The ubiquitous nature of Aeromonas species in aquatic environments provides ample opportunity for animals, particularly fish and amphibians, to come into contact with, and to ingest organisms. Such contact may lead to infection which, depending on the species and the virulence of the strains encountered, may have life-threatening consequences. However, conflicting views have been expressed over the precise role of A. hydrophila as a fish pathogen. Another condition caused by A. hydrophila , known as red-fin disease, is characterized by the presence of surface haemorrhages and scale erosion. Toxic metabolites of A. hydrophila are absorbed from the intestine and induce poisoning.
Capillary haemorrhage occurs in the dermis of fins and trunk and in the submucosa of the stomach.
Hepatic cells and epithelia of renal tubules show degeneration. A eromonas species produce many products that may be toxic to other cells. Some are released from viable cells in soluble form. Others may remain associated with the cell surface, and still others may be released upon cell death. Outbreaks of disease are usually associated with a change in environmental conditions.