Pet Fish Diagnostics

Among ornamental fish, the period of illness (morbidity) is usually very short. One minute they appear fine, the next they may be dead. This is why it is important to quickly arrive at an accurate diagnosis to begin proper treatment and correct your husbandry procedures, if necessary.

As with any sick animal, a good history can be extremely helpful. Your veterinarian will require such important information as:

• How long have you owned the fish?
• How experienced are you when it comes to keeping fish?
• What and how often do you feed the fish?
• Have the fish been treated with any medications?
• Have any new fish been introduced into the aquarium or pond recently, and if so, were they quarantined first?

Armed with answers to these and other background questions, your veterinarian may already have a good idea of what the particular problem is. The next diagnostic step is usually water testing.

While comprehensive understanding of water chemistry is an asset for your veterinarian, it is not necessary for proper and accurate water evaluation. The ability to measure and understand the relation between about a half dozen parameters is adequate. If possible, water quality should be evaluated before any diagnostic procedures are performed on the fish patient.

In many cases, and especially those dealing with your pet fish, the clinician may want to take some tissue samples for examination without killing the animal. Many procedures can be rapidly performed with little risk to the piscine patient. Naturally, larger fish fare better than smaller fish, and the overall condition of the animal will determine how it will respond to biopsy techniques.

Once your veterinarian has ruled out a water quality problem, the next step will likely be biopsies of the gills, skin, and fins. An anesthetic agent such as tricaine methanesulfonate (MS-222) can be used to restrain the fish and make these procedures safe and easy. After your fish loses its ability to maintain equilibrium (keep itself upright), it is removed from the water, the procedures are performed, and the fish is placed in a “recovery” vessel containing clean water and aeration. The fresh tissue samples will then be placed on a glass slide for microscopic examination.

Obtaining blood samples from tropical fish is challenging but not impossible. While difficult to do in fish less than 3 inches long, it is a relatively routine technique in larger pet fish, like koi. A sterile blood sample is a useful way to culture for a suspected bacterial infection and can also provide your veterinarian with valuable information about your pet’s physiology and metabolism. Fresh blood smears can also be valuable in diagnosing blood-borne parasites.

Performing a fecal examination on your fish is a safe and inexpensive way to look for internal parasites. Your veterinarian may ask you to bring in a fresh fecal sample if you are planning a visit to the clinic. You should not collect a fecal sample from the bottom of your aquarium. Fecal material can be quickly colonized by opportunistic organisms that may make diagnosis difficult. The day before bringing your fish to the vet, it can be placed in a plastic bag, clean jar, or small aquarium, and a fecal sample can be collected within a matter of hours in most cases.

One very important area of diagnostics involves the field of microbiology. Cultures of skin and gill tissues are not usually helpful due to the ubiquitous (found everywhere) nature of aquatic bacterial pathogens. However, if a fish has died or needs to be euthanized, valuable cultures can be collected from the internal organs using sterile techniques. The organs most commonly cultured include the kidney, liver and spleen, since these are organs with high blood flow and active cells.

What about X-rays (radiographs)? Why not? In many veterinary clinics, this is becoming a standard part of pet fish medicine. Radiographic findings have been instrumental in diagnosing and treating a number of problems including skeletal disease, swim bladder abnormalities, foreign body obstruction and cancer. Fish are relatively easy to radiograph. Most can be handled without anesthesia by simply removing the fish from a tank or bucket, placing it on a radiographic plate (protected by a plastic bag), and making the exposure. Other more advanced diagnostic techniques that have been performed on pet fish include ultrasonography, computed tomography (CT scan), nuclear bone scans, fluoroscopy (a moving x-ray), and nuclear magnetic resonance (MRI).

In some cases, a full necropsy (animal autopsy) is the only way to arrive at an accurate diagnosis of a disease problem. Necropsies have two obvious advantages over biopsy procedures:

• Tissues can be looked at thoroughly and completely.
• Any and all tissues and organs are available for both gross and histopathological (microscopic) inspection.

Remember, dead fish decompose rapidly. If at all possible, contact your veterinarian as soon as you notice a problem in your fish. Diagnosing a problem early is very important if a sick pet fish is to be treated successfully. In some cases, one or two fish may have to be sacrificed in order to save the rest of the aquarium or pond. A very moribund fish can be euthanized and your veterinarian can obtain fresh and helpful diagnostic samples after performing a necropsy.

Keep a supply of nets, buckets, plastic bags, and water sample jars handy for the day that you may have to transport your fish or water to the veterinarian. You should also find a veterinarian in your area who is comfortable seeing fish patients and keep her or his number in a handy spot near your aquarium.