Toxicity of acidified chitosan for cultured rainbow trout (Oncorhynchus mykiss)

Chitosan is a deacetylation product of chitin. It is used as a flocculent f or sewage and brewery wastes, and as a chelator of heavy metals. In aquacul ture, chitosan has been used as an immunostimulant for protection against b acterial diseases in fish, for controlled release of vaccines, and as a die t supplement. Chitosan has generally been considered to be nontoxic to anim als, but when it was dissolved in acetic acid and added to a culture system at 1.0 ppm to remove organic solids, we found acute toxicity to rainbow tr out (Oncorhynchus mykiss). In controlled experiments to determine the exten t of toxicity, we found that trout died after several hours exposure to 0.7 5 ppm and died in 24 h after exposure to 0.075 ppm. Exposure to 0.038 ppm r esulted in mortality after 6 days exposure, while exposure to 0.019 ppm res ulted in no mortality after 14 days exposure. Histological examination of g ills, skin, muscle, and internal organs indicated significant and consisten t pathological changes only in gills. Lifting of lamellar epithelium, hyper trophy and hyperplasia of lamellar epithelial cells occurred in trout expos ed to 0.019 and 0.038 ppm. In trout exposed to 0.75 or 0.075 ppm chitosan, large areas of lamellar fusion were observed. These results show that solub le acidified chitosan is highly toxic to rainbow trout even at low concentr ations. (C) 2000 Elsevier Science B.V. All rights reserved.