NEUROPATHOLOGY OF ORGANOPHOSPHATE-INDUCED DELAYED NEUROPATHY (OPIDN) IN YOUNG CHICKS

To examine the phenomenon of apparent age resistance of young chicks t o organophosphate-induced delayed neuropathy (OPIDN), groups of either 2- or 10-week-old chicks were exposed subcutaneously daily for 4 days to the neuropathic organophosphate (OP), di-isopropylfluorophosphate (DFP, 1 mg/kg), the non-neuropathic OP, paraoxon (PO, 0.25 mg/kg) or a tropine (20 mg/kg). Subsequently, all birds were examined at post-expo sure intervals (calculated from the last day of exposure) for up to 56 days for neurological deficits and morphological lesions in the centr al and peripheral nervous systems (CNS, PNS). Clinically, none of the birds in the 2-week-old groups, or in the 10-week-old PO or atropine e xposed groups had neurological deficits. However, all birds in the 10- week-old DFP exposed group developed ataxia by 7 days post-exposure (D PE) and then progressive paralysis. Therefore, all birds in the 10-wee k-old groups were killed at 14 DPE. Pathologically, the 2-week-old DFP exposed chicks had increasingly severe lesions of Wallerian-like dege neration predominantly in the spinal cord from 7 DPE and subsequently. In the 10-week-old DFP exposed chicks, the degenerative lesions of OP IDN were first detected in the CNS at 3 DPE and then with equally incr easing severity in the CNS and PNS up to 14 DPE. A higher incidence of neuronal necrosis and chromatolysis in ventral motor horn neurons of spinal cord grey matter and in dorsal root ganglia occurred in both th e DFP exposed age groups compared with those lesions in other groups. These results demonstrate that after neuropathic DFP exposure, 2-week- old chicks develop pathological lesions in the spinal cord without neu rological deficits. In both age groups, onset of degenerative lesions in the spinal cord preceeded those in the PNS. The claim of apparent a ge resistance of chicks to OPIDN needs to be re-evaluated Introduction