Pathological effects in freshwater fish Poecilia reticulata (guppy) and Oryzias latipes (medaka) following methyl bromide and sodium bromide exposure

To study the effects of the fumigant methyl bromide (CH 3Br) and its anorganic residue, bromide (Br −), young guppies and medakas were exposed to these chemicals for 1 and 3 months. The concentration ranges were 10–32 000 mg/l (guppy) and 180–5 600 mg/l (medaka) for NaBr and 0.032–3.2 mg/l for CH 3Br. Besides monitoring of general toxicological parameters, histopathology was performed on total body sections. In addition, a short-term study with the two highest concentrations CH 3Br was conducted for the evaluation of (lethal) effects, in which scanning electron microscopy of the gills was included. In the guppy, a NaBr concentration of 100 mg/l and higher caused paresis and paralysis. Histopathology of these animals, exposed to concentrations of 3 200 mg/l, showed muscle changes indicative of a functional neuromuscular disturbance. In the thyroid, the epithelium appeared to be stimulated (at 32 and 100 mg/l and higher after 1 and 3 months' exposure, respectively), and the thyroid stimulating hormone producing cells in the pituitary were also activated. The development of gonads in both sexes was more advanced than in the controls after the 3-month exposure period, and there was inflammation in and around the larger blood vessels in the abdomen at concentrations of 3 200 mg/l and higher after 3 months. Medakas showed only activation of the thyroid epithelium even at the lowest concentration tested (180 mg/l). In the CH 3Br study, no specific effects were observed in the long-term study of either species. In the short-term study, however, major degenerative and regenerative changes were found in the superficial epithelia, especially of the gills and oral mucosa. These were apparently caused by the local irritating action of this compound. Moreover, at these lethal concentrations, necrotic alterations were seen in the thymic cortex and the testis which were considered stress-related. In this context, the importance of gill lesions is discussed. It is concluded that Br − is goitrogenic in both species at low concentrations, and in guppies, at higher concentrations, it also affects skeletal muscle, blood vessels and gonads. Methyl bromide has no specific action but is irritating for superficial epithelia at relatively high concentrations.