatitis, myositis, and cellulitis of the face. Bacteria were also commonly associated with necrosis and inflammation of the skeletal muscle, olfactory organ, hematopoietic tissues, and vasculature. Sterile cultures of brain and posterior kidney tissues grew bacterial colonies on tryptic soy agar supplemented with 5% sheeps blood at temperatures ranging from 25 to 37ºC. The growth was morphologically classified as non-motile, slender, gram-positive rods.

Genetic Analysis

Sequencing of the 16S rRNA gene determined that 16 fish isolates all were at least 99% similar to one another and to an E rhusiopathiae strain. However, analysis of the gyrB gene—which is proposed as a more suitable gene than 16S for bacterial taxonomy—revealed the fish isolates were genetically distinct from other aquatic and terrestrial Erysipelothrix members. Additionally, isotyping classified the ornamental fish isolates as the Erysipelothrix spaC isoform, while other terrestrial and marine mammal isolates all belonged to either spaA or spaB isoforms.

Challenge Study Confirms Disease

Intraperitoneal (IP) and bath challenges were performed in naïve cyprinid (zebrafish) and cichlid (Nile tilapia) species using one of the ornamental fish outbreak isolates. IP challenge was performed via coelomic injection of 10⁶-10⁷ CFU/fish, and bath challenge was performed in a 1-hour static suspension of 10⁷ CFU/ml water.

Ninety-percent and 10% of the zebrafish died within 10 days after IP and bath challenges, respectively. Contrastingly, 10% of Nile tilapia died after IP challenge only, suggesting that susceptibility to this bacterium varies among fish families. Histologic lesions, which were observed only in zebrafish after IP challenge, included early infection in the skin and skeletal muscle that progressed to systemic infection after 8 to 10 days.

Summary

This study indicates that Erysipelothrix is an emerging disease agent in the US freshwater ornamental fish industry. Molecular data suggest that the spaC isoform infecting fish is genetically distinct from existing E rhusiopathiae members and may represent a new species.

Dr. Stilwell received her DVM from Auburn University, followed by a MS in Fisheries and Aquatic Sciences and a PhD in Veterinary Medical Sciences from the University of Florida. She provides freelance medical writing and aquatic veterinary consulting services through her business, Seastar Communications and Consulting.