Chronic fenthion toxicity in laying hens.

White leghorn laying hens were exposed to weekly dermal applications of either 1 mg fenthion/kg (FEN; low dose) or 4 mg FEN/kg (high dose) for 24 w with the objective of evaluating chronic FEN toxicity. Four of 24 hens at the high dose exhibited transitory loss of proprioception, perching ability, and righting reflex after 8 to 16 w exposure. All hens receiving the high dose lost the ability or desire to jump from a box during the latter half of the FEN exposure period. Inhibition of serum cholinesterase and brain acetylcholinesterase was greater in the high-dose hens. Brain neuropathy target esterase was not inhibited. Behavioral changes were not correlated with changes in brain concentrations of enzymes or neurotransmitters or their metabolites. Muscle fiber abnormalities were more common in the high-dose hens. Muscle electrical activity was recorded electromyographically via telemetry. Fibrillation (denervation) potentials were absent, but amplitude times duration values for motor unit potentials of the peroneus longus muscle for 5 of the 6 4-w evaluation intervals were higher in the high-dose hens. This EMG response suggested presence of a mild neuropathy which was supported by results of ultrastructural examinations of the sciatic nerve. The low dose initially produced 8% stimulation of egg production while the high dose inhibited egg production 10% during the latter 16 w of the study and reduced body weight 8% during this period.