Topical exposure of the eyes to the organophosphorus insecticide malathion: Lack of visual effects

Concern for toxicity following exposure to organophosphorus insecticides le d us to investigate whether topical application of either malathion or mala thion mixed in a protein bait as used for aerial spray applications could b e toxic to the ocular/visual system. Adult male Long-Evans rats were either untreated or treated with malathion alone (two drops per day in each eye), bait alone (six drops per day in each eye) or malathion and bait (six drop s per day in each eye). The dose levels of malathion alone and malathion an d bait were chosen based on pilot work and provided approximately equivalen t amounts of active ingredient. The rats were treated 5 days a week for 4 w eeks. During the final week of treatment, the rats were implanted surgicall y with cranial recording electrodes overlying the visual projection area of the cerebral cortex, Visual pattern-evoked potentials (PEPs) were elicited with vertical sinusoidal gratings at three levels of stimulus spatial freq uency (0.08, 0.16 and 0.32 cycles per degree) and three levels of visual co ntrast (0.15, 0.30 and 0.60), After spectral analysis of the PEP waveforms, the amplitude and phase at the stimulus rate (F1) and the first harmonic ( F2) were determined. Although F1 and F2 parameters were influenced signific antly by manipulation of the stimulus parameters, no significant difference s were observed that could be attributed to treatment with the test substan ces. In addition, an ophthalmological examination of the eyes and a light m icroscopic evaluation of ocular tissues, including retina and optic nerve, revealed no treatment-related lesions. The dose levels used in this study w ere high-approximately 84000 times the exposure per unit surface area expec ted from aerial spraying-and yet the visual function of the treated subject s was apparently normal, This study identified no significant toxicological concerns regarding direct ocular contact exposure to malathion.