Abstract

  The cyanobacterium Microcystis is known to produce toxins called microcystins which are made up of cyclic polypeptides. Their importance is apparent because these toxins are released into the aquatic ecosystem, thus threatening human and environmental health. The study was conducted to determine the pathological effects of microcystin-LR (MC-LR) on the freshwater ornamental fish Carassius auratus (wakin). Purified MC-LR was introduced to the fish by intraperitoneal injection at concentrations ranging from 0.1-10,000 µg/kg. Carassius auratus injected with the highest dose (10,000 µg/kg) of MC-LR died four hours after injection with 100% mortality. Similarly, those given 1,000 µg/kg showed symptoms of acute toxicity within four hours after injection but death occurred 18 hours after injection. Before succumbing to death, the fish showed erratic swimming movement, loss of balance and disorientation. The intraperitoneal LD50 value of MC-LR in C. auratus was found to be 380 µg/kg. Surviving fish developed black spots in areas of the gills, belly, and tail three weeks after introduction of the toxin.
  Histological examination revealed pathological changes in the gills, intestine, liver, and kidney especially in the fish injected with the lethal dose (10,000 µg/kg) of microcystin-LR. The gills showed lamellar hypertrophy and hyperplasia. The intestine was characterized by extensive infiltration of macrophages into the separated epithelium and tunica propria, extensive invasion of neoplastic lymphocytes, and occurrence of hemorrhages within the tunica propria. In the kidney, pycnotic and karyorhetic nuclei were evident in the renal tubules and hemopoietic tissue. Parenchymal architecture of the liver was dissolved and several pcynotic nuclei were present. Fish given 10 µg/kg of MC-LR showed extensive invasion of neoplastic lymphocytes in the intestine, presence of hemosiderin in the kidney, increase in number of melanin accompanied by a decrease of hemopoietic cells. In conclusion, the injuries caused by MC-LR in the liver, kidney, and intestine may have caused the death of C. auratus.