Fish Diseases and Management

5.2.2. Vibriosis

Infections and disease caused by Vibrio spp. have been by far the most numerous of the reported bacterial agents of penaeid shrimp. Vibrio spp. were found to constitute the majority of cultivable bacteria associated with the gut, gills or cuticle of wild or cultured penaeid shrimp. Vibrio spp. are opportunistic pathogens which establish lethal infection as a result of other primary conditions that might include other infectious diseases, extreme environmental stress, wounds etc. Some recently occurring disease syndromes of penaeid shrimp have been caused by Vibrio spp. which behaves more like true pathogens than opportunistic invaders. Important Vibrio species causing infection are V. alginolyticus, V. parahaemolyticus or V. harveyi, V. splendidus, V. vulnificus and V. damsela. Bacterial infections in shrimp may manifest as (1) localized pits in the cuticle, which constitute the condition called bacterial shell disease (2) localized infections of the gut or hepatopancreas or localised infections from puncture wounds, limp loss etc. and (3) generalised septicaemia. All life stages of the penaeid shrimp may be affected.

Vibriosis in grow out ponds of penaeids have been reported from many countries. Major symptoms were cloudiness in hepatopancreas in PL, cloudiness of muscles in sixth abdominal segment, and progressive spots on gills and lymphoid organ. Histopathology revealed extensive necrosis and bacterial invasion of lymphoid organ, with multiple melanized hemocytic nodules. Nodules were found in all the organs while inflammation was found only in lymphoid organ. Affected shrimps lost their escape reflexes, had darkened colour and heavy fouling by epibionts. Less affected were pale and opaque, and gills were brown in colour. Specimens from the pond showed signs of body reddening, extended gill covers, slight melanized erosion of uropods, pleopods and periopods. They also had empty stomach and midgut except for a watery white liquid. Small black spots were present on the lymphoid organ. V. alginolyticus, V. parahaemolyticus and Pseudomonas species were obtained from the haemolymph. Vibriosis occurred during 1989 and in Ecuador which was called Sindroma Gaviota (or Sea Gull Syndrome, SGS) where large number of sea birds were observed to gather on or near ponds and to feed on dead and dying shrimps. Hence it was called as sea gull syndrome.

Reduction of biomass by partial harvest or reducing stocking density and increasing the water exchange helps to control vibriosis in culture ponds. For subsequent production cycles, it is best to dry the pond bottom until the bottom soil cracks. Excessive detritus have to be removed physically by desalting the pond bottom. Quick lime has to be applied at the rate of 0.5 kg/m² of pond bottom. Use of immunostimulants fed through the feed is found to provide better resistance to diseases by stimulating the non-specific defence mechanisms of shrimps. Use of probiotic bacteria is found to compete with Vibrio sp. there by decreasing the load of pathogenic organisms.