Chlorpyrifos: Assessment of Potential for Delayed Neurotoxicity by Repeated Dosing in Adult Hens with Monitoring of Brain Acetylcholinesterase, Brain and Lymphocyte Neurotoxic Esterase, and Plasma Butyrylcholinesterase Activities

Previous work has shown that acute exposures to chlorpyrifos (CPS; diethyl 3,5,6-trichloro-2-pyridyl phosphorothionate) cannot produce >70% inhibition of brain neurotoxic esterase (NTE) and cause organophosphorus compound-induced delayed neurotoxicity (OPIDN) unless the dose is well in excess of the LD50, necessitating aggressive therapy for cholinergic toxicity. The present study was carried out to determine if repeated doses of CPS at the maximum tolerated daily dose without prophylaxis against cholinergic toxicity could cause cumulative inhibition of NTE and OPIDN. Adult hens were dosed daily for 20 days with CPS (10 mg/kg/day po in 2 ml/kg corn oil) or corn oil (vehicle control) (2 ml/kg/day po) and observed for an additional 4 weeks. Brain acetylcholinesterase (AChE), brain and lymphocyte NTE, and plasma butyrylcholinesterase (BuChE) activities were assayed on Days 0 (control only), 4, 10, 15, 20, and 48. During Days 4–20, brain AChE and plasma BuChE activities from CPS-treated hens were inhibited 58–70% and 49–80% of contemporaneous controls, respectively. At 4 weeks after the end of dosing, brain AChE activity in treated birds had recovered to 86% of control and plasma BuChE activity was 134% of control. Brain and lymphocyte NTE activities of treated animals throughout the study were 82–99% and 85–128% of control, respectively. Neither brain nor lymphocyte NTE activities in treated hens exhibited cumulative inhibition. The 18% inhibition of brain NTE seen on days 10 and 20 was significant, but substantially below the putative threshold for OPIDN. Body weight of treated hens decreased 10–25% during Days 4–20 and recovered to 87% of control by the end of the study. Some treated hens developed a slight staggering gait during the first week of dosing, which disappeared by the second week. Throughout the 4-week observation period, all hens appeared normal and were able to perch on a horizontal rod. The results indicate that daily dosing with CPS at a level sufficient to cause significant loss of body weight as well as marked inhibition of brain AChE and plasma BuChE resulted in no significant change in lymphocyte NTE activity, a maximum inhibition of brain NTE of 18%, no cumulative inhibition of lymphocyte or brain NTE, and no clinical signs of OPIDN.