IV- MYCOTIC DISEASES

Fungi were reported to be responsible for many fish diseases. These fungi belong to a wide range of taxa and the most frequent are the water moulds belonging to Oomycetes. They are widespread in fresh water and represent the most important fungal group affecting wild and cultured fish. The saprolegniaceae, in particular members of the genus Saprolegnia, are responsible for significant infections, involving both living or dead fish and eggs of the fish.

The major mycotic diseases , which face the fish production are :

 

Saprolegniasis

Branchiomycosis (gill rot)

Ichthyophonosis

 

 

Saprolegniasis

 

Definition:

It is fungal disease of fishes and fish eggs caused by member of family saprolegiaceal. Characterized by presence of cotton like, white to gray growth on the skin, gills or eyes of fishes or in fish eggs.

Several investigators considered the saprolegniasis as the most significant mycotic disease affecting wild and culture fish causing economical losses, because the disease is difficult to prevent and treat, particularly in the intensive fresh water system, and is reported to be second only to bacterial disease.

Outbreaks of Saprolegnia spp. in farmed fish are usually restricted to chronic but steady losses. However, egg mortality can increase rapidly, causing significant numbers to die that may have a major economic impact. During severe winter, some farmers in the USA have reported losses of up to 50% in farmed catfish, with an annual economic cost of about 40 million dollar In Japan, the infection of coho salmon with Saprolegnia parasitica caused significant losses exceeding 50% per annum. In Egypt, the disease reported in wild fish and culture fish causes mortality of fish and fish eggs.

 

Causative agent:

Saprolegnia is the main genus of water molds responsible for significant fungal infections of freshwater fish and fish eggs. Saprolegnia parasitic, Saprolegnia diclina and Achlya hoferi are the major etiological agents of saprolegniasis. A broad range of media has been used for the culture of fungi from fish. Often, the media are supplemented with antibiotics and a low-nutrient medium is preferred to reduce growth of saprophytic species and bacteria. The media are glucose yeast extract agar, glucose peptone agar, Sabouraudís dextrose agar and corn-meal agar.

The fungus forms long, branched and non septated hyphae. Incubation temperature ranges between 5 and 37oC, however, temperature of 10, 15 and 20oC are the most common.

The hemp seeds also is used for isolation and identification the saprolegnia species. Saprolegnia has complex life cycle, which includes both sexual and asexual reproduction.

1.     Sexual reproduction involves the production of the antheridium and oogonium.

2.     Asexual spores of Saprolegnia release motile, primary zoospores . These are active only for a few minutes before encysted, germinated and release a secondary zoospore. The are motile for long period of time, they are considered the infectious spore of Saprolegnia.

Fish have three types of defenses against Saprolegnia:

1.     Physical renewal of attaches spores by the renewal of mucous.

2.     Morphogen in the mucous inhibited the growth of mycellium but not killing it.

3.     Celluler response in the mucous have direct effect at growing mycellium.

 

Predisposing factors:

1.     Malnutrition among cultured fish.

2.     Presence of toxic substances in food or water or damage to skin, fin or gills from rough handling or external parasites may lead to secondary invasion by Saprolegnia spp.

3.     Physical stresses such as reduced water temperature, high or low pH or high salinity may be responsible for invasion of fish by Saprolegnia.

4.     Over crowding in fish culture increase the incidence of saprolegina infection.

5.     High organic load is also identified as a cause of increase infection by Saprolegnia spp.

6.     Saprolegeniasis may be secondary to other infection e.g. fin rot, ulcer disease and other bacterial disease.

 

Susceptible species:

Most species of fresh water fish are susceptible to take the infection with Saprolegnia, especially those fish in intensive aquaculture. Saprolegnia also infects the fish eggs by adhesion and penetration of egg membrane and can spread from dead eggs to healthy one.

 

   Mode of transmission:

The disease is transmitted by:

1.     Direct contact between diseased fish or fish eggs and healthy one.

2.     Indirect contact through several sources, including, the water supply, transport vehicles, movement of staff between aquaculture facilities and farm equipment, such as nets.

 

Clinical signs:

Saprolegniasis is characterized by:

1.     The appearance of cotton -like, white to grey growth on the skin, gills, fins and eyes or eggs of fish.  These growth is white or grey white in colour.

2.     In severe cases, 80% of body may be covered with fungal growth.

3.     In early infections, skin lesions are gray or white in color with a characteristic circular or crescent shape, which can develop rapidly causing destruction of the epidermis.

4.     Lethargy of fish and loss of equilibrium.

5.     Scales are lifted away from body surface of fish.

6.     Neccrosis of fins and membranous part of gills may occur.

7.     Respiratory manifestations appear on fish when infection is associated with gills.

8.     Infected eggs are opaque in color with growth of fungus on eggs surface lead suffacation and become good medium for growth of the fungi. Saprolegnia lead to death by heamodilution or   osmoregulatory failure.

Time to death by saprolegniasis depends on:

1.     the initial site of infection.

2.     type of tissuse destroyed.

3.     growth rate of fungus.

4.     the ability of the fish to withstand the stress of a fungus invasion.

 

Diagnosis:

1.     Case history.

2.     Observation of a cottony, proliferative growth on the skin or gills alerts the clinician to a possible diagnosis of saprolegniasis.

3.     Direct smear from fungal growth, presence of long, branched non-septate hyphae help in diagnosis of saprolegniasis.

4.     Isolation and identification of saprolegnia using cultural method.

5.     Recently: Polyclonal and monoclonal antibodies raised against various species of saprolegnia indicate that developing a rapid antibody assay for the detection of saprolegnia infection of fish.

 

Treatment and control:

Several investigators treat the saprolegnia infection of fish and fish eggs with formalin, copper sulfphate, potassium permanganate, sodium chloride, malachite green, acetic acid, hydrogen peroxide and iodophors using different doses at different intervals depending on fish species, severity of infection and climatic conditions. Water moulds can not be eliminated from any culture system. For this reason, prophylaxis is the best strategy for prevention and control of the saprolegnia infection, which is summarized in:

1.     Removal the predisposing factors.

2.     Avoiding damage of skin during transportation of fish.

3.     Right kind of food with sufficient amount must be provided to fish.

4.     Over crowding of fish must be prevented.

5.     Preventing the introduction of new fish to the fish farm until known that fish are free from disease.

6.     Disinfection of the equipments and utensils to prevent spread of the infection.