Disruption of the hypothalamic-pituitary-thyroid axis on co-exposures to dithiocarbamate and neonicotinoid pesticides: Study in a wildlife bird, Amandava amandava.

Non-target organisms, including human and wildlife, are susceptible to deleterious effects of pesticide mixtures in their environment. Present study demonstrated the disruption of the hypothalamic-pituitary-thyroid (HPT) axis in a seasonally breeding wildlife bird Amandava amandava on co-exposure to dithiocarbamate mancozeb/MCZ and neonicotinoid imidacloprid/IMI, at concentrations even lower than respective environmentally realistic exposure level of each of the pesticide. Adult male birds (n=8/group) were exposed individually to 0.25% LD50 of each of MCZ (0.14mg) and IMI (2.75μl) followed by co-exposure to their equimixture as MIX-I (0.25% LD50 of each) and MIX-II (0.5% LD50 of each) through food for 30d in preparatory phase of reproductive cycle. Disruptions of thyroid gland and pituitary-thyroid axis were evident. Altered thyroid weight and volume, follicles with inactive colloids and lesions, decrease of height and nucleus-to-cytoplasm ratio of follicular epithelial cells were noted. Plasma levels of T4 and T3 were decreased, more significant in mixture groups than in individually exposed groups. Within co-exposed groups, comparatively high plasma T4 and T3 levels in MIX-II than MIX-I indicated dose non-responsiveness of the pesticides in mixtures; a characteristic displayed by endocrine disrupters. Plasma TSH level was increased in MCZ- and IMI- but decreased in MIX-I and MIX-II suggesting the disruption of the negative feedback and impairment of the HPT axis in co-exposed groups. Effects were more prominent in co-exposed groups due to combinatorial action and cumulative toxicity of pesticides. Considering the role of thyroid hormones in reproductive development, pesticides even in low dose could affect the thyroid homeostasis and reproductive axis.