Assessment of ocular toxicity in dogs during 6 months' exposure to a potent organophosphate

Exposure to anticholinesterase pesticides has been associated with the development of ocular toxicity in humans and animals, ranging from blurred vision to degeneration of the optic nerve. Based on the concern for human safety, the US Environmental Protection Agency has recently required additional studies for this class of compounds, focusing on biochemical, functional and histopathological evaluation of the ocular system. This study was designed to determine the effects on the eye of ethyl parathion, a highly toxic organophosphate, when administered orally to 30 beagle dogs (five of each sex per group) at doses of 2.4, 7.9 or 794 micrograms kg-1day-1 for 6 months. Control animals received corn oil. Routine ophthalmoscopic and slit lamp examinations, refraction and intraocular pressure determinations and electroretinograms were performed as functional assessments at various intervals over the study. Plasma and erythrocyte cholinesterase were determined at weeks 1, 6, 14, 20 and 26, while brain, retinal and ocular muscle cholinesterase were measured at week 26 only. Histopathological examination of the retina, optic nerve, ocular muscle and ciliary body was conducted at termination. Plasma and erythrocyte cholinesterase was markedly depressed at 7.9 and 794 micrograms kg-1day-1 as early as week 1. Retinal cholinesterase was decreased (37-55%) from control values in the 794 micrograms kg-1day-1 group only. Ocular muscle cholinesterase was comparable in treated and control groups at termination. No functional impairment of the eye was noted over the 6-month study.(ABSTRACT TRUNCATED AT 250 WORDS)