Insecticide-induced inhibition of thyroid activity in the fish Oreochromis mossambicus and recovery in insecticide-free water.

The histological and histochemical state of the thyroid gland of Oreochromis mossambicus was analyzed after 20 days' exposure of the fish to BHC (p less than 0.001 ppm). Histological changes after exposure included acolloidal and atrophied follicles and goitre formation. Follicular and nuclear diameter and the E/T ratio were significantly (p less than 0.001) higher than in the corresponding controls. The above insecticide led to thyroid dysfunction. The histochemical characteristics of the gland also changed after BHC treatment. When exposed fish were transferred to normal, clear, dechlorinated water, the altered follicles displayed remarkable recovery of activity and thyroid function returned to almost the same state as in normal controls. Histological and histochemical evaluation at 10-day intervals revealed healthy functional restitution of the gland, indicating that BHC-induced changes are reversible.