Effect of Oregano Vulgare essential oil supplement on growth and histopathological changes of gill and spleen of koi fish (Cyprinus carpio) under long-term exposure to naproxen

Residues of various pharmaceutical substances in surface waters have raised concerns about the possible adverse effects of these pollutants on aquatic organisms in recent years. The study aimed to investigate the effect of feeding with oregano essential oil (OEO) on reducing the damage of gill and spleen tissues in long-term exposure to the non-steroidal anti-inflammatory drug (NSAID) naproxen (NPX). In this study, ۱۸۰ koi fish were purchased from the ornamental fish breeding center. After the adaptation period, they were divided into four groups with three replications. Accordingly, two groups were fed with a basic diet (C, NP) and two groups with an OEO diet (OE, OE+NP) for ۳۰ days. Then, NP and OE+NP groups were subjected to long-term poisoning with NPX for ۱۴ days. Growth factors were examined on the ۳۰th day of the test and sampling was done to examine the histopathology of the spleen and gills on the ۳۰th, ۳۷th, and ۴۴th days. The results showed that feeding with OEO increased the growth performance of fish (P<۰.۰۵) Also, the results of gill pathology examination showed apical hyperplasia, basal membrane hyperplasia, infiltration of blood cells, shortening of the secondary lamellae, protrusion of the epithelium, swelling of squamous cells, fusion of secondary lamellae, and aneurysm, which were reduced in OE+NP treatment. Examination of spleen tissue showed hyperemia, necrosis, vacuolization, hemosiderin, Dilation of sinusoid, and accumulation of melanoma macrophages due to exposure to NPX. Also, the number of side effects decreased in OE+NP treatment. Therefore, the use of this food supplement improves the growth and physiological performance of fish in waters with medicinal pollutants.