An evaluation of 2,4-dichlorophenoxyacetic acid in the Amphibian Metamorphosis Assay and the Fish Short-Term Reproduction Assay

Abstract

2,4-Dichlorophenoxyacetic acid (2,4-D) was evaluated in both the Amphibian Metamorphosis Assay (AMA) and the Fish Short Term Reproduction Assay (FSTRA). In the AMA, tadpoles were exposed to mean measured 2,4-D concentrations of 0 (water control), 0.273, 3.24, 38.0 and 113 mg acid equivalents (ae)/L for either seven or 21 days. In the FSTRA, fathead minnows were exposed to mean measured 2,4-D concentrations of 0 (water control), 0.245, 3.14, 34.0, and 96.5 mg ae/L for 21 days. The respective concentrations of 2,4-D were not overtly toxic to either Xenopus laevis tadpoles or fathead minnows (Pimephales promelas). In the AMA, there were no signs of either advanced or delayed development, asynchronous development, or significant histopathological effects of the thyroid gland among 2,4-D exposed tadpoles evaluated on either day seven or day 21 of the exposure. Therefore, following the AMA decision logic, 2,4-D is considered "likely thyroid inactive" in the AMA with a No Observable Effect Concentration (NOEC) of 113 mg ae 2,4-D/L. In the FSTRA, there were no significant differences between control and 2,4-D exposed fish in regard to fertility, wet weight, length, gonado-somatic indices, tubercle scores, or blood plasma concentrations of vitellogenin. Furthermore, there were no treatment-related histopathologic changes in the testes or ovaries in any 2,4-D exposed group. The only significant effect was a decrease in fecundity among fish exposed to 96.5 mg ae 2,4-D/L. The cause of the reduced fecundity at the highest concentration of 2,4-D tested in the assay was most likely due to a generalized stress response in the fish, and not due to a specific endocrine mode of action of 2,4-D. Based on fish reproduction, the NOEC in the FSTRA was 34.0 mg ae 2,4-D/L.

Behavioural, Haematological and Histopathological Changes in the African Catfish, <i>Clarias gariepinus</i> Exposed to 2,4- Dichlorophenoxyacetic Acid (2,4-D)

“…USEPA (2005) stated that the maximum predicted concentration of 2,4-D in aquatic habitat is between 2000 and 4000µg/L, while the reported maximum concentration in freshwater aquatic ecosystem range from 0.015 to75µg/L (Coady et al, 2013). The estimated half life in the aquatic environment is between 2.5 and 3.2 days (Wilson and Arbruster, 2007) as it is easily degraded in the environment by microorganisms.…”

Weight-of-the-evidence evaluation of 2,4-D potential for interactions with the estrogen, androgen and thyroid pathways and steroidogenesis

“…The FSTRA Coady et al 2013) showed no findings likely to be associated with increased or decreased testosterone or estradiol. Secondary sex characteristics, fertility, gonadal somatic index (GSI), gonadal histopathology, and VTG levels were not affected in this study in males or females.…”

“…Specific data are provided for the quail study because this study was unpublished and it is the only relevant bird study identified. Further details on these studies may be found in Supplementary Appendix III and, for the frog and fish assays, in Coady et al 2013. Regulatory and published studies with Klimisch scores of 2 or higher that are relevant to evaluation of potential endocrine interactions are summarized in Supplementary Appendix III.…”

“…) Ã AMA: amphibian metamorphosis assay (Coady et al 2013); EOGRTS: F1-extended one generation dietary toxicity study (Marty et al 2010); Rat SC (1): OPP 82-1 13-week rat subchronic toxicity study (Schulze 1991a); Rat SC (2): OPP 82-1 13-week rat subchronic toxicity study (Gorzinski et al 1981a); Rat SC 3: 13-week rat subchronic study (non-guideline) (Gorzinski et al 1981b); Rat C: OPP 83-5 two year rat chronic/oncogenicity study (Jeffries et al 1995); Mouse SC: OPP 82-1 13 week mouse subchronic toxicity study (Schulze 1991b); Mouse C(1 F): OPP 83-2 mouse oncogenicity study (females) (Stott 1995a); Mouse C(2 M): OPP 83-2 mouse oncogenicity study (males) (Stott 1995b); Dog SC (1): OPP 82-1: 13-week dog subchronic toxicity study (Schulze 1990); Dog SC (2): OPP 82-1: 13-week dog subchronic toxicity study (Dalgard 1993a) and Dog C: OPP 83-1: dog chronic toxicity study (Dalgard 1993b). †Marty et al 2010 Thyroid weight change not considered exposure related; no dose response, no consistency in direction or biologically significant magnitude of change and no correlating histopathology.…”

“…Based on its production volume, 2,4-D was recently screened using the US Environmental Protection Agency (US EPA) Tier I Endocrine Disruptor Screening Program (EDSP) assays for potential interactions with the estrogen, androgen, thyroid (EAT) pathways or with steroidogenesis (Coady et al 2013. The endocrine activity of 2,4-D also has been comprehensively evaluated in a state-of-art apical in vivo extended one-generation reproduction study (EOGRT; Marty et al 2013).…”