Diseases of Fish
Ruth Ellen Klinger and Ruth Francis-Floyd
Fungi are a group of organisms called heterotrophs 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. However, fungi 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 can also prevent successful
hatching when it invades fish eggs.
Fungi are grouped by the morphology of various life stages. 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.
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. The presence of Columnaris bacteria or external parasites
are also common with Saprolegniasis.
Saprolegniasis is often first noticed by observing fluffy tufts of
cotton-like material--colored white to shades of gray 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.
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. If mortality is in progress, medication is appropriate. Common
treatments include potassium permanganate, formalin, and povidone iodine
solutions. Overtreatment can further damage fish tissue, resulting in
recurring infections. Environmental management is essential for
satisfactory resolution of chronic problems.
Branchiomyces demigrans or "Gill Rot" is caused by the fungi Branchiomyces
sanguinis and Branchiomyces demigrans . Branchiomycosis is a
pervasive problem in Europe, but has been only occasionally reported by
U.S. 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.
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. 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 noninfected
areas. Formalin and copper sulfate 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
Icthyophonus disease is caused by the fungus, Icthyophonus hoferi .
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 feces and by cannibalism of
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 gray-white sores.
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 are encouraged. Ponds with
dirt or gravel bottoms need months of drying to totally eliminate the
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 in the U.S., 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. The best control for all fungal infections is good management:
good water quality, good nutrition and proper handling.
Ruth Ellen Klinger, Large Animal Clinical
Science, biological scientist
Ruth Francis-Floyd, extension
veterinarian, Department of Large Animal Clinical Science, Cooperative
Extension Service, Institute of Food and Agricultural Sciences, University
of Florida, Gainesville, 32611.
This document is copyrighted by the University of Florida, Institute
of Food and Agricultural Sciences (UF/IFAS). Date printed February