Carbofuran-induced histophysiological changes in thyroid of the teleost fish, Channa punctatus (Bloch)

The long-term toxic effects of continuous exposure for 6 months, from January to June, to a safe dose (4.5 ppm) of a commercial carbamate pesticide, carbofuran, on the histophysiology of thyroid in adult and young (yearling) Channa punctatus, are described in this investigation. In both the experimental groups, thyroid histology exhibited various abnormalities, including hypertrophy, hyperplasia and degeneration of follicular epithelial cells, and reduction in colloid content. Apart from this, in young experimentals, fibrosis of thyroid components, formation of cystic cellular masses, and rupture of blood vessels resulting in hemorrhage can also be seen and suggest direct action of this pesticide on the thyroid. On the other hand, in both treated groups, retardation of thyroid function was also evidenced by significantly reduced thyroidal radioiodine (131I) uptake and CR (conversion ratio of PB131I in blood serum in relation to total serum 131I uptake) values. Apparently, these histopathological and physiological changes were markedly pronounced in young experimentals, suggesting greater susceptibility of this group to chronic toxicity of carbofuran than that of adults. Thus, on the basis of these results, it can be inferred that carbofuran pesticide, which is washed into water systems in small quantities, induces adverse histophysiological alterations in thyroid, in an age- and size-dependent manner, possibly by acting directly on thyroid and/or through the hypothalamo-pituitary-thyroid axis, in this species.