Bacterial Diseases of fish
1. Vibriosis:
Vibriosis is also known as Vibrio hemorrhagic septicemia and is often associated with another
disease, the red boil disease, which is caused by Streptococcus sp. The disease has been reported in
Epinephelus malabaricus, E. tauvina, E. coioides and E. bleekeri. Vibriosis has been recorded in
cultured groupers in Brunei Darussalam, Malaysia, Taiwan, Indonesia, Kuwait, Thailand, Singapore
and the Philippines.
Causative agents:
The causative agents of vibriosis are Vibrio parahaemolyticus, V. alginolyticus, V. vulnificus and V.
carchariae.
Stages affected:
The bacteria may affect grouper fry, fingerlings, juveniles, adults and broodstocks.
Gross clinical signs:
The first sign of the disease is anorexia or loss of appetite with darkening of the fish coloration. The
fish may be lethargic, swimming near the water surface. Affected fish may lose equilibrium and
exhibit abnormal swimming behavior. One of the signs of the disease is body ulcer that may be
hemorrhagic. Fin rot, which usually starts with erosion of the tip of the fin and gradually becomes
necrotic, may also be observed. Exophthalmia and corneal opacity are also common signs of the
disease. Internally, bloody discharges may be observed in the abdominal cavity due to internal organ
hemorrhage. In the case of V. carchariae, gastroenteritis manifested by a swollen intestine
containing yellow fluid may be observed.
Effects on host:
Ten to 50% mortality may be observed in affected populations depending on the farm management.
It has also been observed that infected grouper harbor monogeneans and protozoans on the gills
and skin. Vibrio sp. infection is also associated with red boil disease. In relatively colder countries like
Taiwan, outbreaks may occur in summer when ambient temperatures rise.
Transmission:
The spread of the disease has been correlated with high salinity (30-35 ppt). The bacteria enter the
fish through damaged areas. The disease may be transmitted through the water and trash fish given
to the grouper. Infection is enhanced by parasitic infestation and mechanical injuries during
transport, and grading.
Diagnosis:
Squash preparation of affected areas examined under the microscope may reveal the bacteria.
Vibrios are Gram-negative straight or curved rods, 0.5-0.8 µm in width and 1.4-2.6 µm in length. The
bacteria may be isolated from the infected organ using tryptic soy agar (TSA), nutrient agar (NA), or
brain heart infusion agar (BHIA) supplemented with NaCl. Thiosulfate citrate bilesalt sucrose agar
(TCBS), a Vibrio selective medium, may be used to isolate and primarily identify associated main
groups of vibrios. Opening of the abdominal cavity will reveal hemorrhagic internal organs. The
kidney may be swollen and filled with yellow fluid.
Preventive methods:
Rough handling of the fish during stocking, sampling, changing of nets, grading and overcrowding
should be avoided. Good water quality must be maintained.
Control methods:
The disease may be controlled through freshwater bath for 10-15 minutes. Affected fish may be
treated with oxalinic acid mixed with feed at 20 mg/kg of fish. Terramycin added to feed at 7.5 g/kg
for 5 days, reduced to 3.75 g/kg for the succeeding 5 days also proved effective. Prefuran bath
treatment for 1 hour at 2 ppm may also be implemented (see Appendix 1 for points to consider
before using antibiotics and a list of anti-infectives recommended for use in marine food fish
together with the withdrawal period).
2. Pseudomonas Infection
The disease is also known as pseudomonad hemorrhagic septicemia. The only reported case of the
disease is in cage-cultured Epinephelus tauvina in Malaysia.
Causative agent:
The causative agent is Pseudomonas sp.
Stages affected:
The bacteria may affect grouper at all stages.
Gross clinical signs:
Infected fish have extensive hemorrhagic erosions of the body. Ulcerations on the skin, fins and tails
may also be observed. Other common signs of the disease are exophthalmia and corneal opacity.
Effects on host: Twenty to 60% mortality may be observed in affected populations. A secondary
epibiont fouling may also be observed. Internally, there is renal fragility, and dark red multifocal
hepatic discoloration. Histologically, pathological changes consistent with subacute bacterial
septicaemia are observed in the internal organs. Diffuse pericarditis and marked multifocal
endocardial thrombosis and embolism are observed in the heart. Thrombosis and embolism are also
observed in the hepatic vein. Diffuse pancreatic acinar cell atrophy and mononuclear cell infiltration
are observed in the pancreas.
Transmission:
Pseudomonas spp. are ubiquitous in the aquatic environment. Pseudomonas infects fish when it is
subjected to environmental stressors such as extreme water temperature changes, overcrowding,
poor water quality and sub-optimal nutrition.
Diagnosis:
Squash preparation of the kidney and other internal organs as well as the affected areas may reveal
large colonies of Pseudomonas sp. Pseudomonads are Gram-negative, straight or slightly curved rods
but not helical in shape, 0.5-1.0 µm in width and 1.5-5.0 µm in length. The bacteria may be isolated from the infected organ and affected areas using glutamate starch phenol red agar (GSP) or
Pseudomonas-Aeromonas selective agar, a medium that selectively promotes growth of Aeromonas
and Pseudomonas but inhibits growth of other bacteria. Internal examination of affected fish will
show renal fragility and dark red multifocal discoloration of the liver.
Preventive methods:
Avoid the predisposing factors such as extreme water temperature changes, overcrowding, poor
water quality, and sub-optimal nutrition to prevent pseudomonad infection.
Control methods:
Transferring affected fish into another tank with clean water may control the infection.
3. Streptococcal infection
The disease is also known as red boil disease. The disease is often associated with vibriosis. The
disease has been reported in Epinephelus malabaricus and E. bleekeri in Brunei Darussalam,
Malaysia, Singapore and Thailand.
Causative agent:
The causative agent is Streptococcus sp.
Stages affected:
Streptococcus sp. could infect grouper at all stages but is common in fry and fingerling stages. Gross
clinical signs: Affected fish are weak and display disoriented whirling motion. Exophthalmia and
hemorrhages on the cornea, operculum, around the mouth and the anus could also be observed.
Infected fish have red boils in the skin.
Effects on host:
The red boils on the skin enlarge and eventually burst, exposing the necrotic musculature
underneath and form small ulcers, which act as portals of entry for other bacteria. The disease may
also cause systemic infections with few external signs. The infection could cause 10% mortality in
affected fry but is not fatal to older fish.
Transmission:
The bacterium is ubiquitous in the environment, in the water and in carrier fish. Spread of the
disease is associated with the presence of parasites, handling stress and sub-optimal water quality.
Diagnosis:
The bacteria grow well on BHIA, Todd-Hewitt agar, horse agar and TSA supplemented with 0.5%
glucose. The colonies on agar plates appear small (0.5-1.0 mm diameter), yellowish, translucent,
rounded and slightly raised. Streptococcus spp. are Gram-positive bacteria with ovoid or spherical
cells, less than 2 µm in diameter, occurring in pairs or chains.
Preventive methods Avoid the predisposing factors such as the presence of parasites, handling stress and suboptimal
water quality to prevent disease occurrence.
Control methods:
Affected grouper could be treated with oxolinic acid mixed with feed at 20 mg/kg of fish, and
perfuran bath for 1 hour at 2 ppm (see Appendix 1 for points to consider before using antibiotics and
a list of anti-infectives recommended for use in marine food fish together with the withdrawal
period).
4. Flexibacter infection
Flexibacter spp. are long rod-shaped, Gram-negative bacteria with parallel sides and rounded ends,
typically 0.5 µm wide and 1-3 µm long. The bacteria do not possess flagella and move by gliding, thus
are also known as the gliding bacteria. Some Flexibacter spp. are widespread opportunistic bacterial
pathogens. Some species of these yellow-pigmented bacteria have been associated with diseased
fish, including Flexibacter columnare and F. maritimus, as well as other unidentified gliding bacteria
referred to as Cytophaga-like bacteria. Flexibacter spp. are reported to cause bacterial gill disease
and fin rot in groupers.
5. Bacterial Gill Disease
Causative agents:
The disease is caused by Cytophaga sp., Flexibacter sp. or Flavobacterium sp.
Stages affected:
The bacteria usually attack fingerlings. Gross clinical signs: Affected fish become anorexic, lethargic
and dark in color. Fish tend to remain near the surface and may be flaring their operculum. The gills
produce excessive amounts of mucus and the gill filaments may stick together. The gills of affected
fish become yellowish in color indicating gill rot.
Effects on host:
A high mortality rate of >80% may be observed within a week in affected populations. The bacteria
attach to the gill surface, grow in spreading patches and eventually cover individual gill filaments
that result in cell death. Gill lesion may cause respiratory difficulty and the fish eventually dies.
Histologically, fusion of the secondary lamellae, epithelial hyperplasia and presence of rod-shaped
bacteria could be observed in the gills.
Transmission:
The disease starts when the water quality deteriorates after a heavy rain. Silt and suspended organic
particles from run-offs could irritate the gills and increase susceptibility to the disease. Low dissolved
oxygen and high ammonia levels are often observed during disease outbreaks. Stress during grading
makes fish susceptible to bacterial infection.
Diagnosis:
The disease is diagnosed by the presence of brown to yellow brown growth of bacteria in the gills.
Microscopic examination of wet mounts of the gill will reveal bacteria that are in a slow gliding
movement. Flexibacter sp. is a thin, long Gram-negative rod with parallel sides and rounded ends,
typically 0.5 µm wide and 1-3 µm long, that grows in layers, one on top of the other, giving it the
appearance of “columns” or “haystacks” under the microscope.
The bacteria may be isolated using a selective media such as Selective Cytophaga agar and HsuShotts medium. Colonies produced are pigmented yellow with rhizoids or root-like appearance.
Preventive methods:
Maintain good water quality and minimize stress by avoiding overcrowding, low dissolved oxygen
and high ammonia levels. The occurrence of the disease could also be prevented by vaccination.
Control methods:
Transferring affected fish into another tank with clean water may control the infection. Affected fish
may be treated with oxolinic acid mixed with feed at 20 mg/kg of fish and oxytetracycline at 75
mg/kg of fish/day for 10 days. Acriflavin dip at 100 ppm for 1 minute, and potassium permanganate
at 2-4 ppm added to the water and allowed to dissipate over time could also be used to treat
diseased fish
0 Comments