Immune system of channel catfish: An overture on immunity to Edwardsiella ictaluri

Edwardsiella ictaluri is a gram negative rod that causes enteric septicemia of catfish (ESC). The E. ictaluri disease complex includes acute and chronic ESC. One of the most economically important diseases of channel catfish, Ictalurus punctatus, ESC is difficult to control. Although the bacterium is usually susceptible to antibiotics, resistant strains are emerging. Immunization is a more attractive approach to control. Vaccine efficacy is not well documented, in part due to a lack of basic information about immune system components and their interactions after E. ictaluri infection. Channel catfish vaccinated by immersion in E. ictaluri bacterin are only partly protected against ESC. Specific antibody responses are poor in the vaccinates. E. ictaluri infection causes both humoral and cell-mediated immune responses. A vaccine that stimulates both types of immunity may provide better ESC immunity. Diagnosis of E. ictaluri infection is accomplished by detection of specific antibody. Recently the discovery of an immunodominant E. ictaluri exoantigen is allowing the development of accurate and rapid diagnostic tests that can detect infection before clinical ESC occurs. Health management programs for food animals depend on serological tests to detect early infections and carriers; the value of these tests is now realized in the channel catfish industry. This review documents factors that influence the immune responses of channel catfish. These factors include the influence of water temperature, seasons, stressful conditions, diet, and E. ictaluri carrier status.