Pigmented macrophage aggregates: A toxic response in fish exposed to bleached-kraft mill effluent?

An epidemiological study was conducted to evaluate whether the density of pigmented macrophage aggregates (PMAs) and the prevalences of preneoplastic and neoplastic lesions increased in fish captured downstream from a bleached-kraft pulp mill, causing induction of cytochrome P4501A enzymes (CYP1A). White suckers (Catostomus commersoni) sampled 10 and 95 km downstream from a bleached-kraft pulp mill in the St. Maurice River, Quebec, Canada, exhibited higher densities of PMAs relative to age in liver, spleen, and kidney than fish sampled 10 km upstream. White suckers were also sampled at three sites in the Gatineau River, Quebec, Canada, a reference river with a similar upstream/downstream ecological gradient but no pulp mill. In the Gatineau, density of PMAs was lower in the liver of fish sampled at the most downstream site, and there was no difference among sites in spleen and kidney. Higher growth rates have been documented at downstream sites of both rivers and thus cannot explain the increased density of PMAs observed at downstream sites of the St. Maurice River only. Density of PMAs did not differ between sexes or between fish with or without grossly visible parasites or granulomatous histological lesions. Although we cannot exclude the involvement of unknown infectious agents, density of PMAs appears to be a useful marker of bleached-kraft mill effluent (BKME) toxicity. Several known toxic effects of BKME could contribute to this response, including increased rates of lipid peroxidation associated with induced CYP1A and toxic hemolysis. Further field and laboratory studies are needed to evaluate whether the association between BKME exposure, induction of CYP1A, and increased density of PMA is consistent. Preneoplastic lesions were not observed in fish captured downstream of the pulp mill in the St. Maurice River, and only one case of biliary carcinoma was observed, at the most contaminated site.