White-gill disease emerged among cage-cultured in Zhoushan in 2017. Systematic etiological methods such as polymerase chain reaction, histopathological sections, and electron microscopy in sick fish with clinical symptoms were performed to characterize the pathogen. Histopathological results showed that tissue necrosis was present in the spleen, liver, and kidney of sick fish. Moreover, the erythrocytes in the tissues underwent obvious degenerative changes, and a decreased number of red blood cells was observed in the blood. Ultrathin sections used in the electron microscopy showed the presence of microspore-like shapes with diameters from 300 to 600 nanometre. Total DNA extracted from the pathological tissue was used to amplify the 18S ribosomal RNA gene (SSU rDNA). One specific gene was identified to be homologous to that in sp. with 89% amino acid sequence homology. Our study results showed that the recent pandemic of white-gill disease that affected , was different from the previous white-gill disease. A specific and sensitive detection of the pathogen was developed by using nested PCR, which only requires a minimum amount of 0.5 pg of template DNA and produces 10⁴ more copies than the number of copies produced in single PCR amplifications.