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Aeromonas and ulcers

By the koi.wiki editorial team · Updated

Key facts: Aeromonas is a genus of widespread aquatic bacteria that can cause ulcers and Motile Aeromonas Septicemia (MAS) in koi, typically after the fish's skin barrier or immune defenses are compromised by stress, injury or other disease.[1]
Aeromonas bacteria are common, free-living inhabitants of ponds and aquaculture water worldwide, found in the water column, biofilms, mud and even the digestive tracts of healthy fish.[1] The group most often associated with disease in warm-water fish are the motile aeromonads, so named because they swim using flagella, distinguishing them from the non-motile cold-water pathogen Aeromonas salmonicida.[1] Infection by these organisms is called Motile Aeromonas Septicemia (MAS), and it is a frequent cause of ulcers, fin erosion, scale loss and abscesses in koi and other freshwater fish.[1]According to the US importer Kodama Koi Farm, ulcers in koi are often described simply as open sores or reddened areas caused by bacterial infection, commonly attributed to Aeromonas species.[4] A related species profile notes that so-called “ulcer disease” in koi and goldfish is commonly caused by the bacterium Aeromonas salmonicida.[2] Sources therefore differ on which species is most often implicated, reflecting genuine taxonomic complexity in this bacterial group.[1]

What is Aeromonas?

Aeromonas is a genus of gram-negative, rod-shaped bacteria that are oxidase-positive, ferment glucose, and are resistant to penicillin and related antibiotics.[1] They can grow across a wide range of temperatures, from near freezing up to well above human body temperature.[1] Species designations within the genus have been in flux as genome sequencing has advanced, and a single species may include several distinct strains, which has made development of a broadly effective vaccine difficult.[1]Aeromonas hydrophila is described as a heterotrophic, gram-negative, rod-shaped bacterium found mainly in warm climates and in fresh or brackish water, able to survive in both aerobic and anaerobic conditions.[5] A distinct species, Aeromonas salmonicida, is non-motile and grows optimally at 22–25 °C (72–77 °F); it causes furunculosis, a disease associated with sepsis, hemorrhages, muscle lesions and death, and is described as common in goldfish and koi, where it is also called tail rot.[6]

Causes and predisposing factors

Most outbreaks of MAS are linked to an environmental condition that weakens a fish's ability to resist infection, since aeromonads are ubiquitous in pond water, biofilms and organic sediment.[1] Common predisposing factors include handling events, temperature shock, water-quality stress, spawning and aggression.[1] Buildup of organic detritus from feces and uneaten food increases oxygen demand and creates conditions in which Aeromonas and other disease-causing organisms thrive.[7] Bacteria in this genus can also increase in number within recirculating aquaculture systems.[8]Aeromonas hydrophila infections are reported to occur most often during sexual changes, stress, temperature shifts, contaminated environments, and when a fish is already infected with a virus or another bacterium.[5] Aeromonas infections can also be secondary to columnaris disease, viral infections, Saprolegnia (fungus) infection, or Epistylis infections, and the relative contribution of the Aeromonas component to mortality in these mixed cases is often difficult to assess.[1] A related condition, “red sore disease,” involves a combination of Aeromonas bacteria and Epistylis.[9] Secondary Aeromonas infection is also commonly seen alongside fish lice (Argulus) infestations, since parasite damage to the skin can open a route for bacterial entry.[10]

Symptoms

Bacteria typically enter through an injury or a break in the mucus barrier covering the fish, establishing a localized infection marked by breakdown of skin, muscle and connective tissue from proteolytic enzymes.[1] This is often seen as deep ulcers, scale loss, fin erosion or abscesses.[1] Published images document a deep ulcer on the head of a catfish, scale loss and hemorrhage on a koi, and fin erosion with hemorrhage.[1] Internally, the body cavity of affected fish often contains bloody fluid.[1]In fish infected with Aeromonas hydrophila, reported signs include ulcers, tail rot, fin rot and hemorrhagic septicemia, with lesions that can lead to scale shedding, hemorrhages in the gills and anal area, exophthalmia and abdominal swelling.[5] Furunculosis caused by Aeromonas salmonicida is described as producing listlessness, anorexia, darkened pigment, deep or shallow ulcers, exophthalmia, bloody spots and a distended abdomen, with internal signs including dropsy-like gastroenteritis, hemorrhagic septicemia and an enlarged spleen.[6] Similar systemic signs, including hemorrhages and coelomic distension, can also occur with viral diseases such as spring viremia of carp (SVC), and concurrent Aeromonas infection can complicate diagnosis in such cases.[11]

Diagnosis

Because Aeromonas bacteria are normal, widespread inhabitants of pond water, their presence alone does not confirm them as the primary cause of disease; proper diagnosis requires working with a fish health specialist to culture the organism and run antibiotic sensitivity tests.[3][1] A complete diagnostic approach also considers whether an underlying parasite, virus or water-quality problem initiated the outbreak, since bacterial infection is frequently secondary to another stressor.[3]In 2009, a more virulent strain of Aeromonas hydrophila began causing severe outbreaks in commercial catfish operations in the southeastern United States, with no obvious predisposing factor identified other than a distinctive biochemical and genetic profile of the bacterium, which was found to be related to a strain causing high losses of carp in China.[1] This case illustrates that accurate strain-level diagnosis can be important for understanding outbreak severity.[1] General guidance on koi health diagnosis is covered in more detail on the diagnosis page, and background on recognizing illness is covered under signs of illness.

Treatment

Antibiotics should only be used to treat an active, confirmed bacterial infection, never to prevent disease or to treat viral or parasitic disease, and every use of an antibiotic selects for resistant bacteria.[1] Effective use requires that the fish feed well enough to receive a therapeutic dose, that the feed be applied consistently at the correct dose for the full treatment period, and that any required withdrawal period be observed.[1]Most bacteria that infect fish, including Aeromonas species, are gram-negative, and this classification affects which antibiotics are likely to work; antibiotics effective mainly against gram-positive bacteria, such as penicillins, are not considered a good first choice for Aeromonas infections.[3] Before antibiotics are even considered, sources of stress such as poor water quality, temperature change, nutrition and handling should be reduced, and affected fish should also be examined for parasites, since bacterial infection is often secondary.[3] Improper dosing, including the practice known as “shot-gunning” — administering antibiotics in succession without proper diagnosis — increases the risk of antibiotic resistance and can create a “superinfection” that may require depopulation and full system disinfection.[3] Combining multiple antibiotics is also generally not recommended, as it can reduce effectiveness.[3]

Prevention

Preventing MAS requires optimizing management to minimize stress and injury, along with preventive management of parasites and other infectious diseases, use of properly formulated and stored feeds, and conservative management of water quality.[1] Good sanitation that removes organic debris and biofilm, followed by appropriate disinfection, reduces the numbers of Aeromonas and other pathogens that can accumulate in a system.[12] Reducing organic detritus buildup in ponds is likewise important, since decaying feces, uneaten food and dead organisms serve as a reservoir for Aeromonas and other disease-causing organisms while also lowering dissolved oxygen.[7]Controlling external parasites is also a preventive measure, since parasite damage to the skin can open routes for secondary Aeromonas infection; this has been documented with Argulus infestations, and a similar dynamic is described with Epistylis in “red sore disease.”[10][9] During shipping, chemical additives such as acriflavine have been tested against Aeromonas hydrophila in transport water, with acriflavine showing a measurable short-term reduction in bacterial numbers, though without long-term bactericidal benefit once fish were present, and it is not recommended for use for this reason.[13] General biosecurity practices relevant to limiting bacterial spread in ponds are discussed further in sources on pond biosecurity.[12][7]

Common questions

What causes ulcers in koi?
Ulcers are typically caused by opportunistic Aeromonas bacteria that enter through a break in the skin or mucus barrier, often after a fish has been weakened by handling, temperature shock, poor water quality, spawning stress or another infection.[1]
Is Aeromonas always the primary cause of disease?
Not necessarily; Aeromonas bacteria are normal inhabitants of pond water and can also appear as secondary invaders alongside columnaris, viral infections, Saprolegnia, Epistylis or Argulus infestations, so the actual primary cause requires proper diagnosis.[1][10]
Which Aeromonas species causes ulcers in koi?
Sources differ: a general SRAC publication discusses motile Aeromonas species (including A. hydrophila) as causing Motile Aeromonas Septicemia,[1] while an SRAC species profile attributes “ulcer disease” in koi and goldfish specifically to Aeromonas salmonicida.[2]
Can ulcers be treated with antibiotics?
Antibiotics may be used only after a confirmed bacterial diagnosis and sensitivity testing, administered at the correct dose for the full treatment period; misuse increases antibiotic resistance and can lead to a “superinfection” with no remaining treatment options.[3][1]
How can Aeromonas outbreaks be prevented?
Prevention focuses on minimizing stress and injury, managing parasites and other infections, using properly stored feed, maintaining good water quality, and reducing organic detritus buildup in ponds.[1][7]

References

  1. ^ SRAC Publication No. 478: Motile Aeromonas Septicemia (MAS) in Fish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/478%20Motile%20Aeromonas%20Sept…. Retrieved November 2026.
  2. ^ SRAC Publication No. 7201: Species Profile: Koi and Goldfish. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/7201%20Species%20Profile-%20Koi…. Retrieved November 2026.
  3. ^ UF/IFAS EDIS FA084: Use of Antibiotics in Ornamental Fish Aquaculture. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA084. Retrieved November 2026.
  4. ^ HELP, Is My Koi Sick?!? Diagnose Symptoms & Fish Diseases - FREE Health Checklist. Kodama Koi Farm (koi dealer). kodamakoifarm.com/koi-sick-health-diagnosis-symptoms-diseases/. Retrieved November 2026.
  5. ^ Aeromonas hydrophila (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Aeromonas_hydrophila. Retrieved November 2026.
  6. ^ Aeromonas salmonicida (Wikipedia). Wikipedia. en.wikipedia.org/wiki/Aeromonas_salmonicida. Retrieved November 2026.
  7. ^ SRAC Publication No. 4712: Biosecurity in Aquaculture, Part 3: Ponds. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4712%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  8. ^ UF/IFAS EDIS FA100: Fish Health Management Considerations in Recirculating Aquaculture Systems—Part 2: Pathogens. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA100. Retrieved November 2026.
  9. ^ SRAC Publication No. 4701: Protozoan Parasites. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4701%20Protozoan%20Parasites.pdf. Retrieved November 2026.
  10. ^ UF/IFAS EDIS FA184: Argulus (Fish Louse) Infections in Fish. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA184. Retrieved November 2026.
  11. ^ UF/IFAS EDIS VM106: Spring Viremia of Carp Virus (SVCv). University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/VM106. Retrieved November 2026.
  12. ^ SRAC Publication No. 4707: Biosecurity in Aquaculture, Part 1: An Overview. Southern Regional Aquaculture Center. srac.msstate.edu/pdfs/Fact%20Sheets/4707%20Biosecurity%20in%20Aquac…. Retrieved November 2026.
  13. ^ UF/IFAS EDIS FA120: Preparation of Ornamental Fish for Shipping. University of Florida IFAS Extension (EDIS). ask.ifas.ufl.edu/publication/FA120. Retrieved November 2026.