LAUSR

Abstract Although previous studies have shown that blue light emitting-diode (LED) light has photoinactivation activity against bacteria, little is known about its activity against parasites. The objective of this study was to determine the antiprotozoal activity of LED light at 405 and 465 nm and their in vivo efficacies against scuticociliatosis caused by Miamiensis avidus in olive flounder. Briefly, 105 cells ml−1 of M. avidus were exposed to 405 nm, 465 nm, 520 nm, or 640 nm LED to inactivate ciliates. These ciliates were then observed after trypan blue staining, or double staining with propidium iodide (PI) and Annexin V followed by flow cytometry. Additionally, in vivo experiments were performed twice to assess the efficacy of 405 nm LED against M. avidus after olive flounder were challenged with M. avidus by immersion method. Possible harmful effects of blue LED illumination on fish were also observed. LED irradiation at 405 nm and 465 nm, but not at 520 nm or 640 nm, inactivated half of M. avidus population within 6–12 h and 24–28 h, respectively. Flow cytometry analysis revealed that the distribution of 405 nm-exposed ciliates double stained with PI and Annexin V was gradually changed from Annexin-/PI- (live cell) to Annexin+/PI− (early apoptotic cell) section. It was then further moved to Annexin+ - PI+ (late apoptosis or necrosis) section. Relative percent survival (RPS) of fish infected with the scuticociliate was 0% in group exposed to ambient light. It was 60–70% in the group of fish exposed to LED at 405 nm and 30% in the group exposed to LED at 465 nm. To the best of our knowledge, this is first study demonstrating that M. avidus could be killed by LED light illumination due to induction of apoptosis. There were no significant histopathological changes in eyes of fish between control and irradiance with LED at 405 nm, except for the dorsal retina region of 405 nm exposed fish sampled at 14 days. There were no significant differences in plasma cortisol level or expression level of stress-related and biorhythm-related genes (HSP70, melatonin-3, caspase-3) in the kidney. Our results provide insights into possible application of blue LED for treating scuticociliatosis, the most devastating disease in olive flounder farming industry. Significance of this study This is the first study to demonstrate that blue LED light irradiation can inhibit a parasitic pathogen, Miamiensis avidus, by inducing apoptosis. Also, photoinactivation by 405 and 465 nm LED can reduce mortality of live flounder due to scuticociliatosis using in vivo experiments without causing any severe side effects or inducing stress responses. Data obtained in this study indicate that such light therapy may have tremendous benefit, although further research is needed before applying such therapy for intensive fish farming and commercial aquarium facility in the future.