Fungal Fish Diseases and Their Control Methods
Fungal fish diseases can become a problem if fish are stressed by disease, by poor environmental conditions, receive poor nutrition, or are injured. If these factors weaken the fish or damage its tissue, fungus can infest the fish.
Fungi are a group of organisms called heterotrophy that require living or dead matter for growth and reproduction. Unlike plants, they are incapable of manufacturing their own nutrients by photosynthesis.
Fungi are present everywhere; in saltwater or fresh water, in cool or warm temperatures. In most cases, fungi serve a valuable ecological function by processing dead organic debris.
All fungi produce spores and it is these spores which readily spread disease. The fungal spore is like a seed which is resistant to heat, drying, disinfectants and the natural defense systems of fish.
The three most common fungal diseases are discussed here. They are known as Saprolegniasis,
Branchiomycosis and Ichthyophonus Disease
Fungal diseases are the fourth type of infectious disease. Fungal spores are common in the aquatic environment, but do not usually cause disease in healthy fish.
When fish are infected with an external parasite, bacterial infection, or injured by handling, the fungi can colonize damaged tissue on the exterior of the fish. These areas appear to have a cottony growth or may appear as brown matted areas when the fish are removed from the water.
Formalin or potassium permanganate is effective against most fungal infections. Since fungi are usually a secondary problem it is important to diagnose the original problem and correct it as well.
Fungal Fish Diseases
The disease is characteristic white, cotton-like growth of fungus on the skin or fins of infected fish. Known as Saprolegnia sp., this fungus readily invades stressed freshwater fish, but is not known to affect decorative fishes.
Signs of fungal infection in fish are therefore less dramatic and often difficult to distinguish. Whenpresent, the fungus appears on fishes as a fine film, dark pigmented areas, or a coating covering areas of the body or gills.
This coating can easily be confused with similar signs that are caused by the presence of various types of parasites.
1. Saprolegnia sp.
Saprolegniasis is a fungal disease of fish and fish eggs most commonly caused by the Saprolegnia species called “water molds.” They are common in fresh or brackish water.
Saprolegnia can grow at temperatures ranging from 32° to 95°F but seem to prefer temperatures of 59° to 86°F.
The disease will attack an existing injury on the fish and can spread to healthy tissue. Poor water quality (for example, water with low circulation, low dissolved oxygen, or high ammonia) and high organic loads, including the presence of dead eggs, are often associated with Saprolegnia infections.
Among the most common and most easily diagnosed fish “diseases” is saprolegniosis. This is a fungal invader that reveals itself with a profusion of white, gray, green or brown cottony tufts protruding from the fish’s skin. It can appear anywhere on the fish.
Saprolegnia parasitica infection in the fry and fingerlings of major carps is one of the main problems affecting health of fishes.
The disease in fish is characterized by a white to brown cotton like growth consisting of colonies of mycelium and filaments which appear as small to large patches on various parts of the body like fins, gills, mouth, eyes or muscle.
The infection starts due to netting injury and overcrowding or lesions caused by other diseases. Saprolegniosis in fish is primarily caused by one of three genera of aquatic fungi; Saprolegnia spp.
Achlya spp. or Dictyuchus spp. All their basic characteristics are virtually identical (even fungi specialists have trouble sorting them out), so it’s best to think of them as a single fungal invader.
These fungi are found in all freshwaters worldwide and are always present in some form. Healthy fish easily control this fungus disease. The outer slime coat that covers a fish’s skin acts as a shield against fungal invasion.
However, when fish are physiologically stressed or when they are battling some bacterial or parasitic disease they become susceptible to fungal invasion. In this respect, Saprolegniosis is not a primary disease, but becomes a disease problem after some other serious health problem exists.
Once saprolegniosis takes hold it can pose a much greater risk to the host fish than the initial disease. The fungi spread vegetatively below the host’s skin tissues, and underlying muscles, flesh and bone are exposed as the dead skin drops away.
Disease Signs
Saprolegniasis is often first noticed by observing fluffy tufts of cotton- like material–colored white to shades of grey and brown on skin, fins, gills, or eyes of fish or on fish eggs. These areas are scraped and mounted on a microscope slide for proper diagnosis.
Under a microscope, Saprolegnia appears like branching trees called hyphae. With progression of infection fish usually becomes lethargic and less responsive to external stimuli. So fish under such conditions is a target to predators.
Management and Control
Saprolegniasis is best prevented by good management practices such as good water quality and circulation, avoidance of crowding to minimize injury (especially during spawning), and good nutrition.
Once Saprolegnia is identified in an aquatic system, sanitation should be evaluated and corrected. Common treatments include potassium permanganate, formalin, and povidone iodine solutions.
Over treatment can further damage fish tissue, resulting in recurring infections. Environmental management is essential for satisfactory resolution of chronic problems. Bath treatment in NaOH (10-25g/lit for 10-20min), KmNO4 (1g in 100lit of water for 30-90 min), CuSO 4 (5-10g in 100 lit water for 10- 30min).
Treatment of fungal infections of the body and gills involves the use of various fungicides available commercially. Malachite green and methylene blue have been used successfully to control fungal infections in decorative fish.
Various drugs are also useful for controlling both fungal and bacterial infections. Since fungus is a secondary invader, treatment methods must also address the initial cause of the disease, including trauma induced by deteriorated water quality, poor nutrition, or poor handling of the fish.
2. Branchiomycosis
Branchiomyces demigrans or “Gill Rot” is caused by the fungi Branchiomyces sanguinis (carps) and Branchiomyces demigrans(Pike and Tench).
Branchiomycosis is a pervasive problem in Europe, but has been only occasionally reported by other countries fish farms.
Both species of fungi are found in fish suffering from an environmental stress, such as low pH (5.8 to 6.5), low dissolved oxygen, or a high algal bloom.
Branchiomyces sp. grow at temperatures between 57° and 95°F but grow best between 77° and 90°F. The main sources of infection are the fungal spores carried in the water and detritus on pond bottoms.
Disease Signs
Branchiomyces sanguinis and B. demigrans infect the gill tissue of fish. Fish may appear lethargic and may be seen gulping air at the water surface (or piping). Gills appear striated or marbled with the pale areas representing infected and dying tissue.
Gills should be examined under a microscope by a trained diagnostician for verification of the disease. Damaged gill tissue with fungal hyphae and spores will be present.
Read Also : Other Bacterial Fish Diseases and their Control Methods
As the tissue dies and falls off, the spores are released into the water and transmitted to other fish. High mortalities are often associated with this infection.
Management and Control
Avoidance is the best control for Branchiomycosis. Good management practices will create environmental conditions unacceptable for fungi growth. If the disease is present, do not transport the infected fish.
Great care must be taken to prevent movement of the disease to non-infected areas. Formalin and copper sulphate have been used to help stop mortalities; however, all tanks, raceways, and aquaria must be disinfected and dried.
Ponds should be dried and treated with quicklime (calcium oxide). A long term bath inAcriflavine Neutral or Forma- Green for seven days helps this condition. Ponds should be dried and treated with quicklime (calcium oxide) and copper sulphate (2-3kg / ha). Dead fish should be buried.
3. Ichthyophonus Disease
Icthyophonus disease is caused by the fungus, Icthyophonushoferi. It grows in fresh and saltwater, in wild and cultured fish, but is restricted to cool temperatures (36° to 68°F).
The disease is spread by fungal cysts which are released in the faeces and by cannibalism of infected fish.
If a fish appears lethargic and exhibits a loss of equilibrium, it may have a fungal disease: Ichthyophonus. Most cases of dropsy are fatal.
Although Ichthyophonus fungi are generally considered a fungal disease of marine fish, it does show up in freshwater fish from time to time. Infected fish become lethargic, and if the brain is infected, they may exhibit a loss of equilibrium as well as staggered movements.
There is no treatment for this fungal infection, but fluconazole, which is a relatively new antifungal agent active against Saprolegnia fungi, may also be affective against Ichthyophonus fungi.
Fluconazole should be administered at the rate of 6 mg/L (22.6 milligrams per gallon) daily for five consecutive days. If there is no improvement over the course of two weeks, you might consider euthanasia.
Disease Signs
Because the primary route of transmission is through the ingestion of infective spores, fish with a mild to moderate infection will show no external signs of the disease.
In severe cases, the skin may have a “sandpaper texture” caused by infection under the skin and in muscle tissue.
Some fish may show curvature of the spine. Internally, the organs may be swollen with white to grey-white sores. Diseased fish shows curious swinging movements hence the disease is called as swinging disease.
Along with liver, particularly severely affected organs are:- spleen(salmonids), heart(herring), kidney(salmonids), gonads, brain(salmonids), gills(salmonids), and musculature and nerve tissue behind the eyes(sea fish).
Management and Control
There is no cure for fish with Icthyophonus hoferi; they will carry the infection for life. Prevention is the only control. To avoid introduction of infective spores, never feed raw fish or raw fish products to cultured fish. Cooking helps destroy the infective life stage.
If Icthyophonus disease is identified by a trained diagnostician, it is important to remove and destroy any fish with the disease. Complete disinfection of tanks, raceways, or aquaria is encouraged. Ponds with dirt or gravel bottoms need months of drying to totally eliminate the fungus.
In summary, aquatic fungi often are considered secondary tissue invaders that follow traumatic injuries, infectious agents, or environmental insults such as poor water quality. Because many fungi grow on decaying organic matter, they are especially common in the aquatic environment.
Fish egg masses, which usually contain tissue debris and dead ova or embryos, are especially vulnerable.
Fungal diseases are often indicative of a more serious problem. Saprolegniasis is a common fungal disease which affects the external surfaces of fish. It can be eliminated easily after the primary cause of illness has been identified and corrected.
On the other hand, Branchiomycosis, a relatively new problem and has caused high mortalities in cultured fish, and is difficult to control. Ichthyophonus disease is a systemic fungal disease and once it enters the fish, there is no cure.
