Effects of the organophosphate pesticide Counter® on laboratory deer mice (Peromyscus maniculatus)

Laboratory deer mice (Peromyscus manniculatus) received a single oral dose of the organophosphate (OP) pesticide COUNTER® at four dose levels (2.48, 1.69, 0.92, and 0.0 mg active ingredient per kilogram body weight). The animals were sacrificed at 2, 4, 8, 16, 24, 48, and 96 h after dosing to determine brain cholinesterase inhibition and recovery over time. Brain cholinesterase of treated mice was depressed in a dose‐dependent fashion but showed no detectable change over time and did not recover over 96 h. Brain cholinesterase measurement is probably not appropriate for determining COUNTER‐induced death; mice that died did not have lower cholinesterase activity than sacrificed mice. Brain cholinesterase depression may not be a direct cause of death due to COUNTER exposure. Sublethal effects of exposure at the two highest doses included reduction in food consumption and body weight, and behavioral signs indicating intoxication for at least 24 h after dosing. Residue was detected in only one out of 67 brain tissue samples. Response of brain cholinesterase to COUNTER exposure is atypical relative to other OPs.