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Aeronomas Bacteria

article by Dr Paula Reynolds Aquatic Patho-biologist

Aeromonas is a genus of bacteria that includes many species capable of causing disease in Koi. Aeromonas salmonicida and Aeromonas hydrophila are the best known members of the genus but it is the latter of these two organisms that is of most concern to the Koi-keeper. The scientific community has mixed views as to the capacity of Aeromonas hydrophila 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.


Reliable Guidance


The hobbyists need proper guidance to deal with Aeromonas infections effectively as without proper management outbreaks commonly reoccur. However it is far more important to learn how prevent then becoming a reality in the first place. Understanding the many situations in which an outbreak of Aeromonas infection is most likely to occur demonstrates that Koi require stability, and when this is denied them disease often results with Aeromonas a very common enemy.


Causative factors in bacterial disease


  • Recent importation is a major cause of Aeromonas infection

  • Netting, bagging, transportation, rough handling

  • The stress of a new environment

  • Intermittent levels of ammonia or nitrite that goes untested by the hobbyist

  • Prolonged periods of poor water quality when a new filter fails to mature

  • Overcrowding increases the risks of disease transmission

  • Fluctuations in p. H. and other associated water chemistry problems

  • A lack of pond hygiene – No bottom drain fitted or the pond not hoovered

  • A filter system in which fish waste and other decaying matter builds up

  • Parasitic infestation that debilitate the Koi

  • Exposure to harmful pond construction materials, heavy metals or toxins

  • Heavy metals or toxins in the tap water

  • Temperature fluctuations or the inappropriate use of heat

  • Adverse spring conditions- water temperature fluctuations

  • The use of too many chemicals in the pond can leave Koi susceptible

  • Underlying undiagnosed health problem can predispose Koi to Aeromonas

  • Failure to quarantine new Koi creates a further risk from Aeromonas

  • An inappropriate diet leads can predispose fish to disease

  • Injuries to the body caused by sharp objects in the pond can become infected

  • A lack of sufficient dissolved oxygen in the pond allows disease to flourish


Ever present


Avoiding every one of these potential trigger factors is impossible Koi have to be moved or no one could become a Koi-keeper. However, it is possible to look in more detail at each potential risk area and attempt to minimise instability for Koi as each situation arises. It is important to remember that Aeromonas is always present as part of the normal bacterial life of the pond and is found on the fish themselves and internally. Koi can never escape from it and it is the Koi-keeper who keeps this organism at bay by good pond maintenance.


Temperature factors


Aeromonas can cause disease in a pond at a water temperature as low as 40F / 5C but he outbreak is usually less virulent. The higher the temperature the faster the bacterium multiplies and more signs of disease such as ulceration or reddening of the body will be observed. Losses due to septicaemia can be anticipated if the water is over 65F / 17C. This bacterium can multiply at an amazing rate if the temperature and conditions are conducive. In just one day one Aeromonas organism could generate well over 20 million more and 30 million is possible in really ideal circumstances. Each new bacterium will possess genetic material that enables it to resist the chemicals former generations of the species were exposed to in the fight to eradicate them. It is a fact Aeromonas know bout survival.


Ulcerative form of disease


The disease normally presents in the form of ulcers or open sores on the body, although reddening is also a sign of infection. The skin lesions are created by enzymes, which in this case are the metabolites or waste products of the Aeromonas bacterium. Once disease has become systemic and is affecting Koi internally usually only antibiotics are effective or the fish will not survive. However, if the disease is caught in time before what is termed bacteraemia has developed it is possible to control an outbreak with bactericides and topical treatment. The ulcer being open results in vital fluids containing electrolytes being lost from the body, the use of salt at 1/2 an ounce per gallon can help prevent this. Whilst the treatment of ulcers with topical medications will not cure a serious systemic disease it will help to prevent the further invasion of the body by other harmful organisms such as fungus and can encourage healing.


About Aeromonas


Aeromonas hydrophila is by far the most common subspecies causing disease in Koi and is frequently isolated in bacterial cultures along with Aeromonas sobria. Some species work as a team cleaning up after one another we can trust nature never to waste a nutritional resource. From the laboratory perspective Aeromonas hydrophila is a gram negative – oxidase positive rod with a singular polar flagellum. This means that the outer wall of this bacterium unlike some others will retain a specialist stain known as Gram’s stain which colours it blue. A further aid to identification is the confirmed presence of the enzyme oxidase. The term rod refers to the shape of the organism and the flagellum is a tail like protuberance at one end, which gives the bacterium its motility.


Aeromonas species whilst normally aerobic have the capacity to live in both aerobic and anaerobic conditions; we call organisms that can adapt in this way facultative. This means that the bacterium can hide in poorly oxygenated ponds or filtration systems harbouring decaying matter which can become a source of disease emerging at a later date when disturbed. Regular cleaning of the mechanical filtration chambers is obvious because their purpose is to trap debris but systems with voids beneath the media also need routine maintenance.


The filter system


Provision for flushing to waste after all cleaning procedures should be a standard part of all filter systems; a non-return valve incorporated into the filter plumbing prevents contaminated water returning to the pond. Cleaning should never disturb the biological part of the filter. There are biological pond cleaners on the market that use micro-organisms to naturally lower the level of dissolved solids and other unwanted substances in pond water. These can reduce the workload for the Koi-keeper when it comes to pond hygiene whilst at the same time lowering the risks of all disease including Aeromonas.


An uphill struggle in some outbreaks


Koi do not develop antibodies against Aeromonas species so the battle can become a long term one. You may cure an infection of bacterial disease with antibiotics and bactericides only to experience a new outbreak the following year. Koi will not stand up to prolonged use of antibiotics as the kidneys, liver and brain tissues can be damaged by their side effects. Resistance to antibiotics plagues all branches of medicine and is now greatly affecting the Koi industry and antibiotics should only be used under professional guidance. Koi culture is in a serious situation due to continued reliance on curing disease rather than preventing it. Aeromonas is not the only potential bacterium lurking in pond water, but it certainly is the most common.


The ideal


It is possible to have a pond free of the risks posed by Aeromonas without resorting to any artificial aids. Such a pond will have a fully biological filter system with an excessive capacity to support the numbers of fish stocked any borderline system is inadequate. The pond must have sufficient water volume to allow each fish room to thrive 50 inches of Koi to 1000 gallons is an old statistic in Koi-keeping that still stands the test of time. This same pond will only be stocked from a quarantine facility never by direct introduction and it will be hygienically maintained on a very regular basis either through its drainage and filter system or by hovering. The Koi will be well fed on a good quality food including some fresh foods when water temperatures permit but no uneaten pellets will ever pollute this pond. Chemicals will not be used routinely only for a diagnosed problem and test kits will be in regular use to minimise all triggers factors. The kits used to test this pond should include dissolved oxygen because the wise Koi-keeper knows the vital role it plays in the well being of Koi.


Prevention over cure


It is vital for all serious Koi-keepers to realise that prevention is sager and more successful than cure when it comes to all bacterial disease in Koi and this particularly applies to Aeromonas infections. Having to resort to the use of any chemical especially antibiotics can only be seen as a failure to maintain the pond adequately enough to keep bacterial levels at a minimum, however when such drugs have to be used continuously to control disease the long term welfare of the Koi is seriously threatened.

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