Furunculosis

 

Definition:

Furunculosis is an acute, subacute, chronic or latent disease, primarily among salmonid fishes characterized by formation furuncle or boil-like lesions in various tissues of the body.

 

Causative agent:

The disease is caused by gram-negative, short bacilli called Aeromonas salmonicida, which is classified into two strains.

1.     Typical A. salmonicida, isolated from salmonids only.

2.     Atypical A. salmonicida, isolated from salmonids & non salmonids species

It is aerobic but is capable growth as facultative anaerobe and not spore-forming. Non-motile A. salmonicida produce brown-pigment on culture media as trypticase soy agar, furunculosis agar and brain heart infusion agar. The microorganism was isolated from skin lesions, liver, Kidney and blood of infected fish. Colonies develop within 48 hours at 22-25 ?C as small circular, raised and translucent. Colonies will not grow at 37?C.

 

Susceptible species:

All species of the family salmonidae are considered to be susceptible to furunculosis. A. salmonicida has been isolated from fishes other than the salmonids such as carp, catfish, and other fish species. Young fish are more susceptible to the disease than old fish.

 

Incubation period:

It is dependent on water temperature. Susceptible fishes at water temperature of 20 ?C will develop furunculosis within 4 to 20 days .the signs of the disease may never develop among susceptible fishes at water temperature below 8 C.

 

Predisposing factors:

These include:

1.     Physical damage of the skin or gills.

2.     Poor water quality.

3.     Presence of ectoparasites and other diseases.

4.     Both smolting and high temperature.

5.     High stock density.

6.     Rough handling

 

Course of the disease:

1.     Per-acute form: This form is restricted in young fish and leads to rapid death among fish.

2.     Acute form: This form is characterized by sudden increase in mortality with few or no symptoms within 2-3 days.

3.     Subacute form: This form characterised by gradual decrease in mortality. With furuncles formation and hemorrhages at the base of fins.

4.     Chronic form: In this form mortality rate is low with intestinal inflammation and variable lesions.

5.     Latent form: No mortality. But the pathogen present systemically.

 

Mode of transmission:

1.     The infection was transmitted by ingestion of contaminated food.

2.     Water was found to be vehicle for spreading the infection.

3.     A carrier is important sources for the infection.

4.     The infection occurs through eggs.

5.     The infection may occur through injuries of the skin.

 

Clinical signs:

These are different forms showing on diseased fishes:

1.     Per-acute form:         This form is characterized by rapid death of fish especially young fish, whose defenses against a severe bacterial septicemia will be poor Darkening of skin, rapid breathing and slight exophthalmos may be observed on infected fish. Cardiac damage is a possible cause of death.

2.     Acute form: Fish with an acute infection show signs of hemorrhagic septicemia, including body and vents. Skin lesions may be haemorrhagic patches along the side or on the dorsal body surface, hemorrhages at base of fins. Darkening of skin and sluggishness in movement. Hemorrhages scattered over abdominal walls, viscera and heart. Soft and friable or liquefied kidney is observed. Enlarged spleen with round edges. Pale liver with subcapsular haemorrhages. Stomach & intestine may contain bloody mucous. Swim bladder is hyperaemic. Raised furuncles, which usually develop in the dermis due to localization of bacteria rather than the hypodermis. Fish may die within 2-3 days.

3.     Subacute and chronic form: In this form mortality rate is low & more common in older fish. The clinical includes. Slight darkening of skin, inappetance, and lethargy and congested blood vessels at base of fins. Slight exophthalmos & fish may have pale or congested gills. The furuncle may be small and compact or large & soft. They contain dark red pus with numerous bacteria. The mature furuncle bursts leaving deep ulcer or healed furuncle may leave scar tissue. The furuncle may be found in liver, kidney and spleen.

4.     Intestinal form: This form is associated with low mortality. Prolapse of anus and intestinal inflammation may occur.

 

Diagnosis:

1.     Case history.

2.     Clinical signs and postmortem findings.

3.     Isolation and identification of the causative agent.

4.     Serological identification of the etiology by:

a.     Agglutination test.

b.     Precipitation test.

c.      Fluorescent antibody technique.

d.     ELISA.

 

Treatment:

1.     Sulfamerazine: 150-220 mg/kg fish weight/day for 10-14 days.

2.     Oxytetracycline: 50-75 mg/kg fish weight/day for 10 days.

3.     Furazolidone: 25-100 mg/kg fish weight/day for 10 days.

4.     Oxolinic acid: 10-mg/kg fish weight/day for 10 days.

 

Control:

1.     Test and slaughter.

2.     The pond is drained and bottom disinfected with quick lime.

3.     All utensils used around the fishes and equipment must be disinfected.

4.     Transportation of fishes from infected areas to other must be prevented.

5.     Fish eggs should be obtained from fishes free from pathogen.

6.     Movement of the water stream from infected area to non-infected must be prevented.

7.     Immunization of fish against A. Salmonicida has been studied. Laboratory result indicated that fish can produce protective antibodies against the bacterium. (Formalin killed bacterial & mineral oil adjuvant).

8.     Stress factors must be removed.