Toxicology of malathion in bullfrogs and leopard frogs

Abstract

Malathion is an organophosphate pesticide commonly applied to agricultural and wetland areas. There is little information in the toxicological literature regarding the toxicity or residues of malathion in adult ranid frogs. A 96-h toxicity test was conducted with adult southern leopard frogs (Rana utricularia). The 96-h LC50 and 95% lower and upper confidence limits were 56.2 mg/L, 20.75, and 152.41, respectively. Behavior was monitored during the 96-h toxicity test, and frogs in the 10 mg/L malathion test concentration exhibited significantly fewer fright responses than the controls (p=0.05). Brain cholinesterase activity and malathion and malaoxon concentrations in liver and kidney were measured. Malathion was detected in liver and kidney of leopard frogs more frequently than malaoxon. The residues of malathion were investigated in the larger bullfrog (R. catesbeiana) by intravenous injection and monitoring of anesthetized frogs for a period as long as two hours following injection. Blood, liver, and kidney were analyzed for concentrations of malathion and malaoxon, and cholinesterase activity in brain was measured. Because the larval stage is more sensitive than the adult, additional toxicity studies were conducted with bullfrog tadpoles. Effects on growth, development, and righting reflex were determined in a 28-d static renewal test. Effects on learning behavior, gross motor activity, and brain cholinesterase were determined in tadpoles exposed for six days. Survival was significantly decreased (p<0.001) at malathion concentrations of 2500 μg/L and higher; development of tadpoles was delayed significantly (p=0.003) by malathion exposure as indicated by a dose-related decrease in developmental stage. In addition, righting reflex was impaired (p<0.001) in tadpoles in the malathion treatment groups. General activity levels were decreased (p<0.05) in the 3000 μg/L treatment group. Brain ChE activity was significantly less in all malathion treatment groups than that observed in the control group (p<0.05). Learning behavior was not significantly affected by malathion, although this could be confounded by the various physiological effects associated with poisoning. A lowest observed adverse effect level (LOAEL) for effects of malathion on bullfrog tadpoles was 500 μg/L; a no observed adverse effect level (NOAEL) was not identified.