Bacteria are tiny, single-celled organisms which are continually present in the water, soil, and air. Most bacteria are beneficial (e.g., they help digest foods and break down ammonia, nitrite, and organic debris in the environment).
There are a number of different bacteria that cause disease in cultured freshwater food fish such as catfish, salmon, and trout. In aquaculture, many bacterial diseases of fish can be successfully treated with medicated feeds.
However, prevention through good management practices is the best control measure for bacterial diseases. Bacterial diseases of fish are usually a result of stress such as overcrowding. Avoiding these stressors often reduces disease incidence.
Failing to correct stressful conditions while treating sick fish with medicated feed will usually either prevent the medication from being effective or will cause the disease to recur after the treatment is completed.
Bacterial disease:
Some bacteria are considered opportunistic pathogens. These bacteria are often present in the water and inside the fish, and they usually cause no problem. In nature fish are, in most cases, resistant to these pathogens and can seek the best living conditions available.
In aquaculture, however, food fish are weakened by stress conditions including increased fish density, inadequate nutrition, poor water quality (i.e., low dissolved oxygen, or high ammonia and nitrite), parasite infestation, and handling.
Stress suppresses the immune system, increasing the fish’s susceptibility to bacterial infections. As a result, cultured food fish are more susceptible to disease than free ranging animals. Common examples of opportunistic bacteria which can cause disease and death of food fish include:
Aeromonas hydrophila, Cytophaga (Flexibacter) columnaris, and Pseudomonas fluorescens.
Some bacteria are considered obligate pathogens. They can be the sole cause of disease even in the absence of stressors. Aeromonas salmonicida, Edwardsiella ictaluri, Renibacterium salmoninarum, and Yersinia ruckeri are considered by some to be obligate pathogens.
Medicated feed:
Medicated feed is frequently recommended to control bacterial disease outbreaks. Medicated feeds contain an antibiotic and are usually commercially prepared. Antibiotics are drugs that can be taken internally to control bacterial infections.
They do not control parasites, fungus, or viruses. Fish often stop eating as a bacterial disease progresses, so early diagnosis and treatment are essential to ensure that infected fish consume the medicated feed. Once a bacterial disease is detected, and if medicated feed is the appropriate treatment, the feed should be used immediately.
If medicated feed is not readily available in your area, it may be advisable to special order a few bags to keep on hand, stored in a cool, dry place. Delivery of feed may take a few days to more than a week, which will delay treatment and jeopardize the health of the fish.
There are many different types of antibiotics, but the Food and Drug Administration (FDA) has approved only two for use in food fish. These are Terramycin® (Pfizer, U.S. Animal Health Operations, New York, NY) and Romet® (Hoffman-LaRoche, Inc., Nutley, NJ). Older literature lists Sulfamerazine® for the treatment of diseased fish, but it is no longer available. Each is discussed separately below.
Regardless of the antibiotic feed used, treatments should always be the maximum recommended dose and should be fed for the total number of days recommended even if the fish appear to have recovered.
Feeding lower concentrations of antibiotics or decreasing the number of days the drug is fed can allow the bacterial pathogens to develop a resistance to the antibiotic. The antibiotic, then, would not be able to control certain infections that may occur later at your fish farm or hatchery.
Authors:
Robert M. Durborow and Ruth Francis-Floyd
