Correlation of epizootiological observations with experimental data: chemical induction of chromatophoromas in the croaker, Nibea mitsukurii.

Chromatophoromas in the croaker nibe, Nibea mitsukurii, are common neoplasms in feral fish which inhabit the shallow water in a unique geographic distribution along the Pacific coast of Japan. We undertook surveys of the epizootiology of tumor-bearing fish at 25 sites. The highest tumor incidence occurred at the station near the mouth of the Kumano river and was 47% (1,415 of 2,991). The incidence at 2 adjacent survey stations located approximately 18 and 30 km away were 2.7 and 2.5%, respectively. At the other survey stations, no tumor incidences were recorded or they were less than 5%. During the course of experimental studies on the chromatophoromas using tank-reared nibe, we found that nifurpirinol (NP), a drug used for the treatment of fish diseases, might also induce the chromatophoromas in the fish, as well as N-methyl-N'-nitro-N-nitrosoguanidine and 7,12-dimethylbenz[alpha]anthracene. Therefore, tank-reared nibe 5 months of age were divided into 4 groups of about 50 each and exposed to water containing 0, 0.5, 1, or 2 ppm NP, respectively, fourteen times for 1 hour each time. Three hundred nibe were kept as untreated controls. The incidences of chromatophore hyperplasia or neoplasia per group at 13 months were as follows: the untreated: 2.9% (6 of 204), 0 ppm: 5.3% (2 of 38), 0.5 ppm: 73% (36 of 49), 1 ppm: 87% (20 of 23), and 2 ppm: 100% (2 of 2). These and other results led us to believe that 1) NP is a carcinogen, 2) nibe have a high susceptibility to induction of chromatophoromas by chemical carcinogens, and 3) some environmental chemicals are causal factors in the hyperendemic occurrences of the tumors in wild nibe.