Friday, October 12, 2007

Fish DIsease Again

Fish Diseases – Bacteria

1. Cytophagaceae
• Columnaris Disease

2. Enterobacteriaceae
• Edwardsiela septicaemia
• Enteric Redmouth (ERM)

3. Vibrionaceae
• Vibriosis
• Furunculosis
• Motile Aeromonad Septicaemia

4. Pasteurellaceae
• Pasteurellosis

5. Pseudomonadaceae
• Pseudomonas

6. Gram positive Fish Pathogens
• Streptococcal Infections

7. Acid Fast Pathogens
• Mycobacteriosis

Columnaris Disease

• Agent: Flexibacter columnaris
• Morphology: Gram negative, long,thin,aggregates in mounds on slides
• Culture: grows well on cytophaga agar, yellow colonies, fried egg appearance, optimum temp 15C.
• Epizootiology: 36 different species, salmonids and catfish most significant
• Reservoirs: wild fish
• Transmission: horizontal, water, experimental via moribund carcasses
• Environmental factors: elevated temps, temps depends on host species, usually 15C, severity increases with temp, high pH, hard water, organic matter, cleanliness important, crowding

• Patho: lesions confined to head, back, gills, starts as raised whitish spot distal on fins, gills; gradually develops into larger ulcer, bacteria isolated from leading edge of ulcer; skin eventually erodes away, exposing muscle; death rapid if necrosis/lesions are on gills due to respiratory problems, highly virulent strains cause death w/out lesions

• Dx: long, thin gram negative rods from lesions; rhizoid colonies on cytophaga agar; haystacking, clinical signs & serology
• Ctl; improved environment, cooler water temp, increased oxygenation , decreased crowding, organics, reduced stress
• Tx: internal via oxytetracycline in feed, sulfonamides

Edwardsiella Septicemia

• Agent: Edwarsiella tarda
• Morphology: Gram negative, rod
• Culture: gram negative rod, motile by flagella, grows well on most standard media(TSA, BHI), produces small transparent and smooth circular colonies at 35C ferments glucose and produces gas , indole positive (diff. From E.Ictaluri)
• Epizootiology: southern U.S, SE Asia , Pacific NW, many warmwater species of fish
• Reservoir: pathogen of or can be carried by many vertebrates and invertebrates
• Transmission: horizontal
• Environmental factors: higher than 30C water, organics, crowding; salmonids at temps greater than 20C

• Patho: fish large 38 cm( high value), slow progress, low mortality unless fish stressed (5--50%mort) , causes development of gas filled abscesses containing sulfide; mild infections exhibits small cutaneous postlateral lesions, progressing as abscesses in muscles of flank or caudal peduncle, lose control of posterior portion of body
• Dx: isolation from kidney into TSA or BHI; presumptive as gram negative, motile rod, motile via use of flagella, catalase +ve, cytochrome oxidase –ve, & serology
• Ctl: good culture environment
• Tx: oxytetracyline at 2.5g/45 kg feed /fish/day for 10 days

Enteric Redmouth Disease(ERM)

• Agent: Yersinia ruckeri
• Morphology: Gram negative , rod , motile by flagella at 18 - 27C, can become filamentous in order cultures
• Culture: std media, 22-25C, cytochrome oxidase –ve, no gas/H2 S in TSI, ferments glucose
• Epizootiology: originally Idaho, now apparently ubiquitous; mainly rainbows but also cyprinids
• Reservoirs: natural host , carriers with low grade mortality
• Transmission: horizontal
• Environmental factors: large scale epizootics occur due to stress , low DO, poor environment , grading ; reduced 14C
• Pathology: infected fish become sluggish, darker; severe congestion /hemorrhages of tissues of head /mouth, stomach filled w/water colorless fluid, intestines w/yellow fluid , petechial hemorrhages of internal orgs, spleen dark/enlarged
• Dx: isolation from kidney onto TSA or BHI plates at 20-25C for 24-48 h, Gram negative, motile rod, cytochrome oxidase negative , no H2S in TSI & serology
• Ctl: prevented from avoidance , hygiene, detection of carriers; immunization available and best management tool; can disinfect eggs w/iodophires (25 ppm), sulfamerazine + oxytetracycline ( 20g/100kg/f/d; 5g/100kg/f/d for 5 days each)

Pseudomonas fluorescens

• Agent: Pseudomonas fluorescens
• Morphology: Gram negative , rod motile via polar flagellum
• Culture: std media , round glistening colonies w/undulating edge, radial striations, easily seen green pigment under UV light (pseudomonas F agar), cytochrome oxidase +ve, catalase +ve, strict aerobe, grows at 18-25C
• Epizootiology: worldwide, all fish susceptible, problem for aquarium fish
• Reservoirs: mud and water; infected or carrier fish and others( frog)
• Transmission: horizontal
• Environment factors: stress, mainly elevated temps
• Patho: hemorrhages and necrosis of internal organs & external lesions
• DX: isolation from kidney on TSA or BHI &id
• Ctl: remove stressor
• Tx: oxytetracycline at 50-75 mg/kg/f/day for 10 days

Motile Aeromonad Septicaemia (MAS)

• Agent: taxonomy confused, Aeromonas hydrophila (more than 10 other species claimed); G-motile rod w/polar flagella; oxidase +ve, catalase +ve, glucose fermenter
• Culture: TSA, BHI; can grow at 4C but best at 18-25C; white, circular, convex colonies, often confused w/Citrobacter
• Epizootiology:worldwide in fw, all fw species susceptible, others such as frogs, alligators, snails , prawns
• Reservoirs: freshwater w/high organics loads , usually in sewage, normal gut flora of healthy fish; diseased fish/frogs; survivors are carriers
• Transmissions:horizontal
• Environment factors: stress from crowding, variable temps, changes in weather, rough handling, low DO, high organics
• Patho: usually hemorrhages + necrosis internal organs + necrotics lesions on skin/muscle = G-septicemias
• Superficials circular or grayfish- red ulcerations
• Lesions around mouth similar to ERM
• Hemorrhages of fins ,exopthalmia
• Internal pathology: swollen, soft kidney; petechiae of musculature, intestines free of food
• Dx: isolation from kidney into TSA or BHI, Gram negative, motile rod, cytochrome oxidase +ve, ferments in glucose, no fluorescent pigment
• Ctl: prevention via good management
• Tx: oxytetracycline at 50-75mg/kg fish/ day for 10 days


• Agent: Aeromonas salmoncida (produced pigment)
• Morphology: Gram negative, rod, nonmotile
• Culture: TSA/BHI, brown pigment, grows well at 18-25C, small white round raised convex colonies , oxidase+ve, non-motile, ferments glucose but no gas
• Patho: both virulent and avirulent strains, produces endotoxin
• Epizootology:salmonids cultured in fw
• Reservoirs: obligate fish pathogens, found in waters w/infected or carriers fish
• Transmission: horizontal
• Environmental factors: severity increases w/temp, nutrition, handling stress
• Patho: similar to other G-, septicaemia conditions w/hemorrhaging, necrosis of internal organs, external lesions
• External patho: focal necrosis in muscle develops to abscess, hemorrhages at base of fins, bloody discharge from vent, bleeding from gills
• Intenal patho: petechiae in body musculature, congestion of posterior intestine, no inflammatory response
• Dx: isolation from kidney into TSA, BHI; gram negative, non-motile rod, brown diffusible pigment, oxidase +& serology
• Ctl: avoidance via clean water/fish; several vaccines on the market
• Tx: oxytetracyline at 50-75mg/kg fish/day for 10 days


• Agents: Vibrio parahaemolyticus, V.alginolyticus, V. anguillarum
• Morphology: Gram negative, curved/ straight rods, motile with a single polar filament
• Epizootiology: all marine /brackish water fish, including ornament marine fish
• Reservoir: wild fish carriers of pathogenic strains & as a normal inhabitant of the GI tract
• Transmission: horizontal
• Occurance: all ages assoc with high temperatures, rapid salinity changes, handling & ectoparasite damage
Disease is always preceeded by some from of stress
Level of mortalities & acuteness of disease depends on temperature, strains virulence, fish species & stressor
100% mortalities in fry and fingerlings, but in epizootic more than 50% mortality
• Clinical signs:
• Peracute disease: only seen in young fish, anorexia, darkening of hyperaemia of the skin, sudden death
• Acute & Chronic disease: older fish, darkening, lose balance & jump out of water, hyperaemia around vent, sides of operculum , caudal fins & base of fin , haemorrhage of the eyes & gills, petechia on the body surface which develop into haemorrhagic ulcers which may extend down into the skeletal muscles in the chronic forms of the diseases , exophthalmia is occasionally seen.

• Patho: haemorrhages through out the internal organs, hyperamics intestinal tract, enlarged and liquefied spleen, liquefaction of the kidney.

• Histopathology: foci of necrosis in the liver, spleen and kidney
In peracute disease see a severe cardiac myopathy

• Diagnosis: based on clinical signs, pathology and history
Confirmed with isolation
Primary isolation should be made on TSA(1.5%) NaCl at 20-25C and 24-48h
A presumptive diagnosis can be made if the isolate is a Gram negative , short /curved rod, motile , cytochrome oxidase +ve, produces acid but no gas in glucose O/F medium
Confirmed if the TSA isolate is sensitive to vibrostat 0/129
Antisera are available for vibrio in temperate regions

Further biochemical testing for further identification

• Prevention & Control:

Avoid handling & high stocking densities at risk periods of high temperatures or sudden salinity changes
General sanitation procedures
Commercial vaccines available
Treatment: Antibacterial can be incorporated in feeds
But the diseased fish may have stopped eating and bacteria maybe resistant to the antibacterial used