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.