Effect of environmental conditions on the infectivity of Flexibacter columnaris to fish

Columnaris disease, Flexibacter columnaris infection, was first described in North America. Recent publications indicate that its distribution is worldwide. Both warmwater and coldwater fish are infected with F. columnaris. All freshwater fish are probably susceptible to the disease under environmental conditions favourable to the bacterium and stressful to the fish. Flexibacter columnaris cannot tolerate sea water. Generally, outbreaks occur when the average water temperatures reach 15°C. The bacterium apparently enters the fish through the skin and gills. The swarms of bacteria involved give the lesions a characteristic yellow pigmentation. The external lesions destroy the skin, muscle and gill tissues. It is relatively easy to isolate the bacteria from the internal organs of the diseased fish, but there is no evidence of internal tissue damage. In this short review, emphasis is placed on several factors relative to water environment which play a role in the development of outbreaks of columnaris disease: 1 The optimum temperature for an outbreak of F. columnaris infection is thought to be between 20 and 30°C. Mortalities seldom occur at temperatures below 15°C, although highly virulent strains can kill fish at lower temperatures. 2 Flexibacter columnaris can survive longer in water of normal to high hardness, as well as in water containing small amounts of sodium, potassium, calcium or magnesium salts, but it cannot live in sea water. 3 Flexibacter columnaris grows well on paniculate fish feeds so that such deposits in the water may act as reservoirs of infection. 4 An important source of infection is feral fish. In the Columbia River or the Fraser River, resident coarse fish such as suckers, Catostomus spp., are thought to transmit the infection to returning adult salmon. 5 Flexibacter columnaris fails to invade fish in the presence of competitive bacteria like Citrobacter freundii when its initial numbers are approximately one in a 100 less than those of the competitors. 6 Crowding seems to be not only a stressor for fish, but also a factor that increases the chance of adhesion of F. columnaris to the external tissues of fish.