| 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, and related stressful conditions. High concentrations of waterborne bacteria are normally found in ponds and aquaria. Many of these bacteria are opportunistic facultative pathogens, which are
activated by an adverse environment, a debilitated host, or a primary pathogen. In sharp contrast, obligatory bacterial pathogens of finfish require the presence of fish for replication and are unable to survive alone for long in the aquatic environment. |
| Most bacterial pathogens of fish are aerobic gram-negative rods. Diagnosis is by isolating the organism in pure culture from infected tissues and identifying the bacterial agent. |
| Some of the most common bacterial infections that are associated with stressful conditions in freshwater systems are
Aeromonas
hydrophila
,
A
sobria
, and
Pseudomonas
spp
. These opportunistic organisms frequently cause disease in freshwater fish, usually under warmwater conditions, although all species are susceptible. They produce very similar clinical syndromes, sometimes referred to as
hemorrhagic septicemia. Nutritional deficiencies, traumatic injuries, parasitism, and sharp seasonal temperature changes appear to be predisposing factors. The acute form is characterized by signs of a septicemic infection with external reddening, and hemorrhages are found in the peritoneum, body wall, and viscera. 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
is a gram-negative, nonmotile, rod originally described as the cause of a septicemic disease of salmonids (furunculosis) and goldfish (
ulcer disease). It is also a pathogen of many other freshwater and marine fishes and may produce high mortality.
A
salmonicida
is a very important disease of koi and fancy goldfish, and is of particular concern in imported specimens. 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 crateriform abscesses that discharge from the skin surface (furuncles). 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. Vaccination is available for koi, but information on efficacy is limited. 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. Prevention and control of
A
salmonicida
in fish and eggs is also of concern to the salmonid industry. Fish and fish eggs should be obtained from disease-free sources. Eyed fish eggs can be treated for 10-15 min in a solution containing iodine at 100 mg/L at a pH of 7 and a temperature of 10-15°C, although the drug is not approved for use on food-fish eggs. Eggs should be rinsed immediately after treatment. Antibiotic treatment should be based on results of sensitivity testing. Commercial vaccines are available
for prevention of
A
salmonicida
in salmonids. |
|
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, and 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. Oral 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 order Cytophogales (Myxobacterales, slime bacteria, fish slime bacteria) includes an important group of opportunistic pathogens of fish that are common inhabitants of soil and water. The gram-negative, rod or filamentous bacteria have a distinctive gliding motion and form palisading masses on infected tissue. Skin or gill lesions have slimy or cotton-like surface
exudates, which usually cover surface necrosis, ulcerations, and marginal hemorrhages.
Flavobacterium
columnaris
, the member of this group responsible for
columnaris disease
(cottonmouth disease, saddleback), is most common in warm water and warmwater species of fish. A presumptive diagnosis can be made from visualization of typical myxobacteria on wet mounts of infected skin or gill tissue. If the disease is diagnosed early in the course of infection, a treatment with potassium permanganate may be efficacious. If the disease becomes chronic, it
may have become systemic, in which case treatment with oxytetracycline for 10 days is recommended. 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
columnaria
will not grow on Müller-Hinton media. Columnaris disease can be prevented by reducing organic loading and avoiding traumatic injuries.
Cytophaga
psychrophila
causes coldwater (peduncle) disease and fin and tail rot. It most commonly infects coldwater fish but can be found in warmwater fish subjected to low temperatures. The lesions are especially common on the dorsal, posterior surface of the fish under the dorsal fin but may be found on any part of the body. Advanced
cases show necrosis and ulceration of the peduncle. Both
Flavobacterium
and
Cytophaga
infections can be controlled by oxytetracycline.
|
|
Bacterial gill disease is a disease complex 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. Although bacteria in the genera
Flavobacteria
have been implicated as the cause, the association is still somewhat tentative. Opportunistic bacteria, including
Aeromonas
and
Pseudomonas
spp
, may follow as secondary invaders of traumatized gill tissue. Signs of the disease are related to respiratory embarrassment due to impaired gill function. 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 (0.02-0.5%), depending on species and type of system, 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
(corynebacterial kidney disease) is economically important in cultured salmonids. The cause is
Renibacterium
salmoninarum
, an obligate intracellular parasite 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 efficacious 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 bacteria of concern to fish culturists and aquarists include
Streptococcus
and related genera,
Lactococcus
,
Enterococcus
, and
Vagococcus
. Infections caused by these organisms 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 allows 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. Mortality should cease within 48 hr of treatment with the correct antibiotic if complicating factors are not present. If fish are anorectic, therapy may be difficult because erythromycin does not absorb well when administered as a bath. 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
iniae
has been reported to cause disease in fish and mammals, including humans. In aquaculture, it is encountered most frequently in the intensive culture of tilapia and hybrid striped bass. Human cases have been reported rarely; in several cases individuals developed
S
iniae
infection after handling raw fish, primarily tilapia, with open wounds on their hands. Most affected individuals had significant predisposing disease. The most common problem reported was cellulitis, and infections resolved following antibiotic therapy. When
Streptococcus
is isolated from aquatic species, a subculture of the organism should be sent to a laboratory that can determine whether or not it is
S
iniae
. If confirmed, at-risk personnel should be restricted from handling infected fish. |
|
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. The causative bacteria can be any number of species of
Mycobacterium
, including
M
piscium
,
M
marinum
, and
M
fortuitum
. These gram-positive, acid-fast, nonmotile bacteria are difficult to grow but can be isolated using Lowenstein-Jensen media following incubation at 25°C for 3-4 wk. Signs are variable and often resemble those of other diseases, including 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. 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. Because the disease can produce skin lesions and an allergic dermatitis in humans, and because treatment does not eliminate the disease, infected fish are usually destroyed. Aquarists should be informed of potential risks if handling or cleaning contaminated fish or exhibits. An aquarium should be
disinfected before other fish are added. Mycobacteria are not always killed by bleach; disinfection with alcohol or phenolic compounds is recommended. |
|
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. It is unclear whether organisms reported in different groups of fish are separate species. Of the salmonid species, coho salmon seem to be the most susceptible. These organisms do not appear to be a zoonotic threat as they do not seem able to survive at mammalian body temperatures. Rickettsial disease
can result in acute mortality affecting up to 95% of fish with few gross signs. In tilapia, acute mortality events 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 have been described as gray to yellow
mottled areas with ring-shaped foci. 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. Antibiotic control of these organisms appeared promising from in vitro work, but in vivo studies have not been as successful. It may be that intracellular organisms are sequestered from antibiotics. |
| Many other less commonly recognized bacterial diseases have been described in fish. These include pasteurellosis,
Haemophilus
piscium
infection (ulcer disease),
Eubacterium
infection
(fish meningitis), and others. Diagnosis of these diseases requires isolation and identification of the specific bacterial agent. |
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