Immunopathology of 8‐week‐old ring‐necked pheasants (Phasianus colchicus) exposed to malathion

In addition to their acute neurotoxic effects, some organophosphorus compounds have been shown to have immunotoxic properties. Alterations in the immune system may lead to chronic morbidity and/or mortality that is not readily apparent at the time of initial exposure. Because it often inhabits land that is intensely farmed, the ring‐necked pheasant (Phasianus colchicus) may be a suitable biomonitor of agroecosystems, especially those used in the production of corn. We examined immunopathologic effects of a widely used organophosphate insecticide, malathion, on 8‐week‐old, cage‐reared ring‐necked pheasants 3 d after a single oral dose. No differences were seen in hematologic parameters, body weight, or weight of the bursa of Fabricius. Birds given a high dose of malathion (230 mg/kg) displayed significant decreases in absolute and relative thymic and splenic weights (p ± 0.05). Significant changes were also seen in thymic and splenic histomorphometry (p ± 0.05). The high‐dose group and, in some cases, the low‐dose group (92 mg/kg) displayed significant histologic lesions. These findings indicate that a single dose of malathion, at the LD50 level (230 mg/kg) and occasionally at 40% of the LD50 level (92 mg/kg), is capable of inducing quantitative and qualitative changes in the lymphoid organs of the ring‐necked pheasant that may affect immune function.