Experimental Exposure to Imidacloprid in Naturally Parasitized Fish (Squalius cephalus): Behavioral, Physiological, and Immune Responses.

Neonicotinoid insecticides, particularly imidacloprid, are widely used in agriculture and veterinary medicine, but raise major concerns for aquatic ecosystems due to their persistence and toxicity. In this study, wild European chub (Squalius cephalus) were experimentally exposed to 0, 15, or 150 ng·g-1 imidacloprid via oral administration, once a week during six consecutive weeks. Pesticide levels in fish muscle were quantified, and a range of behavioral (hiding response, swimming performance), physiological (thyroid hormone levels, body condition), immunological (neutrophil and lymphocyte counts), and genotoxic (nuclear abnormalities) parameters were assessed. The effects of natural infection with acanthocephalan parasites on ecotoxicological responses were also considered. Imidacloprid accumulated in muscle at the highest exposure concentration (150 ng·g-1). An increase in parasite number was linked to an increase in this accumulation. Imidacloprid exposure increased neutrophil number, reduced lymphocyte numbers, and induced nuclear abnormalities. Parasite biomass was independently associated with a marginal reduction in T3 hormone levels and increased neutrophil counts. Hiding behavior was lower in fish from the highest imidacloprid exposure, but only for those with a high parasite load, probably due to energy impairment. These findings demonstrate that sublethal concentrations of imidacloprid compromise fish leucocyte profiles and behavior, and highlight the importance of considering host-parasite interactions when evaluating their impacts.