THE MERCK VETERINARY MANUAL
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Overview of Fish

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Also see Aquaculture Systems.

Aquatic medicine has emerged as a recognized specialty within the practice of zoological medicine. Fish medicine, an important component of the aquatic specialty, is evolving with distinct subspecialties of aquaculture or production medicine (see Aquaculture Systems), and pet or exhibit fish medicine that focuses on individual animals. Although reference will be made to aquaculture medicine, the emphasis of this chapter is on pet and exhibit fish medicine.

The business of ornamental fish, which includes specimens that may be added to zoological collections, can be broadly divided into freshwater and marine species. The majority of pet fish are freshwater, and many of these are farm-raised either in the USA, Asia, or elsewhere. Many fish sold through the pet trade are imported to the USA. Virtually all marine fish, except for some clown fish (Amphiprion spp), are wild caught. The trade in ornamental fish is a global industry, and fish may be moved through several dealers before reaching a wholesale or retail outlet. The source of fish is an important consideration when designing quarantine protocols and anticipating the types and severity of disease that may occur in recently acquired animals. Marine fish are particularly prone to parasitic and bacterial infection during the quarantine period (first 30 days), including the first few weeks after arrival in a pet store or home aquarium.

Koi and fancy goldfish for ornamental ponds are popular pets and respond well to veterinary care. Many of the highest quality fish are imported from Japan (koi) or China (fancy goldfish) and may have significant value (up to several thousand dollars for show quality koi). Many are large enough for clinical manipulation, are quite hardy, and often have significant emotional value to their owners. These fish are susceptible to several diseases of regulatory concern, most notably s pring viremia of carp (see Spring Viremia of Carp (SVC)) and koi herpesvirus (see Viral Diseases of Fish), which are both reportable.

Clinical management of individual pet fish, exhibit animals, and valuable broodstock has changed dramatically in recent years. Advances include use of nonlethal methods for diagnosing disease and more sophisticated treatment options. Radiology and ultrasound are particularly well suited for disease diagnosis in aquatic species. Development of blood culture techniques to accurately identify bacterial agents and run sensitivity tests prior to the start of antibiotic therapy has been useful in decreasing the need to euthanize, or surgically biopsy, an animal to achieve an accurate diagnosis. Surgical advances, including use of exploratory laparotomy and swim bladder repairs, have salvaged animals that previously would have been euthanized.

The equipment needed to treat fish in a veterinary practice is modest. In addition to equipment already on hand (eg, microscope, glass slides and cover slips, basic surgical or dissecting tools), water quality testing equipment is needed. Basic requirements include a means to test dissolved oxygen, carbon dioxide, chlorine, ammonia, nitrite, nitrate, pH, total alkalinity, total hardness, and salinity. Commercial kits that can do all of these are available at a reasonable cost. Testing for freshwater and marine systems is similar; however, the ammonia test often sold for freshwater use requires the use of Nessler's reagent. There are two disadvantages for the use of Nessler's in a veterinary practice: 1) it contains arsenic and therefore must be treated as hazardous waste, and 2) it does not work in marine systems. Alternatively, an ammonia test using an ammonium salicylate reagent is recommended. A chlorine test is not included in many kits marketed to the aquaculture industry and may need to be ordered separately. Also, a copper test kit should be on hand for use in marine systems. If a practice has enough cases to warrant the investment, an electronic oxygen meter is strongly recommended.

In addition to these basic tools, a practice should have a few fish tanks for use as hospital systems. These can be 10- or 20-gal. tanks with simple foam filters and aerators. A dechlorinator should be on hand if the practice uses city water. In addition, methane tricaine sulfonate (MS-222) and baking soda should be available for sedation or anesthesia of patients. Other useful equipment includes a 1-L plastic graduated cylinder for measurement of water volume, a gram scale to weigh out anesthetic, and a battery-powered aerator if an anesthetized fish is to be moved around the clinic for radiology, surgery, or other procedures. For surgery, a 10- to 20-gal. tank works well as a receptacle. A plexiglass or plastic cover with small holes drilled in it can be placed over the tank to allow water to flow over the fish and back to the tank. The fish can be positioned in a v-shaped foam “bed,” and a small aquarium pump can be used to circulate aerated water, treated with anesthetic, out of the tank and over the gills.

Veterinarians who offer fish medicine should educate themselves about evolving regulatory concerns, including upcoming USDA accreditation programs (see Reportable Diseases and Regulatory Concerns).

Last full review/revision July 2011 by Ruth Francis-Floyd, DVM, MS, DACZM

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