The disease

Herpesvirus epidermal proliferation in carp is a chronic and benign hyperplastic skin disease. Lesions may first develop on one or several locations, commonly on the fins. Initially they are flat, firm, smooth and translucent, but they soon grow thicker, giving the impression of paraffin drops on the skin. Multiple hyperplasia of squamous cells can form islands over the entire body (Fig. 5.12). Further proliferation results in irregularly shaped papillomatous formations, several centimetres wide and 4-6 mm thick. These have a warty surface and a milky to greyish white colour, which is sometimes tinged with pink by capillary dilatation. Proliferations rarely coalesce to cover almost the whole skin. Some cells have cytoplasmic and Cowdry type A inclusions in nuclei, with marginated chromatin. The normal strata are lacking, the number of mucous cells is reduced

Fig. 5.12. Herpesviral epidermal proliferation in carp (HEPC) or carp pox lesions in mirror-type common carp (Cyprinus carpio). Glistening patches of varying size, thickness and shape; some are translucent, others milky.

and serous glands are absent. Mitoses are frequent. Proliferating cells are not invasive and they are not metastatic. The underlying structures appear normal in thin proliferations, but the subepidermis of large papillomatous lesions form finger-like projections to reach and nourish the thick and folded epithelium.

Retarded growth and emaciation were consistently observed in advanced natural cases of 'fish pox'. The tail of such a carp can be easily bent to touch its head. Such flabby fish have a reduced muscular tone and osteomalacia with very low levels of ash, calcium and phosphorus in the vertebrae (Mann, 1951). After recovery, some may have spinal deformities. The causal relationship of this disorder in metabolism of minerals and the CHV has not been established experimentally.

In 2-week-old fry infected experimentally by bath, HSC appeared after 1 week of incubation at 20°C (Sano et al., 1991). Mortality was over 80% in 2-week-old fry and about 30% in 4-week-old fish. In 8-week-old fingerlings, the infection did not cause mortality. At 15°C, mortality is highest and it decreases with increased temperatures to 20 and 25°C (Sano et al., 1993a). Sano et al. (1993b) showed that 2-week-old fry kept at 20°C did not develop HSC, and HEPC appeared 20 weeks after bath immersion. Clinical signs of HSC include loss of appetite, swimming in straight lines and occasional loss of movement. Fish have dark pigmentation, a distended belly, exophthalmia and haemorrhage on the operculum and abdomen. Infected 4-week-old fingerlings only exhibit spiral swimming. Necrosis is the main histological finding; it affects the liver, kidney and lamina propria of the intestinal mucosa. Papillomatous skin neoplasms develop in over 50% of survivors infected as fry-fingerlings and in about 10% of infected 1- or 2-year-old carp. Spontaneous outbreaks of HSC mortality have not been reported.

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