In fresh-water prawn culture, diseases have a greater occurrence in hatcheries than in grow- out ponds. Several diseases have been identified in larvae, juveniles and adults, but many of them are of undetermined aetiology. Even among those for which the aetiology is known, appropriate prophylactic and curative measures have not yet been developed. But most of the diseases appear to be directly or indirectly due to poor sanitation, inadequate water exchange, poor feeds or low oxygen levels.
Among the diseases identified in larval prawns, those caused by microbial epibionts appear to be more common. The epibionts are mainly filamentous and non-filamentous bacteria, algae or aquatic protozoa. The protozoan agents include the genera Epistylis,Zoothamnium and Vorticella. These organismsattach themselves to the epicuticular surfaces, but do not cause any inflammatory responses. Zoothamnium appears to attach itself to the gilllamellae, while other protozoans do not exhibit any site specificity. Bacterial fouling of proximal appendages, gills or the body surface often results in severe mortality. Antibiotic treatment is likely to control the disease. As the disease is triggered by poor biological conditions in hatchery systems, the best control measure is the identification and control of the primary cause.
Brown-spot disease, also known as black-spot or shell disease, occurs commonly in juveniles and adult Macrobrachium. It can be recognized by the presence of brown to black, ulcerative to raised lesions on the body surface or appendages. This disease occurs only in animals which have developed prior cuticular damage due to other causes, including aggression between prawns. Both adults and larvae have been found to develop melanized brown spot lesions. The only control measure suggested is improvement of culture conditions and correction of nutritional deficiencies.
Exuvia entrapment is a disease primarily of late larvae and early post-larvae, with mortality occurring at the time of metamorphosis moult. Affected larvae are not able to free their pereiopods, anterior appendages, eyes or rostrum from the exuvia during ecdysis and consequently die. The aetiology of the disease has not been determined, and the prevention and control measures are limited to the use of algal supplements in larval culture and the maintenance of good-quality water conditions in hatchery tanks.
Idiopathic muscle necrosis of prawns, also known as muscle opacity or spontaneous muscle necrosis, is associated with environmental stresses such as salinity, temperature, hypoxia, overcrowding, etc. The aetiology of the disease is not known. It also occurs in Penaeid shrimps and the only preventive measure presently known is reduction of environmental stresses during culture, especially at times of handling and transfer.
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