Environmental temperature is an important factor in any aspect of fish pathology because the temperature of most fish is essentially the same as that of the surrounding water. Low temperatures usually reduce the incidence of neoplasms in fish exposed to chemical carcinogens (Egami et al., 1981; Hendricks et al., 1984b; Kyono-Hamaguchi, 1984; Curtis et al., 1995). However, the melanosis and melanomas that develop in hybrid Xiphophorus kept at 26.0-27.5°C, do not develop at 31.0-32.0°C (Perlmutter and Potter, 1988).

Certain virally induced thickenings of the epidermis occur seasonally, and temperature is a likely factor in regression of these lesions (McAllister et al., 1985; Sano et al., 1991). There are at least two hypotheses to explain the disappearance of these epidermal lesions: (i) the immune system may be more effective in ridding fish of the tumour cells at certain temperatures; and (ii) effects of the virus on cell proliferation may depend on temperature. If the second hypothesis is correct, the close relationship between the stimulus (virus) and the excessive growth indicates that these lesions are not neoplasms, but rather hyperplastic lesions.

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