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Healthy Gills make Healthy Koi

article by Dr Paula Reynolds Aquatic Patho-biologist

The structure of the gill


The deep red colour of the normal gill filament is a reassuring sight to Koi-keepers as this is a good indication that the gill is healthy. The filaments are comprised of what are termed the primary and secondary lamellae and there are two paired filaments in each gill cavity. Whilst mature fish can develop a ragged edge to their filaments without this being significant, this is rare in juvenile Koi. Any discolouration of the gill tissue or excess mucus covering the filaments suggests a health problem that needs investigation, as the gills are vital to the well being of Koi.


The circulatory system


Once blood has passed through the heart it flows into the gill cavity, and a pressure gradient is created. Blood is pumped in one direction through the gill lamellae whilst the pond water flows in the opposite direction over the lamellae and this forces oxygen from the water into small blood vessels known as capillaries. At the same time, special cells called erythrocytes discharge unwanted carbon dioxide out of the gills. The blood flows from the gills through the arterial system and around the body delivering many vital products including oxygen and picking up unwanted waste materials and carbon dioxide. It is returned to the heart again via the venous system. The smaller blood vessels or capillaries allow the supply of blood into inaccessible areas of the body where larger blood vessels would be unable to reach. The actual route of the circulatory system varies with fish species. Koi being cyprinids are part of the large Carp family but they do not share identical pathways for blood flow.


Diffusion and oxygen


Diffusion is the term for the process equivalent to breathing in humans. However, extracting oxygen from water is more wasteful in terms of energy than taking a breath is to air-breathing animals. This is because Koi have to extract their oxygen from the water whereas humans obtain it directly into their lungs. Koi are able to conserve energy in cold weather by gradually becoming less active as water temperatures decrease. They are able to keep many of their body resources in reserve to support them through a long cold winter by various adaptations in the way that the body functions at higher water temperatures. For example, Koi do not produce the same level of erythrocytes or red cells that carry the oxygen around the body during the dormant phase. In summer, water is warmer and therefore contains less dissolved oxygen and hobbyists have to increase the level with aeration. In winter, Koi need less oxygen and are able to save the energy they spend obtaining it.  


Ammonia and osmosis


The gills are responsible for excreting ammonia not the vent. Ammonia is actually produced by the metabolism of proteins. It is fact that fish generally, not just Koi, require far higher levels of protein in their diets than other animals. This is because carbohydrates do not naturally occur in the aquatic environment so fish have never eaten them or adapted in order to use them as an energy source. Instead, they rely on the amino acids found in protein, converting some for energy and utilising the rest for growth or tissue regeneration, and healing.


Fish have to be able to adapt to changing salt concentrations in the water they live in whilst balancing their body fluid level. These processes are controlled by osmoregulation and this differs in saltwater fish. Unlike certain species, the kidneys in Koi produce large amounts of dilute urine. Water flows in to the body and through the higher osmotic pressure in the circulatory system and the level of urine excreted lowers salt levels in the body. Special cells in the gill lamellae release sodium and chloride to maintain a balanced concentration in the body. Koi being a freshwater species should live in salt free water unless there is a health problem and then short-term use only is advisable.


Fluid balance is critical for health and the kidneys cannot function normally during very stressful events or disease as they both put the body under pressure. Dropsy is often observed in Koi whose kidneys are not functioning properly .Salt at ½ an ounce per gallon is recommended for affected Koi to relieve fluid retention. Further treatment may be needed depending on the underlying cause of the dropsy, as this is a condition not a disease. Freshwater fish expend much energy regulating ion loss and fluid uptake and it is a continual process even when the Koi are at rest or dormant. This is yet another reason why the diet is so important to ensure that the energy required for this and other body systems to function is in reserve. Not being fed enough food in the UK climate is a major factor in well being and the actual ingredients in food is another. For example, feeding a food that is intended for other fish species can result in weak Koi


Bacterial Gill Disease


New ponds with immature filtration systems can be a threat to delicate gill tissue particularly if the hobbyist is a novice and not familiar with the problems posed by ammonia and nitrite as both substances can trigger gill problems. If Koi are not prepared well for shipping or the plane flight is delayed, this can allow the conditions in the transport bag to deteriorate and give rise to gill disease. However, this is not as common a problem in imported fish as it once was due to better preparation of Koi for export and good dealer quarantine here in the U.K. For bacterial gill disease to break out depends on the criteria being present it is very unlikely to suddenly break out in a healthy Koi pond. Usually an affected pond will be heavily stocked and the water temperature high. The gills of the infected fish will possibly have been exposed to pollution, the over use of chemicals or parasites and damage to the filaments will have allowed bacteria to enter the tissue. Any changes in gill tissue are of concern, as healthy gills are vital to survival.


Several causes


It is important to try to discover the cause of changes to the colour or appearance of gill tissue. Water conditions are the most common factor and this is easy to test and correct. If rectified soon enough the gills can fully recover. Is there enough dissolved oxygen available for the Koi? This is a question that is not asked often enough. In many cases, the fish are seen continuously near a source of oxygen and this can indicate a deficit in dissolved oxygen in solution in the water. However, it also suggests the fish are conserving energy for a reason and gill parasites should be investigated. When every Koi in the pond is affected this usually means the cause is the water quality or at least a pond problem. However, when one or two Koi are not behaving normally the colour of the gill tissue is important. When pink the tissue is under oxygenated and this may mean aeration is in adequate but individual fish with a disease can also have pale gill tissue. If the filaments appear coated in a film or mucus then a toxic substance is one possibility but bacterial gill disease can give rise to such signs, diagnosing gill disease is not straightforward.


How serious is necrosis  


Areas of erosion or grey patches in the gill filaments suggests tissue necrosis. This is common following damage by toxins, parasites, or bacteria. However, there are diseases that affect the gill as part of their progress without being an actual gill disease. Koi Herpesvirus (KHV) for example can destroy gill tissue in some outbreaks although this much depends on water temperatures and how the disease develops. It is not a gill disease and in certain cases, the gill appears normal and other signs are observed.


Ammonia and Nitrite


Ammonia and nitrite are both life threatening if Koi are exposed for too long as they can trigger internal changes that result in disease. Ammonia tends to damage the gill tissues directly and the edges of the filaments can become white or grey in colour and very prone to bacterial infection. After a few days of exposure to high nitrite, the skin in Koi can appear to be covered in soap. Methaheamaglobin is a condition in which oxygen cannot bind with the red blood cells that normally transport it around the body. The blood becomes brown and this is reflected in the colour of the gill tissue. Nitrite is the prime cause of what is termed brown blood disease. When the water conditions are not tested and the early signs that the fish are off colour pass unnoticed major organ failure can result.                                                                                  


Gill parasites


Most Koi parasites will enter the gill cavity to see what nourishment is available inside but not all of them take up residence and start breeding. It is those organisms that have to rely on the conditions inside the gill in order to complete their full life cycle that have the potential to do the most damage For example, Dactylogyrus the gill fluke and Ergasilus the gill maggot. Fortunately, one or two gill fluke does not constitute a health risk and it is easy to identify as this organism has eyes that skin fluke does not possess .if the number of gill  flukes  suggest controlling them is  required  it is fairly easy to eradicate  so mortalities are rare. Outbreaks of the gill maggot are unusual in a modern Koi pond being a parasite more commonly found in the more established ponds and lakes although imported Koi occasionally carry it. White spot, chilodonella, and costia can damage the gill to varying degrees whereas Gyrodactylus the skin fluke will not stay long inside the gill and favours the body. Trichodina is only a threat when seriously out of control. Skin fluke and trichodina are not true fish parasites and their continual present in a mucus smear suggests that there is debris in the pond system sustaining their life cycle.  All Koi will carry a few parasites most of their lives without needing any chemical treatment.


Fungal gill disease


Many water-borne organisms are harmless to Koi and live alongside them in harmony. Even those that are a potential threat need appropriate conditions before mounting an attack. We term this kind of organism an opportunistic parasite as it waits in the guise of a friend for weakness in the Koi that enables it to live off the fish and certain fungi are just such organisms. Fungi are not primary pathogens they rely on an entry portal created by poor conditions, parasites, bacteria, or injury in order to invade the body and the gill tissue is no exception. Saprolegniasis is the most common fungus isolated from skin and gill lesions and investigations often reveal several species.


Rarer fungi


Another fungal disease rarely seen in Koi ponds is Branchiomycosis and to complicate matters Saprolegnia species can invade gills already compromised by Branchiomyces species. This disease is usually found in overcrowded aquatic environments such as farms at held at higher water temperatures. It is juvenile Koi that figure higher in mortalities during an outbreak of Branchiomycosis due to the overstocked nature of fish farms were this disease is prevalent. However, newly imported Koi could be affected if the conditions are conducive. The decision to isolate fish suffering from any fungal infection is not straightforward as such fish are a “factory" producing spores which are infective and this is not a healthy situation for Koi so far unaffected. However, the gill tissue has to be damaged by an underlying health problem to be at risk of secondary fungal disease. Treating fungal infection with a fungicide can be effective but it is important to bear in mind the secondary nature of fungal infection. A fungicide may cure the fungus but not the primary problem. It is safer to establish the actual disease as soon as possible.


Treating gill problems


Gill tissue can heal and regenerate as long as the damage is not too extensive and the fish is not subjected to inappropriate chemical treatments. When the gill is compromised high levels of dissolved oxygen are vital to compensate for the inability to diffuse oxygen adequately. Saving fish can depend on investigating the bacterium involved in the disease outbreak and finding a suitable antimicrobial treatment. Speed is essential as mortality levels are high whenever the gill is infected and once the tissue becomes necrotic, there is far less chance of recovery. Adding salt at 1/2, an ounce per gallon can be beneficial as can the use of a gentle bactericide that will safely combine with salt such as acriflavine. When Koi are in isolation an antibiotic suitable for use in water can be tried but antibiotics cannot be introduced into a pond. They will not only damage filter biology and  some actually become toxic to fish as they break down and can only be used in short term baths which are not very successful  . Mortalities are usually high unless early treatment is implemented although gill disease in general does not respond well to any medication, irrespective of the cause.


Getting enough oxygen in summer


Koi need more oxygen than many other fish species during the warmer months of the years. Hobbyists provide aeration but confusion surrounds the production of dissolved oxygen. Fast moving water and waterfalls appear to supply plentiful levels of oxygen in a pond as they move the water. However, oxygen has to be dissolved in the water to be available to the gills, in many ponds the oxygen is not in solution and cannot access the gills adequately. Low levels of oxygen can lead to general health problems not just those affecting the gill and mortalities can be very high in under aerated ponds depending on the conditions and water temperature.


Oxygen is life


An air diffuser creates dissolved oxygen although such devices are not just for summer use. Spring and autumn water temperatures can give rise the need for supplementary aeration and if the pond has to be treated with any product whether a natural material or a medication this can lower the oxygen available to the Koi. Koi need 6mg/L just to look good sat around on the pond floor or sunbathing in the shallows and they can feel lethargic just like humans when oxygen is in short supply. To swim around vigorously and feed normally Koi should have 10 mg/L and 12mg/L. This allows for any shortfall on a warm day as the higher the temperature the lower the dissolved oxygen level will be. The pond filter contains various life forms that all require oxygen as do plants  and  the way in which ponds operate means filter bacteria and plants get their oxygen immediately  and  the Koi being last in the supply chain  have to get  by on  what is left.


Dissolved oxygen is not just vital to keeping Koi alive it also keeps health problems at bay including those that affect the gill. When it is in plentiful supply, dissolved oxygen promotes a balance in the natural biological life of the pond.

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