Bacterial Diseases of Fish

Bacterial Diseases of Fish

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Pseudomonas fluorescens

  • Causes pseudomonas septicemia mainly in general pond fish, seldom in salmonids
  • normally a secondary invader, hard to distinguish from Aeromonas septicemia, not a huge concern in fish
  • Agent: ubiquitous bacterium of soil, water, spoilage-vulnerable foods; G- rod motile via polar flagellum; grows @ 18-25oC; cytox positive, catalase positive, strict aerobe
  • culture: std media, round glistening colonies w/undulating edge, radial striations, easily seen green pigment under UV light (pseudomonas F agar)
  • Epizootiology: worldwide in fw/sw, all fish susceptible but mainly ww, problem for aquarium fish
  • reservoirs: mud and water; infected or carrier fish and others (frogs)
  • transmission: horizontal, no vertical
  • environment: stress, mainly elevated temps
  • Pathology: in catfish, largely hemorrhaging and necrosis of internal orgs, external lesions, loss of pigmentation; individuals can withstand large losses, bone exposure
  • Diagnosis: isolation from kidney on TSA or BHI; confirmation via serology no help
  • Control: remove stressor, drug therapy as with other G-(oxytet @50-75 mg/kg/f/day for 10 days); no vaccine yet


Aeromonas hydrophila (MAS)

  • Causes Motile Aeromonas Septicemia, often referred to as a complex of species, mainly affecting ww fish, opportunistic pathogen, can cause red-leg in frogs
  • Agent: taxonomy confused (more than 10 other species claimed); G- motile rod w/polar flagella; oxidase pos, catalase pos, glucose fermenter; virulence may be via endotoxin
  • culture: TSA, BHI; can grow at 4oC, but best at 18-25oC; white, circular, convex colonies, often confused w/Citrobacter
  • Epizootiology: worldwide in fw, all fw species susceptible (both ww and cw); others such as frogs, alligators, snails, shrimp and humans
  • reservoir: freshwaters w/high organic loads, usually in sewage, normal gut flora of healthy fish; diseased fish/frogs; survivors are carriers
  • transmission: horizontal only from intestinal tract, external lesions, through water, via external parasites
  • environment: stress from crowding, variable temps, changes in weather; rough handling, low DO, high organics
  • External pathology: usually hemorraghia + necrosis or internal organs + necrotic lesions on skin/muscles = G- septicemias
  • superficial circular or greyish-red ulcerations
  • lesions around mouth similar to ERD
  • hemorrhaging of fins, exopthalmia
  • Internal pathology: swollen, soft kidney; petechiae of musculature, intestines free of food
  • Diagnosis: isolation from kidney into TSA or BHI
  • presumptive: G- motile rod, cytox +, ferments in glucose, no fluorescent pigment
  • confirmative: no serology, too many types
  • Control: prevention via good management, injection w/chloramphenicol, no vaccines; therapy via oxytet (50-75mg/kg fish/day for 10 days), chloramphenicol in Europe

Aeromonas salmonicida-furunculosis

  • First isolated from farmed trout in 1894
  • name of disease derived from boil-like lesions known as “furuncles” (not correct)
  • at one time very common, resulted in Furunculosis Committee in England
  • diminished as of late due to better mgmt
  • Agent: comes in three subspecies, the most common is salmonicida (produced pigment), G- nonmotile rod, bipolar staining
  • Culture: TSA/BHI, brown pigment in presence of TYR/PHE, grows well at 18-25oC, small white round raised convex colonies
  • Biochem: G-, oxidase +, non-motile, ferments glucose but no gas
  • Pathogenicity: both virulent and avirulent strains, produces endotoxin
  • Epizootiology: wherever salmonids are cultured in fw (besides Tasmania/NZ), entered Australia via goldfish, brook trout most susceptible
  • reservoirs: obligate fish pathogen, found in waters w/infected or carrier fish
  • transmission: primarily horizontal, contaminated water, eggs, carriers, equipment, clothing, surface of aquatic birds; no vertical demonstrated
  • pathogenesis: acute, subacute, chronic forms (dose, temp, host resistance, virulence of strain)
  • environment: severity increases w/temp, nutrition, handling stress
  • Diagnosis: look at hatchery history with disease; isolation easy from kidney into TSA, BHI;
  • presumptive: G-, non-motile rod, brown diffuseable pigment, oxidase +
  • definitive: serological (rapid slide agglutination, FAT)
  • Control: avoidance via clean water/fish; several vaccines on the market; selective breeding??
  • therapy: oxytet @ 50-75mg/kg fish/day for 10 days, sulfamerazine, sulfonamide (Romet)


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