Mycobacterium chelonae is an ubiquitous environmental acid-fast bacteria that causes disease in a wide variety of teleost species.3 Of the numerous species held in captivity, fish of the Family Syngnathidae, which includes seadragons, seahorses, and pipefish, hold high educational and economic value.4 Furthermore, Syngnathidae are uniquely sensitive to infection and subsequent mortality by Mycobacterium spp.5 At present the microscopic lesions associated with Mycobacterium spp. infection in seadragons have been sporadically described in the literature; however, thorough characterization of the Mycobacterium spp.infections in Syngnathidae have yet to be completely defined.2 In addition, strain variation within bacterial species affecting these animals has not been determined.
In this study, we histologically characterized the unique lesions observed in clinically ill Syngnathidae that were culture positive for Mycobacterium spp. Mycobacterium spp. isolated from these animals were further characterized to the species level by 16S rRNA. The hsp65 protein was subsequently used to evaluate variation among M. chelonae isolates cultured from diseased Syngnathidae and were compared to M. chelonae isolates from diseased non-Syngnathidae teleosts.
From November 2006–March 2013, over 2500 fish were submitted through the University of Georgia Aquatic Pathology Service for necropsy, histopathology, and microbiology. The most prevalent disease process diagnosed from these fish was mycobacteriosis. A subset of fish submitted were of the Family Syngnathidae and represent a total of 211 animals. Thirty-six percent of these Syngnathidae were diagnosed with Mycobacterium spp. by histopathology, special stains, culture, and/or PCR identification. 16S rRNA of 25 of these isolates showed 84% identified as M. chelonae, 8% as Mycobacterium spp., and 8% as M. marinum. Lesions elicited by M. chelonae were observed in many tissues of all Syngnathidae. Most often these lesions were composed of coagulative necrosis of the affected organ, thrombi of free bacteria or large nodular aggregates or sheets of macrophages that have cytoplasm laden with numerous acid-fast bacterial rods. In comparison, non-Syngnathidae fish diagnosed with M. chelonae had lesions in the same organs; however, the majority of lesions were composed of discrete granulomas in various stages of development ranging from solid nodules composed solely of plump epithelioid macrophages to chronic structures with fibrous walls and caseous centers.
M. chelonae isolates affecting Syngnathidae and non-Syngnathidae fish were further characterized by molecular analysis with hsp65 sequencing. Results of these analyses show that 15/17 Syngnathidae isolates had the identical genetic sequence at the hsp65 locus and are also identical to 5/11 non-Syngnathidae isolates. The results of the present study suggest that the many M. chelonae isolates causing disease in both Syngnathidae and non-Syngnathidae fish are genetically similar. Although, one gene is not enough to determine genetic diversity, the difference in histologic presentation between fish species in this study appears to support that Syngnathidae have a different host immune response to M. chelonae when compared to non-Syngnathidae.
The authors would like to thank the Athens Veterinary Diagnostic Laboratory at the University of Georgia College of Veterinary Medicine for their technical support for this project.
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1. Adékambi T, Drancourt M. Dissection of phylogenetic relationships among 19 rapidly growing Mycobacterium species by 16S rRNA, hsp65, sodA, recA and rpoB gene sequencing. Int J Syst Evol Micr. 2004;54(6):2095–2105.
2. Bonar CJ, Garner MM, Weber ES, Keller CJ, Murray M, Adams LM, Frasca S. Pathologic findings in weedy (Phyllopteryx taeniolatus) and leafy (Phycodurus eques) seadragons. Vet Path Online. 2013;50(3):368–376.
3. Decostere A, Hermans K, Haesebrouck F. Piscine mycobacteriosis: a literature review covering the agent and the disease it causes in fish and humans. Vet Microbiol. 2004;99:159–166.
4. Lourie SA, Vincent ACJ, Hall HJ. Seahorses: An Identification Guide to the World's Species and Their Conversation. London: Project Seahorse; 1999.
5. Reavill DR, Schmidt RE. Mycobacterial lesions in fish, amphibians, reptiles, rodents, lagomorphs, and ferrets with reference to animal models. Vet Clin North Am Exot Anim Pract. 2012;15(1):25–28.