Branchiomycosis (gill rot)

 

Definition:

It is a fungal disease involving gill tissues, affecting the most species of freshwater fish. The disease is caused by Branchiomyces sanguinis and Branchiomyces demigrans. It is characterized by areas of infarctive necrosis of the gills, anorexia, and marpling appearance of the gills.

 

Causative agent:

There are two species from genus Branchiomyes, which are responsible for occurrence of the disease.

1.     B. sanguinis: It grows mainly in the blood vessels of gill arches, filaments and in the gill lamellae.

2.     B. demigrans: This fungal species is found in the parenchymal tissues of the gills.

Both species produce branched and non-septate hyphae. The fungi grow at temperature between 14 and 35úC. The optimun growth temperature appears to be between 25 and 32úC. The fungi grow on Sabouraud's dextrose agar medium.

Stress factors:

1.     Elevation of the water temperature above 20úC.

2.     Low dissolved oxygen.

3.     Reduced water flow.

4.     Over crowded conditions.

5.     High levels of nutrients in the water and phytoplankton blooms.

 

Susceptible species:

The most species of freshwater fishes are susceptible to branchiomycosis.

 

Mode of transmission:

Fungal spores are transmitted by water to gills. These spores adhere to the gills, germinate and produce hyphae. The hyphae penetrate gills epithelium or within the blood vessels of gills depending on species of fungi.

 

Incubation period:

Incubation period for disease is related to water temperature. The disease has been rapidly developed within 2-4 days under suitable conditions.

 

Clinical signs:

1.     Fish become weak in movement.

2.     Fish are gathered in groups at water inlet and die.

3.     Fish do not react to the approach of man and can be caught by hand.

4.     There are respiratory distress in infected fish and do not swallow the air.

5.     Fungus develops on or in gill tissue, or penetrates the blood vessels causing obstruction, congestion and necrosis of gill tissues.

6.     Gills may be appearing red from impaired circulation.

7.     Subacute form of branchiomycosis characterized by marbling appearance of the gills due to pale anemic patches in contact with red congested one due to disturbance of circulation in gills.

 

Diagnosis:

1.     Case history.

2.     Clinical signs.

3.     Microscopical examination of wet preparation from infected gill.

4.     Isolation and identification of the causative agent.

 

Prognosis:

Morbidity rate among fish populations with epizootics of brachiomycosis usually reach 100% depending on fish species and susceptibility. Mortality rate may each 30 to 50% of the fish population during late summer epizootics.

 

Treatment and control:

1.     Strict sanitation and disinfection are essential for disease control.

2.     Dead fishes should be collected and daily and burned or deeply buried.

3.     Ponds with enzootic branchiomycosis should be dried and treated with calcium oxide (quicklime) or 2 to 3 kg copper sulphate per hectare.

4.     Diseased fish can be treated with malachite green at 0.1mg/l for extended periods of time or 0.3mg/l for 12 hours.

5.     Transportation of infected fish areas to non-infected areas must be prevents.

6.     Increase of water supply help in control of that disease.

7.     Stress factors must be avoided.

8.     Regulating the feeding rate during warm weather.