Bacterial gill disease (BGD)
Bacterial gill disease is an external infection of hatchery-reared salmonids and occasionally of intensively reared warm-water species. It appears to be caused by one or more species of filamentous bacteria, including Flavobacterium sp. Large numbers of filamentous bacteria can be observed on the gills, accompanied by fusing and clubbing of the gill filaments. The disease generally occurs following the deterioration of environmental conditions, associated with over-crowding and accumulation of toxic metabolic products. Acute or chronic forms of the disease may occur.
Infected fish are usually lethargic and apparently lose appetite. Acute epizootics may result in high mortality of up to 50 per cent in a day. Although extensive clubbing of the gill filaments, lamellar fusion and excess mucus may be found, necrosis of the gill tissue seldom occurs. This is in contrast to columnaris disease, whereextensive necrosis and erosion of the gill filaments can be observed.
The detection of large numbers of filamentous bacteria on the gills under a microscope (under a wet mount or stained with methylene blue) is the recommended diagnostic procedure. Isolation of a pure culture is not considered necessary.
The biology and survival of the aetiological organism are not fully known. Contaminated water or carrier fish are probably the source of infection, but it is almost always associated with deterioration of environmental conditions. Fingerlings are generally more susceptible to the infection and salmonids over one year in age seldom develop the disease.
Application of proper sanitation practices, avoidance of crowding and reuse of water, and maintenance of an adequate flow of clean water should help to reduce the incidence of the disease. A number of compounds have been found to be effective for treatment of BGD, but most of them require multiple applications. Successful treatments are (i) potassium permanganate (KMnO4) at 1–2 ppm, (ii) Hyamine 1622 and 3500, as well as Roccal at 1–2 ppm calculated on the basis of active ingredient, (iii) Diquat at 8.4–16.8 ppm of the formulation, (iv) another quaternary ammonium compound Purina Four Power, at 3–4 ppm as a one-hour flush treatment, and (v) chloramine-T at 10 ppm, in a single one-hour flush treatment.