Lymphocystis is a common non-neoplastic disease of fish and is caused by an iridovirus (Fl├╝gel, 1985). The cutaneous masses typical of this disease are formed by massive hypertrophy of infected cells (Weissenberg, 1965). These lesions might be confused with neoplasia grossly but are clearly and easily distinguished from neoplasia by histopathology. Infected cells increase in size, commonly to 100-500 mm, with the maximum size varying depending on the fish species (Nigrelli and Ruggieri, 1965). Cells have a hyaline capsule, a centrally located and enlarged nucleus, and prominent basophilic cytoplasmic inclusions. Rivers' postulates were fulfilled by Wolf et al. (1966). This disease is widespread geographically and taxonomically (Lawler et al., 1977). It occurs in both freshwater and marine species, but is more common in higher phylogenetic groups (Nigrelli and Ruggieri, 1965; Wolf, 1988).

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