Toxicological evaluation of dietary diazinon in the rat

Female wistar rats were fed a semi-purified diet containing either no pesticide or 0.1 to 15 ppm diazinon for up to 92 days. At specified times, animals were bled from the orbital sinus to facilitate measurement of plasma and erythrocyte cholinesterase activity using a highly sensitive radiometric assay. Additional rats were sacrificed to determine brain acetylcholinesterase activity. General nutritional parameters measured included body weight gains and feed consumption during the growing period. Feeding diazinon at the levels employed produced no visible toxic manifestations. Treated animals showed weight gains and feed consumption which were comparable to appropriate controls. Feeding trials up to 90 days revealed that rats were highly sensitive to diazinon after 31 to 35 days exposure, as judged by reduction of plasma and erythrocyte cholinesterase activities. Brain acetylcholinesterase was judged to be insensitive to dietary diazinon (1.0 to 15 ppm), although moderate reduction (by 6%) of brain enzyme activity was noted among animals fed 10 ppm diazinon at Day 92. For all feeding trials, plasma cholinesterase was a more sensitive indicator of diazinon toxicity compared to erythrocyte or brain acetylcholinesterase. The 'no effect' level of diazinon for the rat was judged to be 0.1 ppm in the diet, which translates into an equivalent daily intake of 9 micrograms/kg body weight/day. This 'no effect' level is 20- to 50-fold lower than levels reported elsewhere in the literature, which may be attributed, in part, to the use of female animals in the present studies.