Abstract

In the ornamental fish hobby and in some parts of the aquaculture industry, individual fish can be worth hundreds to thousands of dollars such that disease-related mortalities are a substantial economic loss. Under these circumstances, non-lethal approaches are desirable. Skin and gill biopsies are routinely obtained from live, sick fish. However, these external examinations only reveal parasitic infections and water chemistry effects. Bacterial cultures were not possible due to the invasiveness of tissue acquisition, which would require euthanizing the animal.

Since 1996, the Dept. of Fisheries and Aquatic Sciences Diagnostic Laboratory has developed a simple, non-lethal approach for diagnosing systemic bacterial diseases in fish. During live examinations, blood was extracted from the caudal vein. One to two drops of whole blood was sterilely injected into 1 mL Brain Heart Infusion (BHI) broth. The inoculated broth would be placed on a rotator and checked for growth every 24 h for four days. If growth occurred, a sterile bacterial loop (1:1) was dipped into the broth and swabbed onto 5% blood agar media. Bacterial identification and antibiotic sensitivities would then be determined from colony growth on blood agar.

Live exam (gill and skin biopsies) with blood cultures were taken from sixteen fish - four goldfish or orandas (Carassius auratus), three gulf sturgeon (Acipenser oxyrichus de sotoi), and nine koi (Cyprinus carpio). Systemic bacterial infections (Aeromonas hydrophila, A. salmonicida, A. sobria, and Streptococcus sp.) were determined in six fish, representing all species examined. Antibiotic treatment was initiated and fish responded positively. Negative growth in BHI was informative as well. The client, who initially would suspect a bacterial infection, could be convinced that other factors, such as poor water quality or parasites, were the primary etiologic agents of their fish's illness.