Abstract
Syngnathidae (seahorses, seadragons and pipefish) suffer significant losses from non‐tuberculous mycobacteria. However, they produce markedly different lesions in response to the disease compared to other teleost species, notably infrequent granuloma formation. This study evaluated 270 syngnathid fish, from which 92 were diagnosed with mycobacteriosis by histopathology, culture or both. Microscopic lesions variably consisted of random foci of coagulative necrosis in multiple organs, containing high numbers of free bacteria and large aggregates or sheets of macrophages with cytoplasm laden with acid‐fast bacilli. Mycobacterial associated granulomas were identified in only six seahorses. Five fish had positive cultures with no observed microscopic changes. RNA ‐seq of the head kidney was performed to investigate the transcriptome of two infected and six non‐infected lined seahorses Hippocampus erectus . Assembled and annotated putative transcripts serve to enrich the database for this species, as well as provide baseline data for understanding the pathogenesis of mycobacteriosis in seahorses. Putative components of the innate immune system ( IL ‐1β, IL ‐6, TNF , NOS , Toll‐like receptor 1, MHC Class I, NF ‐κβ, transforming growth factor beta, MyD88) were identified in the RNA ‐seq data set. However, a homolog for a key component in the TH 1 adaptive immune response, interferon‐gamma, was not identified and may underlie the unique pathologic presentation.