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Fish Parasites



Parasitism as a way of life is just one of a huge range of naturally occurring animal (and plant) associations. In general terms, it is an association between different species in which one, the host, is indispensable to the other, the parasite, while the host can survive quite well without the parasite. In fact, the parasitic way of life involves a whole spectrum of associations, ranging from the very close to the very loose. Not all parasites are parasitic for their whole life, for example, and many spend different phases of their life cycle in very different hosts.

Parasites, of course, usually obtain their food from their host, which can cause problems for the host if it is fed a poorly balanced diet and/or if particularly large numbers of parasites are present. Normally, parasites do not kill their hosts, since this would be akin to suicide. However, under some circumstances, the delicate host parasite balance can be upset, and result in the death of the host. Such situations can occur in an aquarium or ponds from time to time.

Because the parasitic way of life is very hazardous, adult parasites usually produce very large numbers of eggs or larvae. This helps to ensure that at least one or two will be successful in finding a suitable host. Within the confines of a pond or aquarium, where host to host transmission is made easier by high stocking levels, this can result in a rapid build-up of parasite numbers.

Parasitism is a natural phenomenon, an integral part of the "cycle of life". However, when parasites cause problems for the fish hobbyist, it is usually the result of poor pond or aquarium management.


Common fish parasites are those that attach to the body surfaces (skin, gills and so on). These include various protozoa (single-celled organisms) and worm-like flukes. Virtually all these skin-dwelling parasites are invisible to the naked eye.

Other fish parasites include tapeworms (inhabiting the fish’s gut or body cavity), thread-like nematode worms (in the gut), and parasitic crustaceans, such as the disc-shaped fish louse (temporarily attaching to skin).

Often, the first sign of a looming protozoa or fluke problem is when one or more Koi repeatedly rub their flanks against submerged objects, or leap from the water. These symptoms can also arise from a water problem (such as high nitrite level) so always carry out water tests before considering parasites as the cause.

Commercial liquid remedies are available for treating common skin and gill parasites these treatments are added directly to the water so you will need to know the volume of your pond in order to administer the correct dosage. Prompt treatment is important because many skin and gill parasites are highly infectious and potentially lethal. If parasite problems persist then consider having your Koi checked by a fish health specialist or a Koi Vet, this should reveal which type of parasite is to blame, enabling the most effective treatment to be selected.

The risks of introducing parasites into the pond can be greatly reduced by obtaining Koi from a reputable dealer (like Blue Ridge Koi) who practices good husbandry and proper quarantine procedures.

For an example of a Reputable Dealer see how Blue Ridge Koi keeps their Koi in the best conditons possible. For the Quarantine process Click Here, this next link includes their Breeding Process and more.

Protozoan Parasites: These are microscopic, single-cell animals. They can be found on the gills, body surface or imbedded in the flesh. There are many different protozoans, and they cause a variety of fish diseases. Common protozoan parasites are Ich (Ichthyophthirius), Trichodina, Costia (Ichthyobodo necator), Chilodonessa, Glossatella, and Epistylis. They have whip-like flagella or hair-like cilia to facilitate movement, and can only be accurately identified by examining a mucus scraping under a microscope. Immediate treatment with the correct fish medication usually ensures eradication and rapidly restores the fish to good health.



Bacterial Diseases


Most bacterial infections occur when the fish is in an immune depressed state due to stress, or when the bacterial load in a pond overwhelms the fish's natural defenses.

Bacterial gill disease, fin rot, and tail rot are common names given to various diseases caused by Flexibacter columnaris, an organism that primarily affects the gill tissue, leading to fusion of the lamellae. It occurs mostly at water temperatures above 59° F (15° C), when the water quality is poor. Lesions may be found on the gills, fins, mouth, head, and flanks of infected fish.

Aeromonas salmonicida is mostly associated with outbreak of "red-eye" ulcers in summer, when the water temperature rises about 72°F (22°C). Erythrodermatitis of carp are bacteria transmitted by water, therefore, overcrowded ponds and handling infected fish will make the problem worse. Parasites like Argulus and flukes may also introduce the disease into a pond.

Bacterial hemorrhagic septicemia is caused by various motile Aeromonas species, such as Aeromonas hydrophila. Motile bacteria are part of normal water microflora, but under poor water conditions they can overcome the fish's defense system, with tragic consequences.


The signs include: ulcer-like lesions of the skin and muscle, necrosis (death of cells) in the fins and tail, exophthalmus (pop-eye), and dropsy. Antibiotic treatments are not very effective and it is best to remove the infected fish from the pond and dispose of it humanely.

Treatment for bacterial infections: Affected fish should be removed from the pond and treated in a quarantine tank. This will limit the possibility of further infections while allowing the patient to be given special care and treatment. The water quality in the quarantine tank must be maintained with daily partial water changes or adequate biological filtration. Maintain a salt concentration of 0.3-0.6 percent.

Viral Diseases


Viruses are very small infectious agents that multiply only within the living cells of an animal or plant host. Other microorganisms, such as bacteria or fungi, have organelles for their own metabolism, but viruses do not. They must utilize the machinery of the infected host cell for growth and reproduction.

A virus has two parts. The internal part is the virion, or virus particle which is composed of nucleic acid, the same material that makes up genes. The virion is enclosed in an external protein coat called a capsid. Viruses are broadly categorized by the type of nucleic acid they contain; the two basic types of nucleic acid are RNA (ribonucleic acid) and DNA (deoxyribo-nucleic acid). Virologists also classify viruses by their shape, for example, "icosahedral" viruses have 20 sides, and "helical" particles are rod shaped.

Viral diseases cannot be controlled with medication because they use the host's own cells for reproduction and survival. It is therefore prudent to provide "good nursing care" for fish suspected of having a viral infection so that their own natural defense mechanisms can work to eliminate infected cells. This involves maintaining excellent water quality, feeding fish a high quality diet, maintaining clean facilities, and keeping sick or potentially infected stock separate from all other animals.


Spring viremia of carp (SVC) causes loss of young fish in spring when water temperatures rise from 50°F to 68°F (10°C to 20°C). Above 68° F (20°C) mortalities usually cease. SVC is endemic to continental Europe but has found its way to many carp producing countries and is classified as a notifiable disease by the World Organization for Animal Health.

Carp pox, a smooth, wax like area on the skin, has the doubtful honor of being the oldest documented fish disease. It is activated when the water temperature falls below 57°F (14°C). Because of its disfiguring effect, carp pox may reduce the value of a particular Koi. Even though it is not very infectious, fish with carp pox should not be bought, but it is not essential to remove fish carrying it from your collections.

Hikui, or red cancer, a disease of the red pigment cells on the epidermis, has a disfiguring effect on otherwise healthy fish. Areas in the red pattern become discolored and appear ulcerated or blistered, although the disfiguration does not seem to go deeper than the dermis. Sarcoma (cancer) of pigment cells in fish is not uncommon, but hikui is believed to have a viral origin, as antiviral treatments seem to contain the spread. Fish with hikui should not be used for breeding as they may pass the disease to their offspring.

Koi herpes virus (KHV) is a viral disease of great significance to the Koi industry, as there is no cure for it. The disease is restricted to Koi and carp, occurs at temperatures of 64° to 82°F (18° to 28° C), has an incubation period of 7-14 days, and a mortality rate above 80 percent.

An affected fish will appear lethargic and swim with erratic movements until it finally just hangs motionless and dies. Some fish may have sunken eyes. Necrosis of the gill tissue, increased mucus secretion, and external and internal hemorrhaging are the most common clinical signs. The gills, kidneys, and hepatopancreas are carriers of viral particles.

KHV is transported in water and can stay active for about 24 hours without a host, allowing it to spread by seepage through groundwater. Fears that birds and animals can transmit the disease are probably over exaggerated.

To protect against KHV, all newly acquired Koi or pond fish should be quarantined for three weeks at a temperature of 73°F (23°C). If no symptoms are noticed during this time the Koi is probably not infected. To make doubly sure, an expert can perform a PCR test on a small gill snip, or take a swab, to confirm the absence of viral particles. If KHV is detected, the affected Koi should be disposed of humanely.

If any Koi dies unexpectedly, it can be dissected and pea sized samples of the kidney, liver, and gills sent for testing to determine the cause of death.



The most effective way to keep your Koi healthy is to start using KoiZyme in your Pond. Koizyme is 100% successful in excluding Aeromonas and Pseudomonas. Since Aeromonas and Pseudomonas are usually the main cause of fish disease, if you are able to elimate them in your pond, your Koi stay Healthy. Start KoiZyming today!



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