Martindill, Steven William (1994) The significance of pollution and infectious agents in the aetiology of proliferative disorders of marine and freshwater fishes. (PhD thesis), Kingston University, .
Abstract
Liver neoplasia in wild fishes has been proposed previously as a biomarker of carcinogen exposure and effect but the significance of pollution and infectious agents in the aetiology of this condition has not been fully assessed. The aim of the present study was to address this problem. Two field studies were conducted. Firstly, dab livers (n=439) were examined from 6 stations along a pollution gradient in the southern North Sea. Two hepatocellular adenomas and one rare 'hepatocellular carcinoma (HC) were detected in dab livers from the most polluted Stations 3 & 5. One distinctive feature of the HC was the presence of numerous enlarged hepatocytes with a fibrillar-like cytoplasm. Prevalences of preneoplastic foci of cellular alteration (FCA) were significantly higher (p < 0.05) in the livers of dab from Station 3, compared to control Station 7, where prevalences were zero. Other histopathological features such as hepatocellular vacuolation, probably resulting from excess lipid accumulation, and melanomacrophage centre (MMe) proliferation, were also significantly more prevalent (p<0.05) at Station 3 than at Station 7. The second field study examined 400 fishes, primarily freshwater channel catfish, from the Grand River, a polluted tributary of Lake Michigan. One channel catfish bore a fibrosarcoma, but no neoplastic foci were detected in livers. Prevalences of hepatic FCA were also low (0.75%) and this probably resulted from the refractive properties of the sampled species. The present study also examined the effects of known carcinogens in the development of liver neoplasia in European sea bass under laboratory conditions. This represented the first long-term investigation of sea bass as a marine model for experimental hepatocarcinogenesis. Methods for culturing sea bass fry in a recirculating artificial seawater system were established. Fry were then exposed to a single dose of either aflatoxin B[sub]1 (AFB[sub]1) or N-Methyl-N'-Nitro-Nitrosoguanidine (MNNG) by immersion in static solutions for 30 minutes. They were then grown-on for up to 8 months in a system free from infectious agents. The effects of exposure on the specific cytochrome P-450 content of sea bass gill and liver micro somes was assessed spectrally, and the toxicity of the compounds was confirmed in vitro by the neutral red assay. In exposed fry a chronic toxic effect, characterised by marked enlargement of both cellular and nuclear diameters (megalocytosis), was induced in the hepatocytes following exposure to 0.5, 1.0, and 2.0 ppm AFB[sub]1. A dose response was observed up to 1. O ppm AFB[sub]1 Liver lesions worsened up until 6 months post-exposure, when they were visible grossly as pale multicentric nodules. They persisted to the final sampling point of 8 months, when there was also evidence of MMG proliferation and liver regeneration. Although parasites were associated with wild caught fishes, no infectious agents were demonstrated within any preneoplastic, neoplastic, or megalocytic liver lesions examined in the present study by light and electron microscopy. It is concluded from field and laboratory results that liver neoplasia is a chronic, 1?robably non-infectious disease which justifiably should be considered a reliable biomarker of carcinogen exposure and effect in wild fish.
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