Abstract

Microcystins are cyclic heptapeptide, hepatotoxins released into water by lysis of actively growing or dying populations of cyanobacteria. Fish exposed to these toxins either by feeding or uptake through the gills of cyanobacteria and/or their toxins, can exhibit significant morbidity and mortality. In the present study, juvenile hybrid tilapia were intraperitoneally exposed to 700 µg/kg microcystin-LR (MC-LR). Blood and liver tissues were collected from a subset of the control and experimental populations on days 1, 3, 5, 7, and 10 post-exposure. A number of fish appeared agitated and more aggressive toward tank mates, while others became listless, hyperpigmented, and exhibited a loss of buoyancy on day 1 post-exposure. Most of these clinical signs abated by day 4 post-exposure, with all fish appearing normal by day 5 post-exposure. Microscopic examination of livers from days 1 and 3 post-exposure fish revealed hepatocellular degeneration, individualization and necrosis that were regionally extensive to diffuse. By day 5 post-exposure, there was evidence of regeneration with hepatocytes being more basophilic with large round nuclei and prominent nucleoli in close apposition to each other. A few remnants of hepatocellular necrosis and individualization were still present. It was difficult to differentiate the livers of day 7 and day 10 post-exposure fish from the controls as the repair and regeneration process was advanced. The degenerative changes due to microcystin were similar to those reported for other species of fish, but required a higher dose of toxin than reported for other species of fish to produce the same pathologic changes.

* Presenting author