Organ Wet Mount (Squash) Preparations in Fish Pathology: An Underused, Rapid and Low-Cost Tool for Tissue-Level Diagnosis During Necropsy.

Rapid, practical diagnostic approaches are essential for effective fish health management, yet cytological wet mount assessment of internal organs remains underused and inconsistently described in peer-reviewed aquatic veterinary literature. Organ wet mount (squash preparation) cytology offers a simple, low-cost method for immediate evaluation of fresh tissues. Unlike impression smears and aspirates, in which cells are assessed largely in isolation, squash preparations retain structural relationships within the tissue, providing tissue-level information at the point of examination that would otherwise await histopathology. The technique is widely applicable and requires minimal equipment, but guidance on its application beyond gill, skin, and fin samples is limited. A survey of fish health professionals confirmed that internal organ preparations are used less than external wet mounts. This review outlines the use of organ squash preparations during fish necropsy, emphasising technique, interpretation, and recognition of common findings rather than a comprehensive overview of disease. Key limitations, such as post-mortem changes, are discussed, as is the role of targeted special stains and follow-up diagnostics. Greater awareness and more consistent use of organ squash cytology have the potential to improve early detection of disease and support more targeted diagnostic decision-making in fish health investigations.