Performing a basic examination in fish - Veterinary Medicine
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Performing a basic examination in fish
You have the skills to care for fish, so take the opportunity to accept these aquatic creatures as patients. Be sure to explore husbandry issues, since inappropriate care is often the underlying cause of many disorders in fish.


Certain abnormal behaviors, however, are common to all fish. The more important of these abnormal behaviors are

  • Coughing—flaring of the operculum followed by rapid closure in an attempt to dislodge an irritant from the gills, which is suggestive of gill disease.
  • Flashing—rubbing against objects, which is suggestive of an ectoparasitic infestation.
  • Piping—gulping air at the surface, indicating hypoxia due to oxygen-poor water, gill disease, or anemia.
  • Circling (controlled) or whirling (uncontrolled)—suggestive of blindness and neurologic disease, respectively.
  • Drifting—aimless, unpropelled motion indicative of weakness and imminent death.
  • Abnormal posture—floating at the surface, which may suggest disorders such as swim bladder or neurologic disease. Or in the case of goldfish, a lack of fiber in flake food diets often results in poor intestinal motility, resulting in air accumulating in the intestines. This air causes the fish to float abnormally at the surface.


After taking the history, observing the fish, and performing water quality testing, perform a hands-on examination. To do this, first capture the fish with soft mesh or plastic nets to minimize trauma. The physical examination should be performed quickly (in less than one minute) and may require sedation or general anesthesia. Tricaine methanesulfonate dissolved in water is commonly used to sedate or anesthetize fish. The response of fish to tricaine varies among species and water quality parameters; however, the general dose for sedation ranges between 8 and 30 ppm using a buffered 10 g/L stock solution.1 Fish placed in the anesthetic bath should be immobilized in less than five minutes if given the proper dosage. You can add more tricaine gradually as needed to obtain the desired level of sedation or anesthesia. Anesthetic recovery occurs when the fish is returned to water containing no tricaine anesthetic.

Use latex surgical gloves without powder to examine the external surfaces and eyes. Examine the fish for abnormal color changes, eye lesions, swelling of the coelomic cavity (dropsy), and musculoskeletal abnormalities. Examine the gills by lifting the operculum. Spread the fins to examine them, and evaluate the oral cavity. Compare ill fish with healthy tank mates. Sick fish frequently exhibit changes in color and appear lighter or darker than their healthy tank mates. Reddened areas on the body are usually associated with hemorrhage. Dropsy can be related to peritonitis, ascites from renal or liver failure, obesity, or neoplasia. Traumatic eye lesions are common in fish. Exophthalmia is also common and frequently associated with infectious diseases. Musculoskeletal abnormalities are associated with nutritional abnormalities or infectious diseases.

Figure 1: Blood collection by caudal venipuncture using the lateral approach in a koi.
Obtain samples for diagnostic tests quickly and safely. Blood for diagnostic sampling can be collected safely from fish more than 3 in (8 cm) long.2 The estimated blood volume of fish is about 5% of the body weight (compared with 10% of the body weight of mammals).3 Because the hemodynamics of fish differs from that of mammals, about 30% to 50% of the total blood volume can be safely collected from healthy fish.4 For example, a 100-g fish has an estimated total blood volume of 5 ml, so a 1.5- to 2.5-ml blood sample can be collected. However, this amount is rarely needed for diagnostic purposes. A safe and easy to remember sample size is 1% of the total body weight; therefore, a 1-ml sample would be collected from a 100-g fish.

Blood can be collected by venipuncture of the caudal vein or artery in most species. The caudal vertebral blood vessels can be approached ventrally or laterally. The ventral approach involves inserting the needle along the ventral midline near the base of the caudal peduncle. A lateral approach to the caudal vertebral vessels is performed by inserting the needle a few millimeters below the lateral line near the base of the caudal peduncle (Figure 1). Direct the needle toward the midline and under the vertebral bodies.


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