THE MERCK VETERINARY MANUAL
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Bacterial Diseases of Fish

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Epidemics of bacterial diseases are common in dense populations of cultured food or aquarium fish. Predisposition to such outbreaks frequently is associated with poor water quality, organic loading of the aquatic environment, handling and transport of fish, marked temperature changes, hypoxia, or other stressful conditions. Most bacterial pathogens of fish are aerobic gram-negative rods. Diagnosis is by isolation of the organism in pure culture from infected tissues and identification of the bacterial agent. Sensitivity testing prior to antibiotic use is recommended.

A number of bacteria produce a similar syndrome, generically referred to as hemorrhagic septicemia, and characterized by external reddening and hemorrhage in the peritoneum, body wall, and viscera. Morbidity and mortality are highly variable, depending on predisposing conditions such as low dissolved oxygen, other water quality problems, handling stress, or trauma. Ulcerative lesions are common as disease progresses, and mortality can be significant if stress is not controlled. Antibiotic therapy is recommended if fish are dying. Common bacterial isolates from affected fish include Aeromonas spp and Pseudomonas spp, which are more common in freshwater animals, and Vibrio spp, which are more commonly isolated from marine fish. Control is based on removal of predisposing factors. If antibiotic therapy is warranted, the drug selection should be based on sensitivity testing when possible.

Aeromonas salmonicida, a gram-negative, nonmotile rod, is the causative agent of goldfish ulcer disease and furunculosis in salmonids and is a very important disease of koi and goldfish. The disease also occurs in freshwater and marine species other than the groups mentioned above. In the acute form, hemorrhages are found in the fins, tail, muscles, gills, and internal organs. In more chronic forms, focal areas of swelling, hemorrhage, and tissue necrosis develop in the muscles. These lesions progress to deep crateriform abscesses that discharge from the skin surface. Liquefactive necrosis occurs in the spleen and kidney. Diagnosis is made by isolating and identifying a pure culture of the organism from infected tissue. Avoidance through use of good quarantine practices, and vaccination when appropriate, is preferable to treatment. Successful treatment is possible, based on appropriate antibiotic therapy. Blood culture (see Necropsy and Diagnostic Techniques) is an effective and nonlethal method for effective identification and sensitivity testing of A salmonicida isolates from valuable koi. Commercial vaccines are available for prevention of A salmonicida in salmonids and koi, but information on efficacy in koi is limited.

Vibriosis is a potentially serious, common systemic disease of many cultured, aquarium, and wild marine and estuarine fishes; it is less common in freshwater fish. Vibrio anguillarium and other Vibrio spp are responsible for the disease, which produces systemic manifestations, including hemorrhages and ulcerations of the skin, fin, and tail, and hemorrhagic and degenerative changes of internal organs. Diagnosis requires identification of pure isolates from infected tissues. Isolation of V cholera from fish is not uncommon and should not cause alarm as long as the isolate is the non-O type. Preventive measures include minimizing stress and crowding. Coldwater vibriosis (Hitra disease), a serious problem in sea farming of salmonids, is characterized by high mortality, resistance to drug therapy, and stress mediation. The etiologic agent is V salmonicida. Because Vibrio spp are ubiquitous in marine environments, avoidance is difficult. Preventive vaccination with formalin-killed Vibrio is used in the salmonid industry. Antibiotic therapy should be based on results of sensitivity testing.

Yersiniosis (enteric redmouth disease) is a serious acute or chronic bacterial disease of intensively cultured salmonids. The etiologic agent is Yersinia ruckeri. Signs are darkening and hemorrhage of the mouth (red mouth), skin, anus, and fins. Chronic signs are associated with inappetence, exophthalmos, swelling, and degenerative changes of internal organs. Mortality rates are variable but are exacerbated by poor water quality and related stressors. Diagnosis is by isolation and identification of pure cultures of the organism obtained from the internal organs of infected fish. Fish that survive remain carriers and may cyclically shed bacteria, particularly when exposed to stressful conditions and water temperatures of 15–18°C. Depopulation of infected fish and avoidance of introduction of infected fish can be recommended, but preventive vaccination is the usual procedure in endemic areas. Yersiniosis can be treated successfully with antibiotics, which should be selected based on a sensitivity test. Therapy should be continued for at least 14 days.

Edwardsiella ictaluri causes enteric septicemia of catfish, the most important infectious disease in the channel catfish industry. Infection occurs in the spring and fall when water temperatures are 22–28°C, and mortality may be exacerbated by handling stress, chemical treatment, or poor water quality. The disease occurs in 2 forms—the enteric (or intestinal) form and the meningeal form. In the enteric form, infected fish may develop skin lesions characterized by massive petechial hemorrhage around the mouth, operculum, and eyes, or they may develop measles-like red punctate lesions along the body wall. There is a hemorrhagic enteritis, and the intestine may be hemorrhagic and fluid- or gas-filled. Liver lesions are common and may be evident as multifocal areas of necrosis, abcessation, or hemorrhage. In contrast, in the meningeal form, few external signs may be seen in infected fish. The bacteria enter the CNS through the olfactory system, and affected fish develop severe meningitis. In fingerlings, the inflammation may be severe enough to erode the skull, resulting in the characteristic “hole-in-the-head” lesion. Fish affected with the meningeal form may demonstrate bizarre behavior, including spinning, erratic swimming, and general disorientation. Diagnosis is based on bacterial culture and isolation. Brain culture is indicated whenever E ictaluri is suspected. E ictaluri will grow on blood agar incubated at 25°C for 48 hr. Antibiotic therapy should be based on results of sensitivity testing. Vaccination is available for channel catfish fingerlings.

Edwardsiella tarda causes intestinal disease in a variety of aquatic and terrestrial organisms, including fish, reptiles, and mammals (including humans). In catfish, this bacterium causes a disease referred to as emphysematous putrefactive disease of catfish, descriptive of the characteristic malodorous, gaseous lesions. Infection is usually limited to 5–10% of the population, and mortality is chronic. Clinically, affected fish may be unable to swim normally because of abnormal buoyancy created by gas-filled lesions in the skeletal musculature. When lesions burst, they are extremely malodorous. E tarda has been reported in freshwater and marine aquaria across a fairly wide temperature range, and in free-ranging largemouth bass. Clinical signs include significant ulceration of skin as well as systemic disease. Overall mortality rates are generally low, usually <5%. The organism is easily isolated using blood agar incubated at 25°C for 24 hr. Antibiotic treatment based on results of sensitivity testing is effective.

The taxonomic grouping of bacteria causing columnaris disease, coldwater disease, and bacterial gill disease has undergone significant revision in recent years based on genomic studies. The causative agent of each of these important diseases has been moved into the genus Flavobacterium. These gram-negative, rod or filamentous bacteria have a distinctive gliding motion. Skin or gill lesions have slimy or cotton-like surface exudates, which usually cover surface necrosis, ulcerations, and marginal hemorrhages.

Flavobacterium columnarae, the member of this group responsible for columnaris disease, is most common in warmwater species of fish. A presumptive diagnosis can be made from visualization of typical organisms on wet mounts of infected skin or gill tissue. Columnaris disease can be confirmed by isolation of the organism on Ordal's or other cytophage media. Sensitivity tests are difficult to perform because F columnarae will not grow on Müller-Hinton media. If the disease is diagnosed early in the course of infection, treatment with potassium permanganate or hydrogen peroxide may be effective. If the disease becomes chronic, it may have become systemic, in which case treatment with florfenicol or terramycin is recommended. Columnaris disease can be prevented by reducing organic loading and avoiding traumatic injuries. A similar organism affecting marine fish was previously grouped with F columnarae, but has been given its own genus and is now named Tenacibaculum maritinum.

Flavobacterium psychrophila causes coldwater (peduncle) disease, or bacterial coldwater disease, in salmonids and other coldwater species. Rarely, warmwater fish exposed to cold temperatures may be affected. Disease is most severe at water temperatures of 4–10°C, and signs should not be seen at temperatures >18°C. Skin lesions usually begin on the dorsal and posterior surfaces of the fish, but may be found on any part of the body. Advanced cases show necrosis and ulceration of the peduncle, and underlying musculature will be exposed. As the disease progresses the infection becomes systemic, typically involving the spleen, kidney, and liver. Confirmed diagnosis is possible following isolation of F psychrophila using cytophage media (15–20°C for 3–6 days), by immunohistochemistry or PCR. Outbreaks can be controlled with oxytetracycline.

Bacterial gill disease, caused by F branchiophilum, is most frequently reported in young cultured salmonids or fish cultured under conditions of high organic loading. It has been seen occasionally in aquarium fish. It may be initiated by crowding and poor water quality, particularly high organic loads, high ammonia levels, and silt. Gills appear swollen and mottled, with patchy areas of bacterial growth that can be confirmed by microscopic examination of direct gill smears. Hyperplasia, adhesions, and deformity of the gill lamellae can be seen. In young fish affected with the disease, mortality is high and morbidity sustained. Prevention efforts include improving water quality and avoiding overstocking. A single treatment with potassium permanganate, followed by addition of salt to the system (2–5 ppt) may be beneficial in controlling losses, but sanitation is critical for longterm resolution of the problem. Antibiotic therapy may be used as needed to control secondary bacterial problems.

Bacterial kidney disease is economically important in cultured salmonids. The cause is Renibacterium salmoninarum, an obligate intracellular bacteria that is one of the few gram-positive organisms that causes disease in fish. Clinically, infected fish appear lethargic and darkened. Typical lesions include grayish, localized, or conglomerate granulomata in the viscera, especially the kidney or body wall; exophthalmos; blindness; and emaciation. A presumptive diagnosis can be based on visualization of small gram-positive rods in kidney imprints. Definitive diagnosis requires isolation and identification of the bacteria by using a selective medium that contains cysteine and incubating at 15°C for 3–6 wk. R salmoninarum is transmitted both horizontally and vertically, and fish that survive an epizootic remain carriers. Infected female fish should be injected with erythromycin (11–20 mg/kg, IM) 14–60 days before spawning to prevent vertical transmission. Erythromycin (100 mg/kg for 10–21 days) is effective when administered in feed early in the course of an outbreak; however, it is not FDA approved for this use. Obtaining disease-free stock and preventing contamination by infected wild fish are the best preventive measures.

Gram-positive bacterial infections of concern to fish culturists and aquarists may be caused by Streptococcus and related genera, Lactococcus, Enterococcus, and Vagococcus. Infections are uncommon but can cause significant mortality (>50%) when they do occur. Chronic infections may continue for weeks, with only a few fish dying each day. Species known to be susceptible include salmonids, assorted marine fish (eg, mullet, sea bass), tilapia, sturgeon, and striped bass. Susceptible aquarium species include rainbow sharks, red-tailed black sharks, rosey barbs, danios, and some tetras and cichlids. In general, all fish should be considered susceptible. A characteristic manifestation of Streptococcus infection is neurologic disease, often manifest by spinning or spiraling in the water column. Brain and kidney cultures from suspect fish should be incubated on blood agar at 25°C for 24–48 hr. Gram stains of pinpoint bacterial colonies reveal typical chains of gram-positive cocci, which allow a presumptive diagnosis. Confirmation requires definitive identification of the organism. Antibiotic therapy should be based on sensitivity testing. Erythromycin is often the drug of choice but it is not FDA approved for this use. Sources of infection may be environmental or include live foods, such as tubifex worms, amphibians, or previously infected fish. Future epizootics can be prevented if the source of infection is identified and eliminated. Streptococcus innae has been isolated from tilapia and aquarium fish, and has zoonotic potential. Autogenous vaccines are available for use in aquaculture facilities.

Mycobacteriosis is a chronic or acute, systemic, granulomatous disease that occurs in aquarium fish and cultured food fish, particularly those reared under intensive conditions. Predisposing environmental factors include low dissolved oxygen, low pH, and high organic load, all of which are found in recirculating aquaculture systems. Correct use of ultraviolet light as a means of disinfecting system water reduces bacterial counts and can be a useful tool in controlling infection in exhibit animals. The causative bacteria can be any number of species of Mycobacterium, including M piscium, M marinum, and M fortuitum. Syngnathids (sea horses) are particularly susceptible, but the disease can occur in any fish. These gram-positive, acid-fast, nonmotile bacteria are difficult to grow but can be isolated using Lowenstein-Jensen or Middlebrook media following incubation at 25°C for 3–4 wk. Signs are variable and nonspecific; they can include emaciation, ascites, skin ulceration and hemorrhages, exophthalmos, paleness, and skeletal deformities. On necropsy, gross lesions of viscera consist of grayish white, necrotic foci that sometimes coalesce to form tumor-like masses. Granulomas may not be grossly visible, and are often found first on wet mounts of spleen or other viscera from infected fish. A presumptive diagnosis is based on visualization of acid-fast rods in granulomatous material from suspect lesions. Definitive diagnosis requires isolating and identifying the bacteria. There are no effective treatments that eliminate mycobacteria in fish. Mycobacteria can cause zoonotic infections, and aquarists should be informed of potential risks if handling or cleaning contaminated fish or exhibits. An infected aquarium should be disinfected before other fish are added. Bleach is not an effective disinfectant against mycobacteria; disinfection with alcohol or phenolic compounds is recommended.

Rickettsial disease associated with Piscirickettsia salmonis has been described in salmonid species from Chile, Norway, Ireland, and Canada. Rickettsial-like organisms have been reported in tilapia, sea bass, and blue-eyed plecostomus. Rickettsial disease can result in acute mortality affecting up to 95% of fish with few gross signs. In tilapia, acute mortality may be triggered by sudden drops in temperature. Chronic disease is manifest by nonspecific external lesions including anorexia, pale gills, and skin lesions. Internally, lesions are more typical, with granulomatous lesions possible throughout the viscera. The most characteristic lesions may be found in liver and kidney tissue, and appear as gray to yellow mottled areas with ring-shaped foci. Visceral lesions are grossly similar to those seen in advanced cases of mycobacteriosis, and differentiation is important. Histologically, intracellular organisms may be seen in macrophages and hematopoietic tissue in the liver, spleen, and kidney. Blood or tissue smears stained with Giemsa or acridine orange may reveal the intracellular organisms, often appearing as paired, curved gram-negative rods in macrophages or hepatocytes. Rickettsia-like organisms can be isolated using a variety of cell lines; however, confirmation of a suspect case may also be based on serology.

Transmission of rickettsial-like diseases in fish is not understood. In terrestrial species a vector is often required; however, R salmonis has been demonstrated to survive for 14 days in sea water, suggesting that horizontal transmission in the absence of vectors may be possible in aquatic species. Oxytetracycline is the treatment of choice, though it is unclear whether an advanced case can be completely resolved with antibiotic treatment. Rickettsia-like organisms do not appear to be a zoonotic threat, as they do not seem able to survive at mammalian body temperatures.

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

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