Outbreak of vibriosis caused by Vibrio harveyi and Vibrio alginolyticus in farmed seahorse Hippocampus kuda in China

An outbreak of vibriosis characterized by serious ulcers in the skin was reported in cultured seahorses of eastern China in July 2017. Two representative bacterial strains, HM -12 and HM -14, were isolated from the skin, liver and kidney tissues of the moribund seahorses. Both strains were identified as Gram-negative, curved rod-shaped or rod-shaped bacteria with single polar flagellum by gram staining and transmission electron microscopy analyses, respectively. Based on the 16S rRNA sequencing and MLST analysis derived from five housekeeping genes ftsZ, gapA, gyrB, mreB, topA, HM-12 strain was identified as Vibrio harveyi, while HM-14 was confirmed as V. alginolyticus. The experimental infections were conducted by intraperitoneal injection with both isolates into healthy seahorses, and the results showed that both isolates were lethal to H. kuda with the LD(50 )value of 6.067 x 10(3) CFU/g (body weight) and 2.799 x 10(6) CFU/g (body weight), respectively. The infected seahorses showed similar clinical symptoms with those naturally infected. Furthermore, histological analysis of infected seahorses with both strains showed shrunken glomeruli and renal tubules, focal necrosis, aggregated melanomacrophages in kidney; increased vacuolation, hepatocytes disruption and hepatic necrosis in liver; disrupted mucus gland and architecture, condensed basophilic nuclei in V. harveyi infected skin and destruction of the epidermis and disorganized condensed inflammatory cells in V. alginolyticus infected skin. Antimicrobial susceptibility test revealed that V. harveyi and V. alginolyticus were highly sensitive to doxycycline and tetracycline. This study underlines the involvement of V. harveyi and V. alginolyticus in the disease outbreak of farmed seahorse aquaculture and highlights the importance of developing early diagnostics and appropriate prevention strategies to reduce the damage caused by Vibrio spp. in seahorse aquaculture setting of eastern China.