Common Fungal Diseases Of Fish And Their Control Measures
Prabjeet Singh1, Sajid
Mohd Danish1, Nitin Verma1,
Kuldeep Singh Rana1, Shashank Singh1
and Nandkishor Ingole1
1College of fisheries, G.B.Pant
University of Agriculture and Technology, Pantnagar, Uttrakhand,
of food technology, Faculty of Agro-Industry, Prince of Songkla,
of Fisheries, SKUAST-K, India
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. 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
defence 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
Saprolegniasis is often first noticed by observing fluffy tufts of
cotton-like material--coloured 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).
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 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.
Carp with branchiomycosis (gill mycosis): Photo Bayerische
Biologische Versuchsanstalt (Adopted from Fish pathology by
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
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 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 in
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.
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 faeces and by
cannibalism of infected fish.
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).
(Adopted from Fish pathology by Reichenbach-Klinke's)
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.
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. The best control for all fungal
infections is good management: good water quality, good nutrition and
Meyer, F.P., and J. A. Robinson. 1973. Branchiomycosis: a new fungal
disease of North American fishes.
Klinger, R.E and Floyd, R.F. 1996. Fungal diseases of fish. Fact
sheet VM 97, FAIRS, Florida.
Seafood — Fish — Crustacea
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