A study of brown spot disease and the histology of the integument in the White Shrimp, Penaeus Setiferus
Brown spot disease is a syndrome which manifests itself by a progressive destruction of the chitin and protein in the integument of many aquatic crustaceans including Penaeus species. Chitinoclastic bacteria appear to be the causative agents, and prior injury to the integument maybe necessary to provide a route for bacterial infection. This study shows that Vibrio alginolyticus (biotype of Vibrio parahemolyticus), a chitinoclastic and proteolytic bacterium, was capable of inducing brown spot disease, while Aeromonas formicans, a proteolytic but not chitinoclastic bacterium, was unable to do so. Abrasion of the body surface was a necessary pre-disposing factor for infection. In the group of shrimp on which the integument was abraded, and had V. alginolyticus applied to the wound, over 50% developed brown spot lesions. Mo significant lesion formation occurred under other experimental conditions, which included the application of A. formicans to abraded integument, the application of V. alginolyticus to intact integument, and either abraded or non-abraded integument with no bacterial application. Histological work with light microscopy and electron microscopy was used to clarify and characterize the integument, particularly the epicuticle, of K setiferus. Light microscopy and electron microscopy revealed four major types of tissue response in penaeid shrimp to abrasion and infection, in 2A, 43, and 72 hour observations. The first response appears to be an attempt by the shrimp to prematurely slough the old integument and form a new one. This attempt failing, the second tissue response is a heavy migration of hemocytes to the site of infection where they melanize to form a defensive "scab" to prevent farther bacterial penetration. The third tissue response of the shrimp to infection was the migration of fibrocytes into the areas of necrotic epidermis and basement membrane. There, a new sheath of collagen-like fibers were deposited to provide a new boundary whereby normal molting could occur. Finally, the last tissue response occurred independently from the other three. Sometimes the shrimp would react with a deposition of chitin to the already existing integument, perhaps in an attempt to establish a barrier to protect underlying tissues against bacterial infection.