Cholinesterase activity in black-crowned night-herons exposed to fenthion-treated water

The organophosphorus insecticide, fenthion, which was widely used to control mosquitoes, had been implicated in several cases of mortality of wading birds. To obtain information on the possible exposure routes involved, an exposure chamber was designed simulating a shallow wetland environment containing water treated with 1 and 10 times the field application rate of the active ingredient (112 g per ha), in which birds would drink and wade, or wade only. Results obtained from experiments with herons (Nycticorax nycticorax) are presented and discussed. There was no evidence that the birds drank during the experiment, therefore the results probably reflect only skin exposure to the insecticide. There was no significant inhibition of brain acetylcholinesterase activity in birds exposed to the insecticide, although plasma butyrylcholinesterase activity was inhibited. The application rates of fenthion and the types of exposure did not endanger the life of this species.