The neurotoxic potential of trichlorfon, diazinon, phosmet, dichlorvos, phosphamidon and coumaphos was evaluated for their ability to inhibit brain neurotoxic esterase (NTE) activity in adult hens. Leptophos was used as a reference neurotoxic agent. All compounds were administered at high single oral doses and the NTE and acetylcholinesterase (AchE) activities were measured at 24 h and 6 w later. With the exception of leptophos, all compounds produced severe cholinergic signs associated with > 80% inhibition of brain AchE at 24 h. On the other hand, brain NTE activity was 86% inhibited by leptophos and to lesser extents by trichlorfon (76%), phosphamidon (74%), coumaphos (70%) and dichlorvos (70%). However, none of the latter compounds produced clinical delayed neurotoxicity as was observed with leptophos. It was concluded that trichlorfon, phosphamidon, coumaphos and dichlorvos are potentially neurotoxic because of their ability to inhibit brain NTE activity, but the extent of inhibition required for development of clinical delayed neurotoxicity (> 80%) is not likely to occur with any of these compounds due to their severe cholinergic activity.
