Lethal and sub-lethal effects of UVB on juvenile Biomphalaria glabrata (Mollusca: Pulmonata)

Abstract

Although Schistosoma mansoni occurs mainly in the tropics, where intense levels of solar radiation are present, the impact of ultraviolet (UV) light on schistosome transmission is not known. The purpose of this study was to investigate potential effects of UVB (290-320 nm) on juvenile Biomphalaria glabrata, the snail intermediate host of S. mansoni. Albino and wild type snails were exposed to doses of UVB from UV-fluorescent lamps, and the following were measured: survival, photoreactivation (light-mediated DNA repair), effects on feeding behavior, and morphological tissue abnormalities. Irradiation with UVB is lethal to B. glabrata in a dose-dependent manner. Exposure to white light subsequent to UVB irradiation enhances survival, probably by photoreactivation. The shell offers some, but not complete, protection. Experiments in which UVB transmittance through the shell was blocked with black nail polish suggest that injury to both exposed (headfoot) and shell-enclosed (mantle and visceral mass) tissues contributes to mortality in lethallyirradiated snails. Wild-type (pigmented) snails are less susceptible to lethal effects of UVB than albino snails, and they may be more capable of photoreactivation. UVB exposure inhibits snail feeding behavior, and causes tentacle forks and growths on the headfoot. Thus, UVB may influence the life cycle of S. mansoni by both lethal and sub-lethal damage to the snail intermediate host.However, the ability of snails to photoreactivate may mitigate these effects.

Effects of UVB on interactions between Schistosoma mansoni and Biomphalaria glabrata

“…Sources of S. mansoni (NMRI strain) and B. glabrata (schistosome-susceptible NIH albino, and schistosome-resistant Salvador strains), and methods for exposure to UVB and miracidia have been described previously (Ruelas et al, 2006(Ruelas et al, , 2007. As before, UVB was emitted from fluorescent lamps, and radiant exposure (intensity) varied as a function of exposure time, which ranged from 1 min, 26.4 s for 500 J m À2 to 4 min, 19.5 s for 1500 J m À2 .…”

“…As before, UVB was emitted from fluorescent lamps, and radiant exposure (intensity) varied as a function of exposure time, which ranged from 1 min, 26.4 s for 500 J m À2 to 4 min, 19.5 s for 1500 J m À2 . However, unlike the earlier study (Ruelas et al, 2006), snails were provided with food (Romaine lettuce) ad libitum, except during overnight exposures to miracidia, and were not restrained in submerged finger bowls post-irradiation. It should be noted that these UVB lamps do not simulate the spectral quality and intensity of natural sunlight, but deliver regulated doses that allow for quantitative comparisons of UV effects across a range of experimental conditions (Karentz et al, 1991).…”

“…At 1 week post-exposure to miracidia, equal numbers of surviving snails from each treatment category were irradiated, in groups of 5, with 0, 500, 625, 750, 825, and 1000 J m À2 UVB. All groups of snails were subsequently held in 250-ml glass jars at 27°C in the dark to prevent repair of DNA damage by photoreactivation, and were then examined for mortality at 5 days post-irradiation (Ruelas et al, 2006). This procedure was repeated until 45-55 snails from each treatment category had been irradiated at each UVB intensity.…”

“…To test potential effects of UVB on susceptibility, 12 groups of 10 NIH albino snails (3-5 mm shell diameter) were irradiated with 500 J m À2 UVB, the lowest level at which significant mortality occurs in this size range (Ruelas et al, 2006). Snails were irradiated immediately, 1, 3, and 7 days prior to exposure to 50 miracidia, and then were incubated in the dark in 2-L plastic rectangular containers for 1 week post-irradiation.…”

“…Ultraviolet B (UVB, 280-315 nm) radiation is deleterious both to larvae of Schistosoma mansoni and to its snail intermediate host, Biomphalaria glabrata (Ruelas et al, 2006(Ruelas et al, , 2007, and therefore could potentially perturb host-parasite interactions between these two species. In the present study, we investigated effects of UVB on three such interactions: survival of infected versus uninfected snails, innate susceptibility and innate resistance of snails to schistosome infection, and acquired resistance to infection induced by UVB-irradiated miracidia.…”