Fish Diseases and Management

5.2.5.Brown or black spot disease

They are also called as shell disease, rust disease, burned spot disease, necrosis, etc. Infestation is caused by bacteria belonging to Vibrio, Aeromonas and Pseudomonas. Appearance of brownish to black erosion of the carapace, abdominal segments, rostrum, tail, gills and appendages. In larval and post larval stages, the affected part shows a cigarette butt like appearance. The infection is usually initiated at sites of punctures or injuries made from telson or rostrum, cracks on the abdominal segment from sudden flexture of the shrimp body, or from other damage caused by cannibalism. Progressive erosion of these exoskeletal lesions follows upon entry and multiplication of bacterial pathogens. The infection can be controlled by maintaining good water quality and keeping the organic load of the water at low levels by removing sediments, especially dead shrimps and moulted exoskeletons which harbour high numbers of bacteria on the lesions. Minimising handling and avoiding over crowding also helps. Infection can be treated with formalin 25 ppm in static condition for 24 h.