Ultraviolet B irradiation affects resistance of rainbow trout (Oncorhynchus mykiss) against bacterium Yersinia ruckeri and trematode Diplostomum spathaceum.

Ultraviolet B (UVB) radiation is known to have various effects on the immune system of fish, but the effect on the actual disease resistance has remained largely unknown. Here we studied the effect of UVB on the resistance of rainbow trout (Oncorhynchus mykiss) against a bacterium Yersinia ruckeri, the causative agent of enteric red mouth disease, and a trematode parasite Diplostomum spathaceum, which causes cataracts in fish. The fish were exposed to UVB irradiation seven times in 14 days, and inoculated intraperitoneally with Y. ruckeri on day 5 after the first irradiation. On day 2 postinfection (p.i.), the number of viable bacteria in the kidney was lower in UVB-exposed than in unexposed fish. However, on day 8 p.i., UVB-irradiated fish had not been able to clear remaining Y. ruckeri effectively, and had a slightly higher bacterial load than controls. A similar, although not significant, effect was seen in the bacterial numbers in spleen. In the other experiment, fish were exposed to UVB for six consecutive days and then exposed to D. spathaceum. A significantly higher number of parasites was detected in the eyes of irradiated fish, indicating reduced resistance against the pathogen. Furthermore, UVB-irradiation altered the immunological and hematological parameters of fish, which also verified the immunomodulatory potential of UVB in the present study.

Different sensitivity of carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss) to the immunomodulatory effects of UVB irradiation.

In order to study the sensitivity of two fish species, carp (Cyprinus carpio) and rainbow trout (Oncorhynchus mykiss), to the immunomodulatory effects of ultraviolet B (UVB) radiation, the fish were exposed to a single UVB dose of 50, 250, 500 or 1,000 mJ cm(-2). These species represent different phylogenetic groups of fish, and they differ also in their behaviour inhabitating often dark and turbid (carp) or clear and transparent waters (salmonids). Immune responses were studied on day 1 post-irradiation. Unexposed fish, and fish exposed to radiation depleted of UV wavelengths served as controls. UVB irradiation markedly enhanced the blood respiratory burst and cytotoxic activity in carp, but in the head kidney these parameters were significantly suppressed. Rainbow trout respiratory burst was affected only after exposure with the highest dose of UVB. Lymphopenia and granulophilia were noted in both fish blood after exposure. This study indicates that UVB irradiation modulates immune functions in both fish species studied, and that rainbow trout is more tolerant than carp against UVB. Fish are clearly adapted to the environmental UVB levels prevailing in their usual living habitats, but are also a target of undesired effects of UVB on immune functions whenever exposed to increased radiation levels.

Effects of short- and long-term ultraviolet B irradiation on the immune system of the common carp (Cyprinus carpio).

Carp (Cyprinus carpio) were repeatedly exposed to 0, 60, 120 and 240 mJ/cm2 ultraviolet B (UVB) radiation three times in 1 week (short-term exposure) or 12 times in 4 weeks (long-term exposure). The effect of UVB on the functioning of the carp immune system was studied on day 2 after the final irradiation. After short-term UVB exposure, the whole-blood respiratory burst and cytotoxic activity were markedly enhanced, with parallel responses in both the number of circulating granulocytes and in the plasma cortisol concentration of the fish. These changes were not detectable after long-term exposure. The respiratory burst by head kidney granulocytes was suppressed dose dependently after both exposures, but cytotoxic activity was not affected. Exposure to UVB also modulated lymphocyte functions: nonstimulated and PHA-stimulated proliferation of head kidney lymphocytes in vitro was enhanced by both short-term and long-term exposure. LPS-stimulated proliferation was not affected by exposure nor was the number of immunoglobulin-secreting cells in the head kidney. In long-term exposure, the highest dose reduced the level of plasma IgM. This study indicates that UVB irradiation induces immunomodulation in the blood and head kidney of the carp and that the effects of short- and long-term exposure differ from each other. The results emphasize the potentially harmful impact of increased solar UVB radiation on fish immune functions.