Histopathological Changes of Different Organs due to Acute Hypoxia in Zebrafish (Danio rerio)

The production of ornamental fish has increased in recent years. The zebrafish (Danio rerio) is a popular model organism used in scientific research across various fields. Hypoxia, or a reduction in the level of dissolved oxygen in water, is a critical condition that directly affects the survival and maintenance of many aquatic organisms, including fish. In this case report, we describe a zebrafish that experienced hypovolemic shock resulting from hypoxia caused by a sudden drop in dissolved oxygen levels. This case study describes a bioterium in which a power outage caused a drop in dissolved oxygen levels, resulting in a high mortality rate. Histopathology was used to evaluate the tissues, which were fixed in 10% formalin, processed in an automatic processor and embedded in paraplast. Tissue blocks were sectioned at a thickness of 3 µm and stained in haematoxylin and eosin for observation. Microscopically, the myocardium appeared slightly hypertrophic and the pericardium showed dense haemorrhages. The kidney tubular cells showed pronounced oedema with a whitish, foamy cytoplasm. Some tubular cells showed pronounced eosinophilia and necrotic cells were observed detaching from the tubular basement membrane in some tubules. The brain parenchyma showed pericellular oedema, increased Virchow-Robin spaces and mild glial proliferation. Therefore, we conclude that the underlying cause of death was a decrease in environmental oxygen levels combined with pre-existing branchial lesions, which generated hypoxia resulting in cardiac failure due to diffuse haemorrhages.