Isolation and characterization of <i>Nocardia seriolae</i> , a causative agent of systematic granuloma in cultured East Asian four finger threadfin, <i>Eleutheronema rhadinum,</i> and red snapper, <i>Lutjanus erythropterus</i>
Pei‐Chih Liao, Ming‐An Tsai, Ming She See, Pei‐Chi Wang, Shih‐Chu Chen
Abstract
Nocardia seriolae is a common pathogen that causes significant economic losses in cultured fish. An outbreak of this pathogen occurred in Taiwan in November 2018, causing cumulative mortality of 0.86% (300 out of 35,000) in a red snapper, Lutjanus erythropterus, population within 20 days. Another outbreak occurred in January 2019, causing cumulative mortality of 8.29% (2,900 out of 35,000) in an East Asian four finger threadfin, Eleutheronema rhadinum, population within 50 days. The most significant gross pathological changes were white nodules, of varying size, in the spleen, kidney and liver. Additionally, ulcers and skin lesions were found in some East Asian four finger threadfin individuals. Identification of microorganisms isolated from the internal organs was verified using 16S rDNA gene analysis and showed 100% sequence identity with N. seriolae (GenBank accession number AF254417 and AY846841). The antidrug profiles of two N. seriolae isolates showed sensitivity to oxytetracycline, doxycycline, lincomycin, erythromycin and florfenicol. Experimental infection with the isolated strains showed highly similar disease symptoms to naturally infected fish for both N. seriolae strains and caused 100% mortality in red snapper within 13 days. The results revealed that N. seriolae isolated from the field was pathogenic to both East Asian four finger threadfin and red snapper.