Delayed neurotoxicity from long-term low-level topical administration of leptophos to the comb of hens

Abstract

The production of delayed neurotoxicity in hens following percutaneous administration of leptophos [ O-(4-bromo-2,5-dichlorophenyl) O-methyl phenylphosphonothioate] has been investigated. By applying a solution of the insecticide in acetone to the comb, seven groups of three laying hens were given daily a single percutaneous dose of 0.1, 0.5, 1.0, 2.5, 5.0, 10.0, or 20.0 mg/kg of leptophos for 183–323 days. All hens given 0.5–20 mg/kg developed ataxia. The group of hens given a daily dose of 0.1 mg/kg of leptophos showed no abnormalities in gait or behavior. The severity of the clinical condition depended on the size of the daily applied dose. The “latent period” and “total administered dose” before onset of ataxia depended on the daily topically applied dose. The most consistent histopathological changes were degeneration of myelin and axons in spinal cords of intoxicated birds. The severity of change was greatest in hens receiving the highest doses. While plasma cholinesterase was inhibited in all treated birds, plasma acid phosphatase activity was significantly increased. The present investigation shows that long-term low-dose applicaton of leptophos to the comb produced delayed neurotoxicity in hens similar to that reported for the oral administration of this compound.