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Overview of Fish Diseases

Prevention is better and cheaper than cure!

Fish disease can be both highly distressing, and in many cases, costly both in treatment and replacement costs. A key aim is encourage prevention via regular water testing and husbandry. This is particularly important with koi ponds and their often-high stocking levels and reliance on biological filtration for good water quality. In general diseases are often oversimplified, which in turn leads to misunderstandings about diagnosis and treatments.

Opportunistic parasites and bacteria

There are many health problems that are beyond our control. Like any animals, they are susceptible to a range of problems such as tumours, heart and other organ disease, as well as metabolic disorders such as diabetes. However, the overwhelming majority of common health problems involve external parasites, fungus, gill and bacterial infections. An important point is that any body of water, be it a tank or pond will be teeming with millions of opportunistic bacteria and nearly all fish carry small populations of parasites. All animals, ourselves included, are constantly surrounded by germs and bugs, but healthy animals can keep them at bay.

Stress and the immune system

These germs and bugs constantly probe and try to breach the fish’s immune system to gain a strong foothold. In general, for them to be successful there has to be an underlying predisposing factor such as poor environmental conditions, poor nutrition, overcrowding or poor water quality. In addition to causing stress, which will depress the fishes immune system, such conditions will often encourage increased numbers of opportunistic pathogens. Any treatment plan should also tackle any  underlying problems at the same time.

Diagnosis

The main diagnostic approaches are:

  • Observation of changes in behaviour, such as flashing, not eating, clamped fins, heavy respiration, self-isolation, gasping at the surface or water return, loss of equilibrium, jumping – all which indicate the need for further investigation.
  • Water testing and inspection of system to determine water quality
  • Physical examination: This can be carried out in the pond or tank or the 'patient' can be placed in a shallow bowl and examined for fin erosion, cuts, lesions, reddening of the skin, raised scales, larger parasites such as lice or anchor-worm, swollen gills or damaged gills (indicated by trailing mucus or an operculum which does not fully close), swellings, lumps or growths.
  • Alternatively, for a fuller investigation, they can be removed from the water and lightly anaesthetized for easier handling. A closer examination can then be made, especially of the hard-to-see areas underneath and at the base of fins. Additionally the gills can be examined and skin, gill and fin biopsies can be taken for microscopic examination. Where there are body ulcers, bacterial samples can be taken and sent for investigation.

  • Surgical examination is also extremely rare and, at present, has a poor success rate. Blood sampling, which is an important diagnostic tool in veterinary and human medicine, is still poorly developed. Blood sampling techniques are relatively difficult and little research has been carried out as to what would be normal ‘baseline’ figures for various species and the relevance of variations in blood counts.

    Complications

    If detected early enough most simple problems can be easily and speedily resolved. However, there are complications that can arise which can greatly affect both the treatment method and the outcome.

    If the condition is advanced this can lead to stress and a depressed immune system, possibly leading to secondary infections. Other body functions such as wound healing and digestion etc are also affected by stress. These added complications can mean that sick fish may be very slow in responding to treatment.

    Some conditions, particularly severe parasite infestations, can cause severe secondary damage to skin, fins and gills. This in turn may result in secondary infections such as such as ulcers or fin-rot, or excess mucus production and cell hyperplasia.  This secondary damage may influence the treatment regime to be used, or indeed, affect the treatment priority. For example, a parasite infestation affecting the gills may restrict the use of an anti-parasite treatment containing formalin because of the risk of further irritation of the gills.

    Some pathogens, in particular Costia (Ichthyobodo); are very quick to take advantage of sick and stressed individuals. This can cause additional stress and complications.

    A typical example of such complications would be a serious infestation of gill flukes, Dactylogyrus. The irritating presence of these parasites could lead to excess mucus production. The increased mucus layer  would give the parasite some protection from treatments.  Irritation may also cause gill hyperplasia (an increase in normal gill tissue) leading to swollen gills, resulting in a reduced respiratory surface, which in turn would create respiratory difficulties and more yet more stress. Many treatments, for example formalin, might make this situation worse. In the longer-term, unless the situation was resolved gill-rot or bacterial gill disease may occur.

    Treatment methods:

    Broadly speaking there are four treatment methods:

    Tank or pond treatments:

    • The advantages are less stress and no handling.
    • Disadvantages are that biological filtration may be affected,
    • Treatment dosages can be difficult to calculate unless the exact water volume is known
    • Variances in water composition; particularly pH, hardness, and organic load may interfere or affect the chemical activity of the treatment
    • Some treatments, for example antibiotics, cannot be applied via the pond.
    • Overdoses or situations where there is a bad reaction can be difficult to remedy, particularly in large ponds
    • Treatment costs can be high in large ponds

    Bath treatments

    • They can be easily calculated.
    • Higher doses can be used (for shorter periods)
    • Filtration is not affected
    • Less interference with treatment reaction
    • If there is a bad reaction, it is easy to remove the 'patient'
    • Disadvantage is the need for handling and except in the case of short treatment periods, a relatively large treatment tank may be needed if several fish are treated at the same time.

    Food top dressing – usually antibiotics

    • Advantage: no stress or handling
    • Disadvantages include: it can be very difficult to calculate doses or target specific individuals
    • Treatment may wash off the food
    • Very sick fish may not eat

    Individual treatments. e.g. wound dressing or injections.

    • The obvious disadvantage is the need to handle and possible anaesthetize individuals. Can be time-consuming when several are to be caught and treated.
    • Advantages are that treatment is directed and targeted at a specific individual and is usually far more effective, particularly in bacterial infections.

    Treatment limitations

    The presence of water limits the range and type of treatments can be used, which goes part the way to explain why so little research is carried out into metabolic and physiological disorders that are routinely treated and managed in other animals. Even if these disorders could be diagnosed it would be impossible to give a life-time of treatment that may consist of pills, lotions, medicines or injections.

    Hopefully, this overview has made aquarists look at fish disease from a slightly different, perhaps less simplistic, perspective. Disease in any animal is a complex matter. However, over 90% of all health problems can be resolved with a combination of good husbandry, early diagnosis and accurate treatments. It is important to realise the limitations of our own capabilities and recognize that fish are living animals and not innate objects to be the subject of amateurish experimentation.



  • Fish disease problems? Need some help? The help pages take you through the basics of the 'fish health work-up' diagnosis method and basic fish disease treatment methods.

    90% of all health problems and fish deaths are caused by poor fish keeping! The key pages are essential reading for all beginners and intermediates who are serious about disease prevention.

    Wife Beater

    Sample download

    Expanded Content by Dr. Erik Johnson, DrJohnson.com and Used with Permission; Frank Prince-Iles ©2009 All Rights Reserved