Evaluation of an acute oral gavage method for assessment of pesticide toxicity in terrestrial amphibians

Abstract

Development of an acute oral toxicity test with a terrestrial-phase amphibian was considered necessary to remove the uncertainty within the field of agrochemical risk assessments. The bullfrog (Lithobates catesbeianus) was selected for use as it is a representative of the family Ranidae and historically this species has been used as an amphibian test model species. Prior to definitive study, oral gavage methods were developed with fenthion and tetraethyl pyrophosphate. Dimethoate and malathion were subsequently tested with both male and female juvenile bullfrogs in comprehensive acute oral median lethal dose (LD50) studies. Juvenile bullfrogs were administered a single dose of the test article via oral gavage of a single gelatin capsule of dimethoate technical (dimethoate) or neat liquid Fyfanon Technical (synonym malathion), returned to their respective aquaria, and monitored for survival for 14 d. The primary endpoint was mortality, whereas behavioral responses, food consumption, body weight, and snout-vent length (SVL) were used to evaluate indications of sublethal toxicity (secondary endpoints). Acute oral LD50 values (95% fiducial interval) for dimethoate were 1459 (1176-1810, males) and 1528 (1275-1831, females), and for malathion they were 1829 (1480-2259, males) and 1672 (1280-2183, females) mg active substance/kg body weight, respectively. Based on the results of these studies, the methodology for the acute oral gavage administration of test items to terrestrial-phase amphibians was demonstrated as being a practical method of providing data for risk assessments. Environ Toxicol Chem 2018;37:436-450. © 2017 SETAC.

Inefficacy of dietary test substance administration in Amphibian Metamorphosis Assay (AMA) studies

“…It is important to note that dietary feeding in other amphibian studies, including the larval phase in true frogs, may be appropriate (Cary Coyle & Karasov, 2010; Rowe et al, 2009 and Yahn & Karasov, 2021) and the issues presented in the present study specifically relate to Xenopus (Fort et al, 2018). Ranids and bufonids (true frogs and toads) have oral anatomy designed to consume solid food and thus, feed differently than Xenopus which are essentially filter feeders.…”

“…It is important to note that dietary feeding in other amphibian studies, including the larval phase in true frogs, may be appropriate (Cary Coyle & Karasov, 2010;Rowe et al, 2009 andYahn &Karasov, 2021) and the issues presented in the present study specifically relate to Xenopus (Fort et al, 2018) was not used and a solvent-treated (hexane) control diet was not included for comparison to the treatments so that the adequacy of the diet after treatment could be assessed. Therefore, in the context of the present study, none of the studies discussed above should be directly compared to the present study.…”