Parasitic Diseases

Some parasitic diseases closely mimic neoplasia (Ferguson and Roberts, 1976), but more often the resemblance to neoplasia is superficial. Examples of lesions that are readily recognized histologically as non-neoplastic include cutaneous melanosis and inflammation that are caused by a variety of parasites (Fig. 3.2). Certain Myxosporea and Microsporea can form large cysts filled with spores (El-Matbouli et al., 1992; Lom and Dykova, 1992). Grossly, these masses could be confused with neoplasms, but after microscopic examination the cause of the cysts is apparent because of the distinctive appearance of the spores.

Growths consisting of 'X-cells' commonly occur in the skin, gills, or pseudobranchs of certain species in the families Pleuronectidae and Gadidae (Alpers et al., 1977; Eaton et al., 1991a; Watermann et al., 1993) and less commonly in other families of marine fish (Diamant et al., 1994). There is evidence that X-cells are not derived from fish cells, but rather are protistans with characteristics of amoebas (Dawe, 1981; Harshbarger, 1984; Waterman et al., 1993). Virus-like particles have been observed in some X-cell lesions (Wellings and Chuinard, 1964; McArn et al., 1968), but the role of viruses in this disease is uncertain (Watermann et al., 1993). X-cells have cytoplasmic granules, unusually large mitochondria, prominent nucleoli, an extracellular envelope, and a larger size than stromal cells (Brooks et al., 1969). Although the masses formed by X-cells have been called 'papillomas' by some authors, this disease is not neoplastic.

Fig. 3.2. (a) A black growth on the snout of a gizzard shad (Dorosoma cepedianum). This nonneoplastic, inflammatory lesion was caused by digenetic trematodes, Bucephalopsis labiatus. (b) Histologically, the mass consisted of granulation tissue with large numbers of well differentiated melanocytes. Bar = 150 mm.

Fig. 3.2. (a) A black growth on the snout of a gizzard shad (Dorosoma cepedianum). This nonneoplastic, inflammatory lesion was caused by digenetic trematodes, Bucephalopsis labiatus. (b) Histologically, the mass consisted of granulation tissue with large numbers of well differentiated melanocytes. Bar = 150 mm.

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