The fungicide benomyl (methyl 1-(butylcarbamoyl)-2-benzimidazolecarbamate) causes testicular dysfunction by inducing the sloughing of germ cells and occlusion of efferent ductules.

Adult male Sprague-Dawley rats (100 days of age) were given single oral doses of the fungicide benomyl (methyl 1-(butylcarbamoyl)-2- benzimidazolecarbamate) in dosages ranging from 25 to 800 mg/kg body weight. The testis and excurrent ducts were examined at 2 and 70 days posttreatment to determine the chemical effects on spermatogenesis and the epididymis. Primary effects seen at 2 days postexposure were testicular swelling and occlusions of the efferent ductules. Mean testis weight peaked with 400 mg/kg. Premature release of germ cells (sloughing), detected even with the lowest dosage (25 mg/kg), was the most sensitive short-term response to benomyl. Sloughing was found primarily in Stage VII of the spermatogenic cycle at the lower dosages, but at higher dosages sloughing extended into all stages except for Stages VIII-XI. Occlusions of the efferent ductules of the testis were dose-dependent and correlated with testis weight. Sperm and sloughed germ cells were compacted in the ductal lumen of occluded ductules, which were surrounded by two to four layers of polymorphonuclear leukocytes and other inflammatory cells. Although the caput epididymidis contained sloughed germ cells and appeared swollen, evidence of permanent occlusion was not found. The long-term (70 days) effects of benomyl were decreased testis weight (at 400 mg/kg), dose-dependent increases in seminiferous tubular atrophy, and increases in the number of reproductive tracts containing occluded efferent ductules. Fibrosis, granulomas, and abnormal growth were long-term consequences of occluded ductules, which were present 100% of the time in testes containing 26-100% seminiferous tubular atrophy. Only a few testes were found to be completely regressed. Occlusion of efferent ductules and disruption of the seminiferous epithelium by sloughing of germ cells are overlapping dose-dependent mechanisms responsible for short- and long-term effects of benomyl on the rat testis.