Abstract
The National Toxicology Program reported that the daily administration of dichlorvos, in the form of a corn oil gay age, was associated with an increased incidence of mononuclear cell leukemia (MCL) in male but not female Fischer 344 rats. Because of the high degree of variability with which this neoplasm appears in groups of control laboratory rats, supportive evidence is frequently sought to facilitate interpretation of increased incidences of the disease in chronic toxicity and carcinogenesis studies. In the National Toxicology Program study MCL was increased in only one sex of rats (m ales), did not occur earlier in DDVP-dosed than in control animals, and there was no evidence of a dose-response relationship. The U.S. Office of Science Technology Policy has recommended that the presence or absence of treatment-associated changes in tumor progression be employed as an adjunct in the interpretation of common tumors in rodents. The present study employed evaluative criteria described by the National Toxicology Program to determine the effects of DDVP on the progression of MCL in affected male rats. The results revealed that DDVP had no effect on either the severity or progression of MCL in male rats. Since there is no corroborative evidence supporting the interpretation of the increased incidence of MCL in male rats, this isolated DDVP-associated finding in a single sex of rats should not be considered to be evidence of a carcinogenic effect.