Skin Disorders in Fish

Skin disorders in fish are especially harmful. Any surface injury to the skin makes osmoregulation (fluid balance) more difficult and can lead to circulatory malfunction. The skin and mucus are extremely important protective barriers for fish. They seal the fish so fluid balance is more easily controlled. The mucus allows fish to slip through water more easily, so less energy is used while swimming. There are also several protective compounds in the mucus that protect the fish from bacteria and other organisms in the water. Various types of parasites, from tiny single-celled protozoa to larger lice and worms, can cause skin disorders in fish, as can bacteria, viruses, and other organisms.

Sunburn can be a problem in fish that swim near the surface. Access to shade should be available to fish housed in outdoor ponds. Plants can be an excellent source of protection from direct sunlight.

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Disorders Caused by Bacteria

Flavobacterium columnare are bacteria that cause columnaris disease (sometimes called saddleback or cottonmouth disease). Signs may include skin lesions with slimy or cotton-like excretions. It is common in warm-water fish. Early treatment with potassium permanganate can help, but if the disease is not recognized in the early stages, antibiotic treatment is generally needed.

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Read also: Disorders and diseases of fish

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Disorders Caused by Viruses

Carp pox is one of the oldest recognized fish diseases. It is caused by cyprinid herpesvirus-1. It is primarily a disease of koi. The skin abnormalities are smooth and raised, possibly with a milky appearance. They do not usually cause problems, but they can be a site of secondary bacterial infection. Carp pox is of particular interest to koi enthusiasts because the high value of these fish is based on appearance. Quarantine is essential, and any infected koi should be removed. Surgery to remove the pox lesions is not helpful.

Lymphocystis disease is a viral infection that can affect both saltwater and freshwater fish. In home aquariums, painted glass fish are especially susceptible. The main sign is cauliflower-like growths on the skin or fins. Microscopic examination of tissue is necessary to confirm that the problem is lymphocystis. These growths do not usually cause a health problem but do affect the appearance of the fish. Infections usually are not life-threatening and resolve without treatment.

Disorders Caused by Protozoa

A common protozoal infestation in home aquariums and ponds is ich, or white spot disease. This disease is caused by Ichthyophthiriusmultifiliis in freshwater fish and by Cryptocaryon irritans in saltwater fish. The organisms attach to the fish and burrow into the skin and gills. The resulting cysts appear as visible white spots. Microscopic examination of diseased tissue is required to confirm the diagnosis. As the protozoa begin to reproduce, they create a wound in the fish and sink to the bottom of the tank or pond to reproduce. One parasite can produce hundreds of new organisms. This rapid and massive reproduction is what makes this parasite so deadly. The damage caused by large numbers of these parasites can make a fish more susceptible to infection by other agents or cause a loss of bodily fluid. Treatment is with either copper sulfate or formalin, both of which can be obtained from your veterinarian or pet store. As with all medications, label instructions should be followed carefully. The treatment must be applied at specific intervals based on the water temperature, so advice from a fish health professional is recommended.

Another serious disease in aquarium fish is called velvet (also known as rust or gold dust disease). It is caused by protozoan parasites that attack the gills and skin, causing fine yellowish spots that are smaller and harder to see than the ones that occur with ich. Sometimes these appear as a thin, velvety film covering the skin. Other signs include loss of appetite, lethargy, and a tendency for affected fish to scratch against rocks or other hard objects. High death rates are common. Both freshwater and saltwater fish may be affected. In freshwater fish, the cause is Piscinoodinium (also known as Oodinium). In saltwater fish, a related parasite called Amyloodinium is responsible. The parasites attach to the skin and gills. Piscinoodinium and Amyloodinium can be identified by microscopic examination of gill, skin, or fin tissue. Your veterinarian may recommend chloroquine, an effective treatment for ornamental fish (chloroquine cannot be used for food fish). Instructions for giving it must be carefully followed. A recheck of the fish in 7 to 10 days may be needed.

Other protozoan parasites such as Chilodonella, Brooklynella, and the trichodinids may infest the gills or skin of aquarium fish. Infected fish may seem to have excessive amounts of slime or mucus. Microscopic examination of diseased tissue is required to confirm the diagnosis. Signs include dulled coloration, a light gray-white covering of mucus on the body of the fish, gill damage, and general weakness. Fish often rub against objects to relieve itching. Other observations may include rapid breathing, piping (swimming near the surface of the water, trying to gulp air), flashing (scratching), and loss of condition. Once the diagnosis is confirmed, fish can be treated. Formalin is often effective. The trichodinids in particular are often associated with overcrowding or poor sanitation, in which case cleaning the system is an important component of the treatment plan.

Tetrahymena corlissi and Uronemaspecies are parasites that are usually found on the skin, gills, or fins, but they can also be found inside fish, including in skeletal muscle and the fluids of the eye. These parasites are usually found in water that has a high level of organic matter. If the parasites are only on the surface of the fish, the infestation can be cleared with good sanitation and chemicals. If the parasites have moved inside the fish, the condition is not treatable and is often fatal.

Ambiphyra and Apiosoma are parasites that attack the skin and gills. These parasites are more common in pond fish than in aquarium fish and usually do not affect saltwater species. Low numbers of these parasites are not a problem, but high numbers can damage the skin and gills, which compromises breathing and can leave the fish susceptible to other infections. Treating involves using formalin, copper sulfate, potassium permanganate, or salt. Excessive crowding and poor sanitation are predisposing factors and should be avoided.

Ichthyobodospecies are common parasites of the skin and gills of aquarium, pond, and saltwater fish. They can be difficult to see but look like a flickering flame when infected tissue is examined with a microscope. Affected skin may appear to have a steel-gray discoloration, and mucus production called “blue slime” or “gray slime” may be seen. Behavioral signs include lethargy, poor appetite, piping (swimming near the surface, trying to gulp air), flashing (scratching), and overall weakness and loss of condition. Microscopic examination of infected tissue is required to confirm the diagnosis. Poor sanitation, crowding, or overfeeding can contribute to proliferation of the parasites and should be corrected. Salt, formalin, copper sulfate, or potassium permanganate baths can be effective treatments after underlying problems have been corrected.

Disorders Caused by Larger Parasites

The anchor worm (Lernaea) is a parasite that buries its head into the muscle tissue of a host fish. Despite the name, anchor worms are not worms, but crustaceans. These parasites can damage gill tissue. Pond fish are the most common hosts. The fish may scratch against objects to try to knock the parasite off. The parasite can easily be seen, appearing as whitish-green threads hanging off the red, inflamed skin of the fish. The parasite can be removed manually. Treatment is recommended to eliminate the parasite from the pond or aquarium.

Fish lice (Argulus) and leeches are parasites that attach themselves to a host fish, penetrate the skin, and feed on the blood. The fish scratch against objects to try to remove the lice. Lice look like small, clear disks that lie flat against the skin. Leeches are worm-like parasites that contract when touched. Treatment involves removing the parasites from the fish. The tank should be treated to kill any lice larvae that may be present. Leech infestations can be seasonal, but eggs may have been laid in the system, resulting in reinfection after the adult parasites have been eliminated. Treatment is usually recommended if the system has no contact with surface water.

Gyrodactylus and Dactylogyrus are tiny flatworms that are skin and gill parasites of goldfish, koi, and other fish. The parasites are usually too small to be seen without a microscope. Fish become pale and can have skin sores with scattered hemorrhages and ulcerations. The death rate can be high when fish are heavily infested. Formalin or praziquantel are the treatments most often used for these infestations, and quarantine is a good practice to help prevent introduction of these parasites into a healthy environment.

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Some saltwater parasites (Neobenedinia and related capsalid parasites) that have a similar effect as Gyrodactylus and Dactylogyrus do in freshwater fish. The marine organisms can be more virulent, however, because they are much larger and therefore can cause more damage to the fish. They also lay very sticky eggs that can be easily spread by nets or other objects to uninfected systems. Praziquantel is an effective treatment for adult ornamental fish (not for food fish). Reinfection remains a threat if eggs have been released into the system.

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