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Bacterial Infections

Bacteria come in many different shapes and sizes. They are microscopic in size and unlike parasites, cannot be seen with a normal microscope that is used for the identification of parasites. Stains or sometimes culturing are applied to identify the species. They are single-cell organisms with an outer cell wall which allows liquid and nutrients to pass through. Most bacteria multiply through binary fusion. It has been confirmed that under favourable conditions, some bacteria can generate well over 20 million organisms in a single day. Bacteria are naturally occurring in all lakes and ponds. The scientific community has mixed views as to the capacity of bacteria to cause disease. Some regard it as an opportunistic pathogen causing only secondary disease in koi that are already in a susceptible condition. Others claim that it is capable of causing primary infection in its own right. Koi ponds for a variety of reasons are susceptible to outbreaks of bacterial disease.

The key to treatment of bacterial disease is an understanding of the pathogen, early diagnosis and treatment. The following shocking photo, illustrates the severity of a full-blown bacterial infection.


For the pond owner, there are four types of infection caused by bacteria:

Fin-rot where erosion of fins take place.

Photo: Jack Birkenbach

Photo: Jack Birkenbach




Body ulcers- open, shallow to deep lesions on the head and body.




Bacterial gill disease where the gills are the primary target.


Systemic infections (Septicaemia) where the bacteria multiply in the blood and affects internal organs.



Many pathogens such as parasites, fungi, viruses and bacteria can cause skin disease in fish but bacteria are of particular interest because infections are common and often deadly if untreated. Although sometimes it may be necessary to start a course of antibiotic treatment based on personal experience, it is also prudent to have bacterial identification and antibiotic sensitivity tests carried out at the same time. At worse this simply confirms the treatment choice, while possible saving valuable time and money if the initial course of action is ineffective.Some of the most obvious signs of bacterial skin disease include exophthalmia (pop-eye), abdominal swelling, reddened lesions, ulcers on the body, reddening of the base of the fins and darkening of skin colour. The distribution of the skin lesions may include the head, face, operculum, mouth, back, trunk or body wall including the lateral line, and tail base. Other symptoms may include loss of appetite, weight loss and decreased activity. The following factors may contribute to bacterial disease:

  1. Recent importation or new fish released into the pond without quarantine.
  2. Insufficient dissolved oxygen in the water.
  3. Netting, transporting and handling.
  4. Unacceptable levels of ammonia and nitrite.
  5. Stress of a new environment.
  6. Prolonged levels of poor water quality.
  7. Overcrowding
  8. pH fluctuations
  9. Poor pond hygiene
  10. Build up of waste in the filter system.
  11. Parasite infestation
  12. Temperature fluctuations.
  13. Exposure to harmful pond material, toxins and heavy metals.
  14. Warmer water after adverse winter conditions.
  15. Overdosing with chemicals.
  16. Inappropriate diet.
  17. Injury caused by sharp objects in the pond or injury caused by predators.

Some bacteria produce toxins that are excreted into the blood and tissues of the host. The only symptoms that this koi exhibited, was a redness and swollen tissue around the mouth. It died within three days after the first symptoms became noticeable.

Bacterial infection fatal

Now look at the vent area


Sometimes a substantial bacterial ulcer may cause water to penetrate the fish by means of osmosis at a faster rate than the kidneys can excrete it, causing exophthalmia or protruding eyes. The effect of the osmosis can be reduced by the addition of salt to the pond water. The following two photos of the same fish clearly demonstrate the effect of an ulcer where water penetration caused the eyes to bulge from the internal pressure caused by the excess fluid.



There are numerous species of bacteria which may infect fish. Most of the bacteria that are commonly isolated from fish are Gram-negative bacilli. Below are the major bacterial pathogens that have been associated with koi disease.

Gram-negative bacilli
Aeromonas salmonicida is a non-motile (cannot move on its own), rod-shaped bacterium that produce skin nodules and ulcers in fish. The ulcers are generally surrounded by reddened tissue from which scales can be easily removed. Muscle infection and necrosis are common, allowing infection to penetrate up to the underlying bone. The infected tissue exhibit necrosis but few inflammatory cells will be noticeable. This condition is created by enzymes which are the waste products of the Aeromonas bacterium. Once the infection has become systemic, usually only antibiotic treatment is effective, or the fish will not survive.

Aeromonas hydrophila is a motile (mobile) Gram negative bacillus. It is free living and always present in the water. It has a tail-like protuberance which gives it mobility. As an opportunist, infection with this organism is thought to be at least partly associated with overcrowding and high levels of stress in the koi population. The symptoms are similar to those associated with Aeromonas salmonicida.

Fish do not develop antibodies against Aeromonas species and it has been reported that resistance may develop to certain treatment. The battle is therefore a long term one.

Flavobacterium columnare (previously known as Flexibacter columnaris) is a long slender bacillus. Symptoms associated with Flavobacterium columnare include bacterial gill disease and “saddle back” disease. The bacteria will excrete cartilage and protein degrading enzymes that will cause erosion of the gills as well as skin lesions. Infection of the gills may cause acute mortalities while skin infection may have a more prolonged effect. Skin lesions during the early stages range from shallow white erosions to eventual yellow/brown ulcers. Because this bacterium is a natural resident of ponds where healthy fish are found, it is thought that the disease is triggered by high stress conditions. A related bacterium, Flavobacterium psychrophillum causes fin rot that progress to involve the tail base (peduncle)

Gram-positive bacilli.
Of the gram-positive bacilli that the pond keeper may encounter, the most important is Mycobacterium fortuitum. It is Gram-positive and non-motile organism. Mycobacteriosis is usually characterised by protracted development of the disease and slow wasting or emaciation of the fish. Granules are also formed in the tissue and organs of the fish which may be detectable on the skin.


No effective treatment for this disease is known and it may be transmitted to humans. The photos below highlight the danger of handling infected fish.

granuloma1 granuloma2

It is recommended that infected fish be handled with gloves. The photo below demonstrates a classic example of Mycobacteriosis.


Treatment of bacterial infections.

  1. For any bacterial infections, treatment takes the form of anti-bacterial drugs that are normally obtainable from a Veterinarian. In most cases where the infection is confined to the epidermis of the fish, topical “off-the-shelf” treatment will be effective. It is always recommended to take a swab of the infected area and have it cultured to determine the exact treatment for the particular bacteria. My experience is that by the time the results are known, the condition of the fish has deteriorated to the extent that treatment will not be effective.
  2. Potassium permanganate treatment of the infected fish or pond water may reduce the bacterial load and offer the fish some relief. The cause of the infection should however be addressed.

Now also see Case study

Last Updated on Monday, 15 September 2014 21:53