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

Abstract

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.