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Bacterial disease is extremely common in fish and is most frequently associated with bacteria that are ubiquitous in the aquatic environment acting as opportunistic pathogens secondary to stress. Less commonly, bacterial disease is caused by primary or obligate pathogens. Bacterial disease in fish is complex and involves the interplay of various factors including the environment, the host (immune system function, host susceptibility, host's injuries, etc.), and pathogen-specific factors such as virulence. Stress can result in immune suppression and is critical in the pathogenesis of bacterial disease in fish with poor environmental conditions as the most common stressor involved in precipitation of bacterial disease. Water quality should routinely be assessed when investigating any disease outbreaks in aquatic organisms. Major bacterial pathogens in fish can be divided into the following four major groups and one newly emerging group of pathogens:
- Ulcer-forming or systemic, gram-negative bacteria. This group includes bacteria in the genera Aeromonas, Citrobacter, Edwardsiella, Flavobacterium, Pseudomonas, and Vibrio. This is the most common group of bacterial pathogens that affect fish.
- External, gram-negative bacteria. This group of bacteria most commonly causes external infections. Some of these bacteria may also cause systemic infections. Included in this group are Flavobacterium columnarae, Flexibacter maritimus, yellow-pigmented bacteria, Cytophaga spp, and others.
- Systemic, gram-positive, rapidly growing bacteria. These bacteria generally cause systemic infections and include Streptococcus spp and related species.
- Slow-growing, acid-fast bacteria (mycobacteria). These bacterial pathogens cause systemic, chronic, granulomatous disease. The most common pathogens in this group are Mycobacterium spp.
- Itracellular rickettsial pathogens (newly emerging).
Clinical signs of bacterial disease may be peracute (mortality without gross evidence of disease), acute (severe and sudden in onset), or chronic (a long-lasting condition that can be controlled but difficult to cure). and varies with the particular pathogen and various host related factors. With misuse of antibiotics, antibiotic resistance is becoming more important in treating bacterial diseases in fish.
Bacteremia is defined as the presence of bacterial organisms in the bloodstream, whereas septicemia implies that bacteria are actually present and multiplying within the body. Both conditions are likely to be lethal.Back to top
This is the most common group of bacterial pathogens that affect fish. It includes bacteria in the genera Aeromonas, Edwardsiella, Flavobacterium, Pseudomonas, and Vibrio, and others. Clinical signs of ulcer-forming and systemic infections caused by gram-negative bacteria include lethargy, anorexia, abnormal swimming patterns or spinning, hemorrhagic lesions on the skin, ulcerative skin lesions, abdominal distension, ascites, abnormal position in the water column, exophthalmia ("pop-eye"), skin darkening, gill necrosis, and high mortality. With gill involvement, respiratory signs such as increased opercular rate and piping (gasping for air at the water surface) may be seen.
Motile aeromonads are the most common bacterial pathogens of fish and may result in a syndrome called motile aeromonad septicemia (MAS). MAS is most commonly caused by ubiquitous aquatic bacteria of the Aeromonas hydrophila complex, including A. hydrophila, A. sobria, and A. caviae. A. hydrophila is the most common isolate and is more commonly isolated from freshwater than marine fish. MAS is almost always secondary to an underlying stressor and is most commonly found in conjunction with eutrophic (rich in nutrients, lack of oxygen) water quality conditions. In fish ponds, this aeromonad is commonly isolated from clinically ill fish in the warmer months of the year.
Common clinical signs include cutaneous hemorrhages and ulcers that can be deep through the dermis to connective tissue and muscle (ulcerative dermatitis (UD)), visceral hemorrhages. Osmotic distress due to loss of epidermal integrity may result in fluid retention, edema, dropsy or ascites, and exophthalmia. Numerous pathogens have been isolated from these cutaneous lesions including A. hydrophila and A. salmonicida (non-motile aeromonad) as well as Pseudomonas spp, Citrobacter spp, Chryseobacterium spp, Delfia spp, and Shewanella putrefaciens.
A. salmonicida is a non-motile species of Aeromonas that causes a chronic-to-subacute bacterial infection resulting in cutaneous ulcerations of the skin. There are three reported subspecies of A. salmonicida: A. salmonicida subsp. achromogenes, A. salmonicida subsp. masoucida, and A. salmonicida subsp. salmonicida. A. salmonicida subsp. salmonicida is usually associated with systemic infections and is the causative agent of furunculosis in salmonids. A. salmonicida subsp. achromogenes is more commonly associated with ulcerative skin lesions in non-salmonid species such as goldfish, common carp, and eels. It is also the causative agent of carp erythrodermatitis and has been implicated as the causative agent of UD in koi. A. salmonicida is considered an obligate pathogen of fish, but carrier fish can occur.
Systemic antibiotics are often used in the treatment of this condition. Prolonged bath immersion treatment with salt at 0.1% to 0.3% is typically used to decrease osmotic stresses. In some cases, debridement of the ulcer and removal of necrotic tissue or scales may be necessary. Various topical treatments such as silver sulfadiazine can also be helpful. No treatment should be initiated without assessing and addressing water quality concerns.
Fish injected with pathogenic bacteria Aeromonas hydrophila showing one or more typical signs of infection according to the stage of disease: hemorrhagia, shallow to deep necrotizing ulcers, and abdominal distension with sero-hemorrhagic fluids exuding from the vent.
Kumar N et al. PLoS One. 2014; 9(4): e93499.
Aeromonas+fish in PubMedBack to top
Most Pseudomonas-associated septicemia cases in ornamental as well as sometimes in wild species are due to P. fluorescens. Infections are more common in warmer water temperatures and are typically secondary to environmental stressors. There is still disagreement among fish health specialists if Pseudomonas represents a primary or secondary pathogen or a non-pathogenic environmental contaminant.
Fish with septicemia are generally anorexic, lethargic, and depressed.
Pseudomonas+fish in PubMedBack to top
Bacteria of the genus Vibrio are ubiquitous in the marine and estuarine environment and most commonly cause disease in marine fish. In a survey of 129 cases from tropical marine fish, nearly 50% had either Aeromonas or Vibrio infections. Species that have been shown to cause disease in fish include V. anguillarum (most common), V. salmonicida, V. ordalii, V. alginolyticus, V. parahaemolyticus, V. vulnificus. V. anguillarum and V. ordalii are the two most commonly identified species from marine ornamentals. Clinically, vibriosis is very similar to motile aeromonad septicemia (MAS) and results in hemorrhagic septicemia with cutaneous hemorrhages and ulcers. As with other bacterial infections, underlying environmental stressors are often present.
Vibrio+fish in PubMedBack to top
Synonym: Pasteurella piscicida
One common species recently classified in family Vibrionaceae causing pathology in marine tropical fish is Photobacterium damselae subsp. piscicida. It is found in a variety of freshwater and marine tropical and temperate species. It can manifest in an acute outbreak with varying mortality, or the disease can be marked by an insidious chronic form with pseudotubercle (nodules and granuloma) formation in the kidney and spleen with internal necrosis. This disease was formerly known as fish pasteurellosis. Outbreaks are associated with high temperatures and can cause catastrophic losses in several species of marine fish. This disease has been treated using various oral antibiotics.
Photobacterium+fish in PubMedBack to top
Synonyms: Paracolobactrum anguillimortiferum, Edwardsiella anguillimortifera
Edwardsiella tarda is a gram-negative rod associated with septicemia with or without ulceration in a wide variety of freshwater and marine species. Disease associated with Edwardsiella tarda often occurs during the summer in ponds. Reports of disease in ornamental fish have been reported in coral reef grunts and squirrelfish, and a keyhole angelfish. This bacterium can infect all poikilothermic vertebrates and is zoonotic. In disease outbreaks, antibiotics are commonly added to feed.
External signs of olive flounder infected with Edwardsiella tarda. A: External lesions of diseased fish. B: Abdominal distension and rectal hernia. C: Exophthalmia and opacity of the eye. D: Peripheral hyperemia in mandible lesion.
Park SB et al. Vet Res. 2012; 43(1): 67.
Edwardsiella+fish in PubMedBack to top
This is the most common group of bacterial pathogens that affect fish and includes bacteria in the genera Aeromonas, Edwardsiella, Flavobacterium, Pseudomonas, and Vibrio, and others. Clinical signs of ulcer-forming and systemic infections caused by gram-negative bacteria include lethargy, anorexia, abnormal swimming patterns or spinning, hemorrhagic lesions on the skin, ulcerative skin lesions, abdominal distension, ascites, abnormal position in the water column, exophthalmia ("pop-eye"), skin darkening, gill necrosis, and mortality. With gill involvement, respiratory signs such as increased opercular rate, piping (gasping for air at the water surface) and respiratory distress may be seen.
Synonyms: Flexibacter columnaris, Flavibacterium columnare, Cytophaga columnaris, Chondrococcus columnaris, Bacillus columnaris
Flavobacterium columnare is common pathogen of freshwater ornamental fish such as guppies, platies, mollies, and swordtails. F. columnare seems to be an opportunistic pathogen. Predisposing factors include crowding, poor water quality, shipping, inadequate nutrition, and parasitic or other bacterial disease.
Columnaris results in cottony proliferative lesions on the skin and fins. Common locations for the lesions include perioral, periocular, fins, dorsum, and tail regions. The synonym "cotton-wool" disease describes the fluffy white cotton-like masses, patches, or plaques often seen with F. columnare. Given this clinical appearance, columnaris disease is often misdiagnosed as fungal disease in aquarium fish; wet-mount examination of affected areas can be used to differentiate between these two conditions and rule out parasitic infestations. F. columnare may also affect the gills resulting in respiratory signs.
Treatment of columnaris disease can be achieved with antibiotic bath treatment with oxytetracycline repeated daily for 10 days. Other treatment options include potassium permanganate as a prolonged bath, copper sulfate, and diquat herbicide dosed at 2 to 18 mg/L for four-hour bath immersions. Treatment should be repeated daily for three to four treatments with large water changes after each bath treatment. Systemic antimicrobials may be needed in more severe infections. One study found that a single hydrogen peroxide (H2O2) treatment of 3.1 mg/L or more for one hour effectively eliminated external bacteria in the green swordtail Xiphophorus helleri.
Flavobacterium columnare in PubMedBack to top
The most common bacteria in this group that cause disease in fish are Streptococcus spp.; other gram-positive genera that are closely related to Streptococcus and cause disease in fish include Lactococcus, Enterococcus, and Vagococcus.
Streptococcus & kin
Clinical signs are similar to those involving systemic gram-negative infections such as skin discolorations, exophthalmos, ascites, skin ulcerations, and hemorrhages. Neurologic signs are extremely common in fish with streptococcal infections; abnormal swimming behavior such as spiraling or spinning is often reported. High mortality may also occur. On gross necropsy, Streptococcus can cause granulomas in the kidney, spleen, or liver.
Antibiotics that may be effective against Streptococcus and related species include erythromycin (1.5g/lb of food fed for 10–14 days), amoxicillin or ampicillin, and florfenicol. Immunostimulants added to the feed, such as beta-glucans and nucleotides, have been shown to increase survival for Streptococcus-infected redtail black shark populations. In addition to being a human pathogen, Streptococcus iniae causes disease in cultured marine and freshwater fish and is difficult to eradicate from hatcheries and grow-out facilities.
Streptococcus+fish in PubMedBack to top
Mycobacteriosis in fish is caused by nontubercle-forming Mycobacterium species that are ubiquitous in the aquatic environment. A recent report found 75% of water samples from decorative aquaria to be positive for Mycobacterium spp. Environmental factors that favor growth of mycobacterium include low dissolved oxygen, low salinity, low pH, warmer water, and high organic loads. The two most common species associated with ornamental fish disease are M. marinum and M. fortuitum.
The primary route of infection is through the intestinal tract. Fish can be infected by consuming contaminated feed, by way of cannibalism of infected fish and carcasses or aquatic detritus. Mycobacteriosis is zoonotic and can cause "fish tank granuloma" in people.
Clinical signs of mycobacteriosis are usually nonspecific and can include ulcerative skin lesions, reduced appetite, emaciation, lethargy, exophthalmia ("pop-eye"), swollen abdomen, anorexia, fin or tail rot, and skeletal abnormalities. This disease is usually chronic, slowly progressive, and causes low-to-moderate mortalities. Mycobacteriosis is the most commonly diagnosed chronic wasting disease of aquarium fish. On internal examination, granulomas will develop in the liver, kidney, spleen, heart, muscle, gill, and other tissues. Granulomas are typically pale gray to tan but are only visible to the naked eye in more advanced cases.
There is no effective cure for mycobacteriosis in fish. In aquaculture, retail, and wholesale situations, depopulation and disinfection is often recommended. Treatment is often different in public aquaria or zoos and private collections, and mycobacteriosis is often managed differently at these institutions and locations. Various antibiotics such as rifampin, erythromycin, streptomycin, kanamycin, doxycycline and minocycline have been suggested as possible treatments, but a clinical cure is unlikely.
Macroscopic characteristics of M. marinum chronic (A) and acute (B) infections. (A) Intraperitoneal infection with strain Mma11 induces skin ulcerations at 7 wpi (arrows). These ulcerations were usually not located at the site of infection. (B) Intraperitoneal infection with strain Mma20 causes a swelling of the abdomen and severe hemorrhages within 2 wpi (arrow and white dashed line). No ulcerations were observed in control animals.
van der Sar AM et al. Infect Immun. 2004 Nov;72(11):6306-12.
Mycobacteria+fish in PubMedBack to top
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