A wide variety of wild and domestic birds are susceptible and outbreaks of fowl cholera are not uncommon among wild waterfowl. Fowl cholera can be acute or chronic, generalized or local and is characterized by sudden onset with high morbidity and high mortality. Fowl cholera is said to be a disease of overcrowding, spread by contact between healthy and ill birds, in an environment contaminated with Pasteurella organisms.
Stress appears to be an important factor in breaking down the bird's resistance.
Cold, damp weather predisposes birds to this disease and outbreaks are generally seen during the winter months in overwintering waterfowl flocks. The only existing enzootic areas of fowl cholera in the world are the Muleshoe National Refuge in Texas, in north central California and in the rainwater basin in Nebraska where periodic winter outbreaks occur. Sporadic outbreaks have occurred in other areas of the U.S. during the winter months, with Wisconsin and Illinois being the states closest to Michigan with fowl cholera cases. Fowl cholera has occurred only once in Michigan and that was in a green heron. The mechanism of spread of avian cholera among waterfowl is unknown.