Use of LC50 in aquatic regulatory toxicology-Disharmony in global harmonization of hazard classification of chemicals

In regulatory aquatic toxicology, acute toxicity studies with chemicals are conducted with a species of fish, crustacea, and or alga. The LC50/EC50 obtained from these studies is used for the hazard classification and labeling of the chemicals. The methods like probit or logit analysis and Litchfield and Wilcoxon method are prescribed in the OECD guidelines to determine the LC50. In the present study, LC50s were calculated using probit analysis, Litchfield & Wilcoxon method, and also using the method by Trevan (the inventor of median lethal dose) using three sets of concentration-mortality data of fish acute toxicity tests. The slopes of the concentration-mortality curves, fiducial limits (95% confidence interval) of LC50s, and ‘mode’ of the concentration-mortality curves were compared. Though the methods used in the study resulted in more or less similar LC50s, the LC10 and LC90, slopes and ‘mode’ differed considerably, indicating that LC50 does not reveal the exact toxicity profile of a chemical. The LC50 calculated using Finney’s probit analysis provides better information on the toxicity profile of a chemical than the LC50calculated by Litchfield & Wilcoxon method. While interpreting LC50, the mortality occurred below 16 % (eg.,LC10) and above 84 % (eg.,LC90), slope and ‘mode’ of the concentration-mortality curve may also be considered. It is worth having a relook at the current practice of hazard classification and labeling of the chemicals based only on LC50 in regulatory aquatic toxicology.

Histopathological changes in the gills of zebrafish (Danio rerio) and bullfrog tadpoles (Lithobates catesbeianus) caused by the use of formaldehyde

Formaldehyde is a carcinogenic and aggressive agent mainly to epithelial tissues. However, for rearing aquatic organisms its use is common for the treatment of fungi and parasites, and the use of incorrect doses can harm the health and life of these animals. The fish species Danio rerio and the tadpoles of the species Lithobates catesbeianus are internationally recognized for use in aquatic toxicology tests. We aimed identify the effects caused by formaldehyde on the gills of these two experimental models used in aquatic toxicology, verifying the susceptibility of both species. Acute and chronic tests with formaldehyde were conducted for experiment. We found that the formaldehyde in the higher concentration caused injuries to the gills of both fish and tadpoles, with a loss and displacement of epithelium, vascular congestion, telangiectasia and lamellar epithelial lifting/edema (possible aneurysm), hyperplasia and hypertrophy of epithelial cells, lamellar fusion in addition to the proliferation of mucus-secreting cells and chloride cells. Despite structural differences, the histological changes caused by chronic exposure to formaldehyde in sublethal concentrations were similar in both organisms and we recommend reviewing its use in prophylaxis and in prolonged treatments with this chemical.