how is the above mentioned disease is caused in this particular fish?
It is a bacterial infection of freshwater and marine fish caused by the pathogen.
Rainbow trout
Enteric redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen . It is primarily found in rainbow trout and other cultured salmonids. 
when does a rainbow trout become a steelhead UNANSWERABLE what does this trout look like? Adult freshwater forms are generally blue-green or olive green with heavy black spotting over the length of the body. Adult fish have a broad reddish stripe along the lateral line, from gills to the tail, which is most pronounced in breeding males. what is their lifespan? 11 years is there any disease associated with this fish species? Yes, Enteric redmouth disease. where is this trout fish commonly found? he native range of is in the coastal waters and tributary streams of the Pacific basin, from the Kamchatka Peninsula in Russia, east along the Aleutian Islands, throughout southwest Alaska, the Pacific coast of British Columbia and southeast Alaska, and south along the west coast of the U.S. to northern Mexico.
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Rainbow trout [SEP] Redmouth disease
Enteric redmouth disease is a bacterial infection of freshwater and marine fish caused by the pathogen . It is primarily found in rainbow trout and other cultured salmonids. The disease is characterized by subcutaneous hemorrhaging of the mouth, fins, and eyes. It is most commonly seen in fish farms with poor water quality. Redmouth disease was first discovered in Idaho rainbow trout in the 1950s.