Stress and Koi

Author: Chris Neaves

Modern life styles have made us acutely aware of the effects of stress and the problems it produces for the individual and society. Stress is blamed for nearly every malady afflicting man, beast and koi. Stress is also used to explain diseases that are either undiagnosed or misdiagnosed. Prolonged stress can even cause death. Stress has a dramatic and very real influence on our lives and the lives of our koi.

Relatively little emphasis is placed on the role of stress in the health of koi. It is spoken about but often glossed over because the way fish are handled and shipped is a major cause of stress but cannot be avoided. However, what is of importance is there is a direct relationship between stress in fish and the breakdown of the immune system.

We are all familiar with the immune system of the body. In the last ten years or so the immune system has been thrust into the lime light of publicity with the world wide AIDS epidemic affecting mankind. AIDS affects the immune system so that the body cannot fight infection. Most AIDS deaths are caused by secondary infections, which the body simple cannot fight.

The immune system is the body's first and it must be emphasised - most important natural defence mechanism against disease pathogens. The ancestors of koi developed in a relatively stable environment over millions of years. In this relatively stable environment there was never any need to develop a highly effective system against chronic environmental changes and various stressors that cause stress in fish. Because of this, the immune system is, unfortunately, very sensitive to environmental changes.

The environment and koi are intimately interwoven. Koi are literally sacks of water separated from the surrounding pond water by a permeable membrane (the skin). Their bodies are composed of approximately 80% water. Any changes to the environment have a direct and immediate effect on our koi. Changes greater or more sudden than what the fish has evolved to cope with - will result in stress. It is the response of the immune system to these changes that ultimately determines whether our fish, will survive an encounter with stress and the after effects thereof.

The environment, stress, infection and disease are inseparably interrelated. Studies on both cold water and warm water fish show that antibody production is lowered in the periods of stress. Ironically this is the important time when the body needs these proteins for defence.

Stress affects the immune response in koi in much the same way as higher animals. However, in pond water there is much greater chemical and physical variability than exist in the terrestrial environment. More pathogens (things that cause an infection) are found in water than in the atmosphere.

Koi in our ponds are continuously affected by environmental fluctuations such as temperature and pH changes, ammonia, nitrite, nitrate levels, oxygen levels, etc. Other stressors such as excessive or rough handling, toxic materials dissolved in the water, drug treatments, water chemistry, population density, macro-organisms such as parasites all have an effect on fish and their immune systems. Fortunately, immune systems are constantly at work as an intimate and inseparable function of life.

Koi live in a balance or equilibrium with their water environment. It is important to point out that this equilibrium between the natural health and stability of koi and the continual attack by micro and macro-organisms is a normal on-going fact of life. Fish, like humans have evolved to successfully cope with everyday living on this planet. It is abnormal for a koi to become sick and diseased. Many people in the hobby place great emphasis on “sick” koi and diseases. It is no wonder that when a normal slight variation in behaviour is observed this is seen as an open invitation to spend hundreds of rands treating the whole pond system - “just in case”. The emphasis should be on the fact that normally koi are not sick and normally the environment is healthy and normally koi will live a long time.

In a stressful situation a part of the brain that is responsible for controlling the most basic of functions: hunger, thirst, sex drive and in mammals, body temperature is stimulated. Once stimulated, chemical signals are released into the body that begin a chain of events. However, the body follows a natural course of reactions until theses chemical substances are finished or re-absorbed - even if the stressor is removed.

One of the initial reactions to stress is the release of two hormones from the adrenal glands. The first is called epinephrine and is responsible for the most primitive "fight or flight" reaction that occurs in all creatures. This hormone produces a number of significant changes in the body that prepare the creature to stand its ground and fight or beat a hasty retreat.

The koi readies it's self for action as the epinephrine hormone races through the body. The hormone does this by increasing the heart rate, which in turn, results in the blood pressure and respiration rate increasing. Glycogen from the liver is mobilised so that the body has an immediate energy source. The increased blood pressure and heart rate provides the whole body with extra oxygen. There is a short explosion of maximum energy during the fight or whilst escaping from the immediate threat.

If the stress is prolonged, these reactions can exhaust the body and in will themselves become stressors. An exhausted koi will take far longer can to regain its equilibrium than realised. Take note of the reactions of a koi that has been chased around a tank or pond for some time. When left alone it will retreat to a corner exhausted and breathing heavily as it's body battles to return to normal.

The other hormone released by the adrenal tissue in response to stress is called cortisol. Cortisol, like the first hormone, epinephrine, produces many changes in the body. Again under prolonged stress, these changes can lead to metabolic imbalances, such as increases in protein breakdown and elevated thyroid hormones, which can further increase the demands on the body, leading to biochemical exhaustion. Studies on various fish indicate that cortisol levels in the body increase rapidly and dramatically when fish are crowded together or handled.

The entire stress situation must take cognisance of the fact that the heart of a koi is a simple four chambered structure with only two valves and therefore, not a very powerful organ. Circulation is slow and the tissues farthest from the heart function relatively inefficiently due to low levels of nutrients, oxygen and a slow removal of waste products. Another organ affected by stress are the gills which are extremely sensitive. Literally only a cell thick, they are easily affected by stress and secondary infections. (more about gill and stress in another article)

The hormone cortisol not only affects the body in certain ways but it also disrupts the normal functioning of the immune system. It is this disruption of the immune system that leads to the diseases associated with stress, particularly bacterial infections. Specifically, cortisol is known to interfere with the immune process known as phagocytosis.

Phagocytosis, (or "cell eating"), refers to the actions of certain white blood cells. They are produced by the body as the first line of defence against unwanted bacteria or fungus. They literally "consume" bacteria or other foreign materials that enter the body. The white blood cells, called macrophages (meaning big eaters), are mobilised by chemical signals to move into the area where bacteria have managed to penetrate the bodies basic defences.

These white blood cells are remarkably similar to single cell amoeba and move in a remarkably similar manner. Some scientists even speculate that these phagocytic cells may have originally evolved from amoeba living in the tissues of primitive animals such as sponges and jellyfish. It has been suggested that over billions of years of evolution, these amoeba lost their independence and eventually became a part of the immune system.

The battle begins once these phagocytic cells arrive at the scene. They surround the invading bacteria and engulf them. Within the cell are membrane-bound bags of enzymes called lysosomes. Lysosomes fuse with the bacteria held captive within the phagocytic cells and release their digestive enzymes, which then proceed to break down the bacterial cells. Some phagocytic cells also produce sodium hypo chlorite, known as household bleach, to kill the engulfed bacteria. Once the bacteria are digested, the residual material is excreted from the cell into the bloodstream for processing by the kidney into urine.

Unfortunately, many bacteria have the ability to overcome this response either by releasing toxins that destroy the ingesting white cells or by producing an agent that dissolves the fibrin wall and allows them to spread throughout the body. Adding to the war in the body, the hormone cortisol, released under stress, interferes with important process of phagocytosis. It chemically changes the lysosomal membranes and prevents their fusion within the bacterial containing cells. The digestive enzymes cannot be released and the bacteria are able to remain alive in the cells and spread throughout the body.

In some cases of stress the gut seizes up and anaerobic fermentation and enzyme action attack the gut wall allowing disease organisms to enter the tissues and bloodstream.

Research has shown that dietary supplements of vitamin E and A can increase antibody levels and enhance the immune response. Some scientific publications suggests that vitamin C may prevent this chemical change of lysosomal membranes by cortisol, allowing enzyme release and phagocytosis. It is also known that vitamin C can increase of fishes resistance to bacterial infection.

Vitamin C becomes practically useless shortly after food is made. Within 90 days of manufacture there is a significant degradation of some forms of Vitamin C. After about 120 days most of the activity is gone.  However, higher quality fish foods use a more expensive stabilised form of vitamin C which has an exceptionally long shelf life thus prolonging its benefits to the fish.

Trace metals also participate in this immune function. Selenium, for example, is an immune helper involved in the phagocytic process in a manner similar to vitamin E. Other metals such as cadmium and zinc, interfere in this process. Research in the area of heavy metal involvement in the immune response of fish is expanding and should shortly provide much needed information on fish nutrition.

It is certain that fish maintained in healthy, natural environments and on fresh, high-quality diets are less susceptible to parasite infections and disease infections. A reduction in parasite load may be an important factor, because parasites can spread bacterial infections. They act as carriers for pathogenic bacteria as well as creating the wounds that allow bacteria easy access into the body. Conversely we should not medicate to the point that the fish have nothing on them as anti-biotics and “medications” are indiscriminate in their action. Koi need specific micro-organisms to survive.

Should the immune system fail and the fish become infected, the standard procedure is to treat with antibiotics. The unwitting person, in an effort to help diseased fish, may, in actuality, depress the immune system response even more by administering antibiotics. Certain antibiotics are known to interfere with the immune response in koi and increase bacteria resistance to treatment.

Environmental changes are stressful to koi and lower their resistance to infection and disease. Experience has shown that a wide variety of bacterial, parasitic and other diseases become a problem only if koi are being held under environmental conditions that are unfavourable to them.

The hobbyist must always keep in mind that fish diseases are usually not caused by any single event but are a result of the interaction between several 'agents'.

A fish must interact with a pathogen (something that causes a disease) in a stressful environment for an out break of disease to occur. Here are some examples of diseases and the stress factors that contribute to the infection.





Low oxygen / handling.

Bacterial gill disease:

Crowding / chronic low oxygen / high ammonia/nitrites,/ medication irritation / pH / introduction of new fish without quarantine

Columnaris (Body fungus):

Crowding / increase in temperature.

Bacterial diseases:

Inadequate pond cleaning leading to increased bacteria load in pond water / handling / crowding / low oxygen / exposure to heavy metals / pesticides / handling after winter / introduction of new fish with out quarantine


Handling / low oxygen / environment

Costia, Trichodina, Ich, Chilodenella

environmental stress factors / introduction of new fish with out quarantine


low oxygen / excessive size variation.


Handling after over wintering at low temperatures.(Spring Viraemia of carp)

Fin & Tail rot

Crowding / improper temperatures / nutritional imbalances / poor water quality.

Theories on koi disease must always take into account the fact that in any koi community a large percentage of normal healthy individuals continually harbour potentially pathogenic microbes without suffering any symptoms themselves. Put another way there are always 'carriers' in any pond which have latent infections. The carriers are not infected by the disease they carry but are capable of transmitting the infection to other koi. These infections are often activated by stressful situations and environments.

Systemic bacterial infections of koi can be reduced considerably if the fish are not subjected to handling stress in the early spring when water temperatures are rising.

References suggest that the metabolic changes of both koi and the bacteria it carries are dependent on temperature. When the temperature rises after winter the bacterial activity increases and the metabolic activity of koi increases. However, literature suggests that the bacterial activity increases far faster than that of the koi often resulting in disease if the koi are handled too soon in the spring.


Chris Neaves

The Immune System of Koi - Further Studies

A study by Perimutter, Alfred, Daniel Sarot, Man-Lin Yu, Rocco Filazzoia and Seely on the Effects of Crowding on the Immune Response of Fish indicates that there are "other" hidden factors at work than first meet the eye.

These are not readily visible or apparent to most koi keepers. Sometimes we simply notice that our koi die without explanation. Perhaps the study by Perimutter et al can throw some light on this phenomena.

Their study may indicate that fish kept in overcrowded conditions suffer impaired immune systems as a result of biochemical agents released into the water by the other fish living in the pond.

This may be natures way of reducing a large population to more acceptable levels. The studies suggest that fish can release immune suppressing pheromones (hormones) in overcrowded conditions. This make the other fish in the system less able to fight disease.

To investigate their suspicions three separate experiments were set up. These involved different treatment and control groups.

The first two experiments had 30, 15, 5 and 5 fish respectively. Four similar control groups were set up. Fish were assigned randomly among the groups. All ponds were the same size.

The water in the treatment groups had methylchloroform added to remove organic substances. The control groups were not treated but simply monitored.

Ammonia, nitrate, dissolved oxygen, carbon dioxide and pH were monitored regularly.

The immune systems were challenged with infectious Pancreatic necrosis virus. All fish in the treatment and control groups were injected with the virus two weeks after the experiment began. Two week later they were injected again.

The researchers made antibody tests. These antibody measurements showed that maximum antibody reaction occurred three weeks after the second injection of the virus.

Looking through the results shows that the fish from the experimental groups had two to four times the antibody levels than the fish from the control groups.

As the level of crowding increased so the difference between the groups increased. The implication was that the methylchloroform was removing something from the water that affected the fish immune response.

This unknown factor was directly proportional to the crowding level. The greater the crowding level the greater the immune suppressing effect. The evidence suggested an immune suppressing pheromone was present.

To check their findings the authors then decided to refine a third experiment. This third experiment was run twice with each of the four treatment and control groups. These being 30, 15, 5 and 5 fish again.

The results were as follows.

1. The groups of fish at low stocking densities had twice the antibody level of the control groups.

2. The groups with slightly higher stocking densities had four times the antibodies present.

3. However, as the stocking densities increased the effect of the immune suppressing agent became more pronounced. There were less and less antibodies recorded in the test groups.

4. It is worth noting that even at low stocking densities there was a lower antibody count. This indicates that even at low stocking densities there is immune response suppression.

Conclusions: Low stocking densities can help minimise bacterial and viral disease problems. It is presumed that the fewer the fish in the water the less the concentration of pheromone present that suppress the immune system.

The lower the stocking density the lower the bacterial load on the pond.

The offending pheromone can be removed with proper chemical and physical techniques.

The importance of water changes should never be under estimated. Regular water changes will dilute the biological 'soup' the fish swim in. To save water utilise the pond water for watering the garden then supply the fish and pond with fresh water. Both will benefit as will your pocket.

The use of activated carbon has been shown to remove organic agents such as pheromones.

Skimming the frothy bubbles or scum that forms on the surface of the pond (Dissolved Organic Compounds or DOC) from time to time will also lower the level of pheromones. The DOC should be removed from the system and not simply skimmed back into the filter.

A last consideration. Perhaps we should select and remove koi from our collections regularly. As they grow and as we add new ones to the system voluntary removing the old ones which are not of good quality will have positive results.

1. The collection will be continually up-graded and improved.
2. The collection will be selectively 'culled' by the owner and not by mother nature taking things into her own hands when disease breaks out in overcrowded pond.


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