Basic diagnostic techniques for fish
BASIC DIAGNOSTIC TECHNIQUES FOR FISH
Julius M. Tepper
Long Island Fish Hospital
1 Saddlebrook Lane, Manorville, NY 11949
As with all our diagnostic protocols for different species in veterinary medicine, the first step is to observe the patient (or patients) in their natural environment. In fish medicine, this means slowly approaching the pond or tank to note the movements and interactions of the fish. Stressors, such as water quality and temperature issues, parasites, feeding behavior abnormalities and cohort interactions may manifest themselves in gross or subtle changes in fish behavior. The direct physical exam can then be done after capture and before microscopic exam sampling. Many species are sedated for this sampling. The general condition of the skin, eyes, mouth and fins are noted. A small sample of peripheral gill tissue is excised (gill snip) and placed on a glass slide. A drop of tank water is applied and covered with a coverslip. These will then be examined to determine the microscopic condition of the gill tissue and check for the presence of parasites. Small samples of gill tissue, obtained non-lethally in this way, can also be submitted to the lab for histopathology and viral antigen identification. Similarly, several samples of skin mucus are obtained by gently but firmly scraping craniocaudally with a coverslip (preferably plastic) in high- probability areas. These are typically areas of minimal water movement, such as behind the pectoral fins. Scrapings should also be done at the periphery of any skin lesions found. A fin clip at the tip of any fin can be useful to identify bacterial, fungal and/or parasitic lesions. Blood analysis can be useful to identify internal disease conditions, though less so than in homeotherms. This is because parameters will vary depending on water temperatures, making standardized norms harder to establish. In some species, important viral screening can be done via blood tests. Many different locations are sampled, depending on the species and size of the patient. In typical pet fish species, such as koi and goldfish, the caudal vein is accessed via a ventrodorsal puncture, similar to the technique used for reptiles. In large specimens, the lateral cutaneous vein can also be sampled via an anterolateral needle insertion. Fecal exams are often done to identify parasites or ova. Radiology can be useful to visualize internal abnormalities. Of particular interest is the condition of the swimbladder, which may be affected when buoyancy disorders occur. A horizontal beam lateral x-ray, with or without sedation, is useful to visualize fluid in the swimbladder. Ultrasound is also a beneficial diagnostic procedure to identify solid masses and when fluid is found to be present in the swimbladder, to help guide the aspiration procedure.