Ichthyophonosis

 

Definition:

It is the fungal disease of both freshwater and marine fish. Affecting any species of both types. Caused by Ichthyophonus hoferi. Disease characterized by rough or granulomatus of the skin and white to gray-white lesions in the internal organs and different parts of the body.

 

Causative agent:

The disease caused by Ichthyophonus hoferi.  This fungus is an obligate parasite with complicated life cycle. Ichthyophonus hoferi is spherical or oval in shape, yellowish-brown and has granulated cytoplasm. The fungus lives as an intercellular parasite in the tissues of various organs. Parasites become encysted by host tissues. If the cyst walls become very thick, the fungus cannot reproduce and dies. If the fungus grows faster than cyst wall, it produces numerous daughter cells.

I. hoferi can be cultivated on broth gelatin-agar or Sabouraud's dextrose agar with 1% bovine serum. Growth can occur between 3 and 20C. The optimum temperature is 10úC. Growth appeared within 7-10 days after inoculation.

 

Susceptible species:

Infection with I. hoferi has been reported in many species from freshwater and marine water fishes. The fungus has been found in crustaceans, amphibians, reptiles & fish eating birds.

 

Mode of transmission:

1.     Oral route: Ingestion of infected raw fishes or fish products or other food transmits disease.

2.     Skin abrasions or damaged gills help in transmission of the infection.

3.     Carriers help in spreading of the infection through faecal discharges.

4.     The disease can transmitted by contact between diseased fish & healthy one.

 

Clinical signs:

1.     Fish with slight or moderate infection. There is no external signs can be observed.

2.     Fish with advanced infection have rough or granulomatus skin, the term sandpaper effect is used to describe the granulomatus appearance. Its caused by development numerous infective unites under the skin & underlying muscle. These infective unites are swelling & necrotic lesions. Some may rupture through the skin leaving small openings. These are located on the latero-ventral tail region. Those are black in colour & a raise above the skin.

3.     Those lesions found in the internal organs as white nodules similar to the granulomatus lesion of mycobacterium infection.

4.     These nodules found in heart, liver, kidney, spleen, and brain and filled with cellular debris and fungus.

5.     Infection of liver or kidney lead to abdominal distension & exophthalmia.

6.     Infection of swim bladder, which led to damage of it, fish lies on the bottom of the pond & died.

7.     The infection of the brain causes marked changes in the behavior of fish.

8.     Curvature of the vertebral column may also occur due to muscle spasms.

 

Diagnosis:

1.     Case history.

2.     Clinical signs and P.M.

3.     Microscopical examination of infected tissues to detect the fungi (spores).

4.     Histopatholgical examination: Host response to the parasites is variable but severe granulomatous response is the usual finding, with large numbers of epithelioid cells and macrophages and occasional giant cell. In the early stages, calls of the inflammatory series are seen in large numbers. The granulomata usually have well-developed capusle of connective tissue.

5.     Isolation and identification the causative agent.

 

Treatment and control:

1.     There are no therapeutic procedures for treatment of ichthyophonus disease.

2.     Prevention of infection is the best method for controls the disease.

3.     Feeding of raw fish or infected food must be prevented.

4.     Any infected fish or dead fish must be destroyed.

5.     Complete disinfection of raceways or aquaria with chlorine or other throng disinfection.

6.     All nets, brushes and utensils used around infected fish must also be disinfected at the same time.

7.     Removal of fishes and water for several months or years of drying may be necessary to eliminate the pathogen form soil of fish farm.