Preliminary hematological reference values and morphology of blood cells in farmed sex-reversed male Nile tilapia (Oreochromis niloticus) from Southern Thailand

Background and Aim: Hematological assessment is a fundamental tool for evaluating fish health, yet its application in teleost species remains constrained by the reliance on manual methods because of the presence of nucleated blood cells. Consequently, accurate interpretation of blood cell morphology and the availability of reliable reference data are critical for diagnostic standardization. This study aimed to characterize blood cell morphology and establish preliminary hematological values in farmed sex-reversed male Nile tilapia (Oreochromis niloticus) (NT) raised in Southern Thailand, thereby supporting laboratory competency and aquaculture health management. Materials and Methods: Blood samples were initially collected from 46 sex-reversed male NT. Samples were evaluated for quality, and those containing blast cells, apoptotic cells, hemolysis, or suspected parasitic inclusions were excluded. A total of 14 high-quality samples were included for detailed analysis. Blood smears were prepared using the push technique and stained with Wright’s stain. Hematological parameters, including packed cell volume (PCV), hemoglobin (Hb) concentration, red blood cells (RBC) count, white blood cell (WBC) count, and differential leukocyte counts, were determined using standard manual methods. Morphological and morphometric evaluations of blood cells were conducted using light microscopy and image analysis software. Results: Erythrocytes, leukocytes, and thrombocytes were identified, with leukocytes classified into neutrophils, basophils, eosinophils, lymphocytes, and monocytes. Lymphocytes were the predominant leukocyte type, followed by neutrophils, whereas eosinophils and basophils were rarely observed. Monocytes were the largest cells (mean diameter: 15.56 ± 1.8 μm), while lymphocytes were the smallest (6.89 ± 0.99 μm). Hematological analysis revealed PCV ranging from 0.23 to 0.45 L/L, Hb concentration from 76 to 119 g/L, RBC counts from 0.94 to 2.72 × 10¹²/L, and WBC counts from 4.17 to 23.01 × 10⁹/L. An Anaplasmataceae-like inclusion body was observed in one sample, although confirmatory diagnosis was not performed. Conclusion: This study provides preliminary hematological and morphological reference data for farmed NT, contributing to improved diagnostic accuracy and laboratory standardization. The findings support fish health assessment, facilitate future research, and enhance sustainable aquaculture practices and food security. Keywords: blood cell morphology, hematological parameters, Nile tilapia, piscine hematology, reference values, sex-reversed males, Southern Thailand, white blood cells.