Bioassay of tetrachlorvinphos for possible carcinogenicity.
A bioassay of technical-grade tetrachlorvinphos for possible carcinogenicity was conducted by administering the test chemical in feed to Osborne-Mendel rats and B6C3F1 mice. Groups of 50 rats of each sex were administered tetrachlorvinphos at one of two doses for 80 weeks, then observed for 31 additional weeks. Time-weighted average doses were either 4,250 or 8,500 ppm. Matched controls consisted of groups of 10 untreated rats of each sex; pooled controls, used for statistical evaluation, consisted of the matched controls combined with 45 untreated male and 45 untreated female rats from similar bioassays of four other test chemicals. All surviving rats were killed at 111 weeks. Groups of 50 mice of each sex were administered tetrachlorvinphos at one of two doses, either 8,000 or 16,000 ppm, for 80 weeks, then observed for 12 additional weeks. Matched controls consisted of groups of 10 untreated mice of each sex; pooled controls, used for statistical evaluation, consisted of the matched controls combined with 40 untreated male and 40 untreated female mice from similar bioassays of four other test chemicals. All surviving mice were killed at 90-92 weeks. The mean body weights of the treated rats and mice were generally lower than those of the matched controls; however, the mortality rate was affected adversely by tetrachlorvinphos only in the male rats. Survival of all groups of rats and mice was adequate for meaningful statistical analyses of the incidence of tumors, except for a matched-control group of female rats for which the survival was abnormally low. In rats, C-cell adenoma of the thyroid showed a significant dose-related trend in the females, using pooled controls (controls 1/46, low-dose 2/50, high-dose 7/46, P=0.013), and by direct comparison, an increased incidence in the high-dose group (P=0.027). High incidences of C-cell hyperplasia in treated males and females further indicated a chemical-related effect on proliferative lesions of the thyroid. Cortical adenoma of the adrenal also showed a significant dose-related trend in the females, using pooled controls (controls 0/50, low-dose 2/49, high-dose 5/50,P=0.017), and by direct comparison, an increased incidence in the high-dose group (P=0.022). Hemangioma of the spleen occurred in male rats at a significantly higher incidence in the low-dose group than in the pooled controls (controls 0/52, low-dose 4/48, P=0.049); however, neither the incidence in the high-dose group (0/47) nor the test result for dose-related trend was statistically significant. In mice, hepatocellular carcinoma in males showed a highly significant dose-related trend, using either matched controls (controls 0/9, low-dose 36/50, high-dose 40/50, P<0.001) or pooled controls (controls 5/49, P<0.001). This finding was supported by direct comparisons of low- and high-dose groups of males with matched- or pooled-control groups, which showed highly significant increases in incidences of the tumor in the treated groups in all instances (P<0.001). In females, the incidence of hepatocellular carcinoma was not significant; however, the incidence of neoplastic nodule was significantly higher in both the low- and high-dose groups than in the pooled controls (controls 1/48, low-dose 14/49, P<0.001; high-dose 9/47, P=0.007), using pooled controls for tests for both doses. Because of this higher incidence in the low-dose group than in the high-dose group, there was a significant departure from linear trend (P=0.006). Granulomatous lesions of the liver were found in high proportions in both treated rats and treated mice, but none were found in matched controls. It is concluded that under the conditions of this bioassay, the administration of technical-grade tetrachlorvinphos in Osborne-Mendel rats was associated with proliferative lesions of the C cells of the thyroid and cortical adenomas of the adrenal in females. In female B6C3F1 mice, the incidence of neoplastic nodule of the liver was associated with treatment, and in male mice tetrachlorvinphos was carcinogenic, causing hepatocellular carcinoma of the liver.