Haematological and histological changes to Oreochromis niloticus juveniles exposed to sub-chronic doses of cypermethrin in a flow-through bioassay

Abstract Cypermethrin (CYP), a type II pyrethroid widely used for insect control in agriculture, has contaminated aquatic ecosystems, posing threats to fish health. Researchers evaluated how sub-lethal toxicity influenced white blood cell count (WBCC), red blood cell count (RBCC), haematocrit (PCV %), and haemoglobin (Hb) levels (g/dl). Mean corpuscular haemoglobin (MCH), mean corpuscular haemoglobin concentration (MCHC), and mean corpuscular volume (MCV) served as erythrocyte indicators. Fish were collected from ponds and acclimatized in the laboratory. Haematological samples were collected after intervals of 2, 6, and 8 weeks; gill and liver tissues were examined after 8 weeks from control and CYP-exposed fish at low (0.5 µg L⁻¹) and high (1.8 µg L⁻¹) concentrations. RBCC, Hb, PCV, MCV, and MCH levels significantly declined (P≤0.05) after 2 weeks in treated groups. WBCC significantly increased in a time-and dose-dependent manner. Percentage lymphocytes exhibited a mixed trend, while neutrophils increased at the highest CYP concentration. CYP exposure caused mucus secretion, vacuolation, oedema, hyperplasia, and hypertrophy in gill primary lamellae. Liver tissues showed steatosis, coagulation necrosis, and sinusoidal gaps. These findings indicate that sub-lethal CYP exposure negatively impacts fish health.