Type E botulism in salmonids and conditions contributing to outbreaks

Type E botulism was first recognized as a major cause of fish mortality in the United States in 1979. Between 1979 and 1983, fish botulism was confirmed in 18 different outbreaks resulting in losses of over 2.3 million juvenile salmon and steelhead trout. These outbreaks occurred in both excavated earth-bottom and asphalt-lined ponds. The source of the toxin was from the growth of Clostridium botulinum type E in the dead fish that were not removed from the rearing ponds because of pond depth and turbid water. Live fish developed botulism and died after they cannibalized the dead fish and ingested lethal concentrations of toxic flesh. Based upon laboratory and field studies, a botulism cycle is described and the conditions contributing to the length of the cycle and fish mortality are discussed.