Pseudomoniasis in Fish

Koi with side ulcer possibly from pseudomoniasis

Jessie Sanders

There are many bacteria species present in your fish system. Most are benign and some even beneficial for your fish, including those residing in your biological filtration. Other bacteria wait until your fish's defenses are lowered so they can take advantage of a weak immune system. It is very rare for a bacteria species to cause serious problems in your fish on their own. Usually, they are secondary infection after the fish has been damaged by a traumatic injury, transportation stress, or a parasite infestation. However, some species of Pseudomonas are able to cause debilitating disease in perfectly healthy fish.

What Is Pseudomoniasis?

Pseudomoniasis is the name of a bacterial infection caused by a Pseudomonas species bacteria. There are many various species of Pseudomonas in the aquatic environment, and the majority are not going to cause your fish any issues. Most aquatic bacteria live on the detritus that collects on the bottom of the aquarium or on the surface of plants and even the aquarium glass. They do not affect the fish unless some other problem or lesion occurs in the fish.

The most common reason for Pseudomoniasis is that the fish's immune function is weakened by another primary stressor.

Symptoms of Pseudomoniasis in Fish

Pseudomoniasis in fish resembles many other bacterial infections. These generalized signs are not specific to a Pseudomonas infection.

Clinical Signs

  • Fin erosion and redness at the base of the fins
  • Ulceration or red spots on the skin
  • Lethargy
  • Decreased appetite
  • Sudden Death

Causes of Pseudomoniasis

Most Pseudomoniasis infections occur when your fish is subject to another primary stressor. There are particular Pseudomonas species that have a special affinity for a specific fish species that can cause disease with little to no decrease in immune function. Pseudomonas aeruginosa is a part of the normal fish microbiota, however the bacteria have become highly opportunistic and pathogenic, causing serious illness including, hemorrhagic septicemia, gill necrosis, abdominal distension, and internal organ failure.

Pseudomonas spp. are present in almost all aquatic environments and become pathogenic when a fish becomes debilitated. Once a fish's defenses have been lowered, many pathogens, including Pseudomonas bacteria, can spread and flourish. If a fish's immune systems are continually weakened, or the bacteria is too plentiful, your fish will not recover without treatment.

Diagnostic Processes

Most critical to the diagnosis of Pseudomonas is to identify the primary stressful event. Although rare, primary bacterial infections are possible, but it is important to evaluate your fish's environment regardless.

Examples of stress in fish include poor water quality, inadequate or incorrect nutrition or aggression or bullying issues. Possible solutions, depending on the primary issue, may include better aquarium or pond maintenance, removing fish from the environment, purchasing new food, and adding decor items or other environmental improvements. Work with your aquatic veterinarian to get advice on how to improve your fish's surroundings to limit stress.

Additional diagnostics include sampling open wounds for bacterial culture and antibiotic sensitivity testing. However, external wounds are a breeding ground for many different bacteria species, so the results may not be too specific as to the actual cause of the disease.

The best diagnostic data is obtained from euthanizing a moribund fish, that is, a fish showing severe clinical signs and close to death, and submitting the body to a lab for bacterial examination. Tissues collected from the kidneys and spleen provide a great deal more information about what bacteria species is the true causative agent of disease. Then, if there are other fish in the aquarium, also suffering from similar symptoms, it will make it easier to treat them with the appropriate antibiotics.


Once the primary stressor has been identified, it should be eliminated as soon as possible. Depending on the initial stressor, you and your veterinarian can make plans to remove it from your system and prevent it from coming back. If this stressor is not removed, such as correcting poor water quality, antibiotic treatment will only provide temporary relief.

Based on the bacteriology results, an antibiotic sensitivity test can be performed. Using isolates of the infective bacteria species, different antibiotics are tested and the most effective antibiotic for that specific bacteria is identified. This is critical for many Pseudomonas species since there is known resistance to multiple antibiotics.

Once a suitable antibiotic has been identified, your veterinarian will determine the best treatment method. Depending on the size, status and total number of fish affected, the drug may be given as a bath, in food, or directly injected into the fish.

Over the counter antibiotic-like products are usually not effective against Pseudomonas species bacteria.

How to Prevent Pseudomoniasis

The best method to prevent the spread of Pseudomonas bacteria in your aquarium is to adhere to strict quarantine protocols. This way, if a fish that is stressed out from capture, transport, and the new environment starts to show clinical signs, they will not spread the disease to the rest of your fish.

Is Pseudomoniasis Contagious to Humans?

Pseudomoniasis is not typically transmissible to humans. Immunocompromised individuals, children and the elderly should limit their contact with fish systems, just to be on the safe side. The most common fish bacteria with zoonotic potential is fish Mycobacterium, which can cause severe skin lesions in humans.