Vaccination of fish against Aeromonas hydrophila infections using the novel approach of transcutaneous immunization with dissolving microneedle patches in aquaculture

The aim of this study was to develop and evaluate a novel route of administration for vaccinating fish against Aeromonas hydrophila infection using a dissolving microneedles (MNs) patch. The A. hydrophila JUNAH strain was inactivated with formalin and used as a vaccine antigen. It was mixed with dissolvable carboxymethyl cellulose (CMC) as the matrix material to produce the MNs patches. When examined with a scanning electron microscope, each patch has 282 uniformly distributed, pyramid-shaped needles on a circular base. In the skin insertion experiment, the MNs patches were confirmed to be capable of penetrating the skin of the fish. Through agglutination assay and analysis of non-specific parameters like lysozyme and superoxide dismutase, it was verified that the antigen embedded into the patch induced adaptive and innate immune responses in the fish. In the challenge experiment, the group inoculated with the MNs patch and the group injected with formalin killed cells (FKC) showed a similar survival rate. Our results suggest that the FKC-loaded MNs patch is a wholly viable method alternative to injection for the vaccination of fish.

Enhanced bath immersion vaccination through microbubble treatment in the cyprinid loach

Immunization by bath immersion is likely the simplest method of fish vaccination. Although the route of immunogenicity has not been fully identified, immersion vaccination is clearly a useful labor-saving technique. In this study, microbubble (MB) treatment was assessed for its ability to improve the efficacy of bath immersion vaccination in the cyprinid loach. MBs are commonly defined as minute particles of gas with a diameter of less than 100 μm, which generated free radicals. Here, the efficacy of MB treatment for vaccination enhancement in the cyprinid loach was assessed in direct challenge experiments using the virulent Aeromonas hydrophila JUNAH strain; assessments comprised agglutination titer assay and non-specific parameter analysis. Agglutination titers were high in loaches that were immunized via injection with inactivated cells (FKC group); however, non-specific immune activation parameters (e.g., lysozyme, superoxide dismutase, and phagocytic activity) were more increased in loaches that were immunized via bath immersion with MB treatment. Moreover, MB-treated loaches showed comparable survival rates, relative to loaches immunized via injection with formalin inactivated cells. Thus, higher levels of non-specific immune parameters suggest increased efficacy of this vaccine approach. Improving the effectiveness of bath immersion vaccine will increase its affordability and ease of application in aquaculture.

Morphometric and Histological Changes in Cyprinid Loach, Misgurnus anguillicaudatus, in the Early Growth Period

In this study, we measured the morphometric and histological changes in the cyprinid loach, Misgurnus anguillicaudatus, during the early period of growth. Eyes, yolk length, yolk height, and yolk volume of the larva decreased for 16 days post hatching (DPH) (P<0.05). During 60 DPH (P>0.05), the most anterior extension of the head × the posterior end of the supraoccipital, the most anterior extension of the head × the origin of the dorsal fin, the most anterior extension of the head × the origin of the pectoral fin, the posterior end of the supraoccipital × the origin of the pelvic fin, and the origin of the dorsal fin × the ventral origin of the caudal fin gradually decreased, whereas the most anterior extension of the head × the dorsal origin of the caudal fin, the origin of the dorsal fin × the origin of the anal fin, the origin of the dorsal fin × the origin of the pectoral fin, and the insertion of the dorsal fin × the origin of the pelvic fin gradually increased (P<0.05). In the cyprinid loach, the retina is composed of six layers: the epithelial layer, ganglion cell layer, inner nuclear layer, inner plexiform layer, outer limiting membrane, and rod and cone layer (RCL). After hatching, part of the RCL gradually increased in density. The kidney and midgut epithelium were already formed in the cyprinid loach just after hatching and grew gradually in subsequent days.