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OVARIAN NEOPLASIA IN ORNAMENTAL HYBRID CARP (NISHIKIGOI) IN JAPAN *

Massive abdominal enlargements were observed in 21 adult carp 4--6 years old living in the breeding ponds of carp fisheries in northern Japan. The abdominal enlargement rapidly increased, and the affected fish died within a few months. At necropsy, single or multiple tumors, 1.5--25 cm (average, 10.6 cm) in diameter, were found in the abdominal cavity. Ovarian tissue was identified close to the tumors in 15 cases, and there was no evidence of anatomic continuity between the tumors and other viscera. It seems likely that all of these tumors arose in the ovaries. Histologically, the 21 tumors observed exhibited great variation, but they were composed mainly of various types of cells similar to those of human dysgerminoma, granulosatheca cell tumors, or embryonal carcinomas. Squamous cell nests were found with mesenchymal elements in one tumor. Thus, ovarian tumors in carp may have diverse histogenetic origins and probably arise both from germ cells and from ovarian mesenchyme. Areas of cellular pleomorphism, which suggested a malignant character, were seen in some of the tumors, but no evidence on metastasis was found. Tumor-bearing carp had generally been raised in ponds supplied by wells or underground springs in mountainous areas. Evidence of a possible environmental factors(s) is yet to be demonstrated.