Unclassified Viruses Associated with Neoplasms

Papillomas of brown bullheads have been reported to contain virus-like particles measuring 50 nm in diameter (Edwards et al., 1977). However, other studies have failed to confirm this observation (Harshbarger et al., 1993; Poulet and Spitsbergen, 1996). An RNA-dependent DNA polymerase activity, presumably reverse transcriptase, was present in brown bullhead papillomas, but no other indication of a viral agent was found (Poulet et al., 1993). Chemical carcinogens have also been suggested as causes of papillomas on brown bullheads (Black et al., 1985a). During October, papillomas were present on 60% of brown bullheads in Silver Stream Reservoir, New York, which had relatively low concentrations of contaminants (Bowser et al., 1991); PAH levels in sediment were similar to those at reference sites used during studies of neoplasms in fish from Puget Sound. No brown bullheads with papillomas were found during July, suggesting that there is a pronounced seasonal fluctuation in prevalence of papillomas on brown bullheads in some waters. The widespread occurrence of papillomas and carcinomas on brown bullheads from both polluted and unpolluted sites suggests that the causes of these lesions are complex (Poulet et al., 1994).

Papillomas occur on Atlantic pleuronectids (Sindermann, 1990). Small (30 nm) cytoplasmic virus-like particles that apparently contained DNA were found in cutaneous growths on winter flounder (Pleuronectes [=Pseudo-pleuronectes] americanus) from the northwestern Atlantic (Emerson et al., 1985). Particles resembling adenovirus were observed in hyperplastic epithelial cells and papillomas of dab (Pleuronectes [= Limanda] limanda) from the North Sea (Bloch et al., 1986). These papillomas were distinguished from hyperplastic lesions by the epithelial folding and extensions of dermal tissue characteristic of papillomas. The adenovirus-like particles measured about 80 nm in diameter and were present in nuclei of epithelial cells near the surface of the lesions.

Papillomas of European eel (Anguilla anguilla) have often been considered to be caused by viruses (Pilcher and Fryer, 1980). These lesions are typically located on the jaws and other parts of the head, and the disease is sometimes termed stomatopapilloma or 'cauliflower disease'. Although viruses have been isolated from eels with papillomas, they can also be isolated from eels without papillomas. The role of these viruses in the pathogenesis of papillomas is questionable (Wolf, 1988), although an interaction between a virus and unidentified environmental factor(s) could be involved with tumour formation (Peters, 1984; Roberts, 1989).

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