Research on epidermis ulcer syndrome caused by Vibrio harveyi in Hippocampus erectus

Hippocampus spp. is unique marine fish with special body morphology and life history traits. Compared with other marine fishes, the most special part of Hippocampus spp. is male breeding and strict monogamy. In addition, its unique body morphology, including curvaceous torso and horse-shaped head, has given them higher ornamental value. The trade volume of Hippocampus spp. is huge in the world, and the majority of the trade in Hippocampus spp. is for traditional Chinese medicine. In recent years, the destruction of wild habitats and overfishing lead to a sharp decline of the wild populations. Aquaculture of Hippocampus spp. has been considered as an effective way to solve the problem of decline of the wild populations and the unsustainable trade for traditional Chinese medicine. Therefore, the development of the Hippocampus spp. aquaculture is essential. However, the threat of various diseases has seriously affected the healthy development of the aquaculture industry. Since 2017, in the breeding process of the H. erectus in northern China, epidermis ulcer syndrome has frequently occurred, the morbidity is rapid and the mortalily is high. Epidermis ulcer syndrome has become a high incidence disease in the aquaculture process. Diseased H. erectus showed obvious symptoms, such as large rot area in body surface, darker and lackluster of the gills and liver, diseased H. erectus would die within 3 days. If the diseased H. erectus were not removed in time, a large number of seahorses would be infected. Although most fish diseases can be treated with antibiotics, the overuse of antibiotics may produce many potential threats, which are not conducive to the sustainable and healthy development of the aquaculture industry. Therefore, it is essential to further study the pathogens of various fish diseases and then we can develop biological control on this basis. Accordingly, the priority of research should be disease treatment and health management, especially disease treatment. Specific vaccines can be developed from isolated pathogenic bacteria, which provide effective way for disease treatment. In this study, dominant strains were isolated from the lesion tissue and intestine of H. erectus with epidermis ulcer syndrome. In order to determine the pathogenic bacteria, we carried out artificial infection, the infection results suggested that strain HDM-2 was the pathogen causing epidermis ulcer syndrome, and the LD₅₀ of strain HDM-2 was about 2.89×10⁸ CFU/mL. We identified strain HDM-2 with physiological and biochemical identification and molecular biological identification methods, and we observed strain HDM-2 under transmission electron microscope. 16S rDNA gene sequence analysis by BLAST in GenBank suggested that the similarity between strain HDM-2 and V. harveyi reached 100%. The observation results showed that HDM-2 was about 2.2 μm×4.75 μm, with peritrichous flagella and capsule. In order to study antibiotic susceptibility of strain HDM-2, we implemented the antibiotic sensitivity test by using Kirby-Bauerdiffusion method, and results showed that strain HDM-2 was highly sensitive to florfenicol. In order to study the pathogenicity of the pathogen to the host, we carried out histopathological observation of the diseased H. erectus. Observation results suggested that skin, liver and gills were severely damaged by HDM-2. This study would lay a foundation for the prevention and further study of the epidermis ulcer syndrome of H. erectus.