Pathology of fathead minnows (Pimephales promelas) exposed to chlorine dioxide and chlorite

Fathead minnows (Pimephales promelas) were exposed to the biocide chlorine dioxide (0.13 and 0.19 mg l−1) for up to 12 h and to its primary decomposition product, chlorite (177 and 304 mg l−1), for up to 96 h followed by recovery periods of up to 14 days. Chlorine dioxide exposure produced dose-dependent gill pathology including epithelial lifting, hypertrophy, hyperplasia, lamellar fusion, and necrosis. Complete recovery, even in fish with severe hypertrophy and lamellar fusion, was achieved within 4 days. Chlorite did not produce gill pathology even at a lethal exposure level (304 mg l−1 for 96 h) but did elicit a chronic inflammatory response with a marked increase in circulating and fixed phagocytes within hematopoietic and vascular tissues. This study indicates that chlorine dioxide is approximately 1000 times more toxic to fathead minnows than chlorite. Further, exposure of fathead minnows to these distinct but related compounds is consistently associated with very different pathologies.