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Spontaneous convulsions in Sprague-Dawley rats in carcinogenicity studies.

Spontaneous clonic convulsions in Sprague-Dawley (SD) rats are occasionally observed in chronic toxicity studies and carcinogenicity studies. To estimate the incidence and features of the spontaneous convulsions, data from 11 oral gavage carcinogenicity studies using Charles River SD rats, which were conducted at our laboratory between 2003 and 2010, were collected (N = 2990 for each sex). The total incidence of animals which showed spontaneous clonic convulsions at least once during the 2-year study period was 2.4% (2.9% in males and 1.9% in females). In those carcinogenicity studies, the earliest observation of convulsions was 25 weeks of age in males and 20 weeks of age in females, and the average age at the first occurrence of convulsions in all animals was approximately 66 weeks of age. Some animals showed convulsions only once, but others on several or numerous occasions. Most convulsions were observed during the oral gavage procedure, especially when holding the animals. Approximately 0.3% of animals died immediately after the seizure. No related histopathological abnormalities in the brain have been recorded for such dead animals following routine examination.