also known as: Flexibacter columnaris, Bacillus columnaris, Cytophaga columnaris, cotton-mouth disease, mouth "fungus", cottony mouth disease, rainbow trout fry syndrome, cold-water disease CWD
Flavobacterium columnare is a Gram-negative bacterium, best known for causing cold-water disease (CWD). The bacterium can infect a very wide range of cooler temperature freshwater fish species. The bacterium can infect most host tissues, but is often diagnosed by gill and mouth infections with yellowish secrtions and bacterial masses that form microscopic "columns". CWD occurs in water less than 60°F (16°C) and is worse at lower temperatures. Flavobacterium columnare infections can cause disease in both cultured fish and free-ranging populations, with hatchery-based young trout and salmon species especially vulnerable. Flavobacterium can be diagnosed using culturing in reduced nutrient media.