Blue light-emitting diode photoinactivation inhibits edwardsiellosis in fancy carp (Cyprinus carpio)

Although the use of blue light (400-500 nm) to reduce bacterial pathogens in fish farms has various advantages over chemical microbicides, there has been little research on applications of blue light in aquatic organism disease management. The aims of this study were to verify the bactericidal effects of light conditions (light spectrum and intensity) on Edwardsiella piscicida demonstrate the efficacy of blue light irradiation in reducing edwardsiellosis in fancy carp, and analyze the potential harmful effects of blue light on carp. E. piscicida at a concentration of 10(5) CFU ml(-1) were exposed to 405 or 465 nm light for an exposure time estimated to inactivate 99% of the bacteria. In vivo efficacy test of blue light emitting diodes (LEDs) was done using a cohabitation challenge in which infection rate and viable E. piscicida count in the rearing water were monitored. The potential harmful effects of the light conditions were investigated by observing histopathological changes in eye tissue, and gene expression of heat shock protein 70 and corticosteroid 11-beta-dehydrogenase isozyme 2 in kidney tissue. The percentage of E. piscicida inactivation resulting from various intensities of blue LED wavelengths demonstrated a strong correlation between light intensity and irradiation time. Furthermore, blue LED irradiation decreased the number of E. piscicida in rearing water as well as the proportion of infected fish. Histopathological examination revealed melanin granules (rod outer segment) and photoreceptors were temporarily thickened in retinas, but there were no significant differences between control and light irradiation groups after 28 days of exposure. This study demonstrates that blue light irradiation is capable of inactivating E. piscicida and reducing its infection of fancy carp without causing adverse side-effects. Significance of this study: This study provides data on photoinactivation of blue LED light on the fish pathogen Edwardsiella piscicida, and the optimal conditions of light intensity and exposure time for inactivation. We also demonstrate that appropriate blue LED exposure reduced viable E. piscicida in rearing water and infection rates without causing adverse side-effects. The use of high-intensity narrow-spectrum blue LED light, without the addition of a photosensitizer, has great potential as an eco-friendly option for treating bacterial diseases in aquatic animals, and will help to reduce the use of antibiotics.