Aquaculture plays a major role in augmenting food production, livelihood and thereby the economy of a country. Diseases are the outcome of imbalance between environment, host and pathogen. During the past three decades, viral and bacterial diseases have emerged as serious impediments to shrimp farming. Concomitant with the expansion and Intensification of shrimp aquaculture, large number of diseases, especially those caused by viral infection, have been recorded in farmed shrimp. The impact of disease is a combination of several factors such as mass mortality of shrimp, reduction in feed acceptance or morbidity.
Skeletal Muscle necrosis in Crustaceans
The several cases of muscle necrosis has reported in farmed Pacific white shrimp (Litopenaeus vannamei). These cases were associated with low mortalities at harvest based on anecdotal evidence from shrimp farmers.
The disease was described as an idiopathic muscle necrosis. It was visually characterized by extensive necrotic areas in striated tail muscle tissues that appeared white and opaque. Furthermore, histological analysis of suspect samples with macroscopic lesions revealed a loss of sarcomeric structure accompanied by coagulative muscle necrosis and hemolytic infiltration.
Histological evidence has suggested two forms of muscle necrosis in crustaceans: one involving extensive hemocytic infiltration and the other a non-inflammatory necrosis in which a weak immune reaction is observed. Hemocytic infiltration can occur in response to injury caused by pathogens. The unusual cellular immune response in L. vannamei is an infectious etiology.
The two viruses both primarily targeted the skeletal muscle and resulted in very similar gross signs ( white or opaque tail) and histopathological changes (focal to extensive areas of muscle necrosis and the formation of prominent lymphoid organ spheroids) in Penaeid shrimp.
The two muscle necrosis diseases are Infectious Myonecrosis (IMN) and Penaeid White Tail Disease (PWTD). IMN affecting cultured L. vannamei is caused by a double-stranded RNA virus named infectious myonecrosis virus (IMNV). The etiological agent of PWTD in shrimp (Litopenaeus vannamei) is Nodavirus (PvNV). Microscopic examination illustrates acute necrosis in the skeletal muscles of shrimp caused by IMNV on the left and PvNV on the right.
Infectious Myonecrosis (IMN)
Infectious Myonecrosis (IMN) is a recently identified disease in cultured L. vannamei in northeast Brazil. Also, IMNV has been reported in South-East Asia, including Indonesia, India and
Sri Lanka. IMN causes significant disease and mortalities in juvenile and sub adult pond-reared stocks of L. vannamei. Outbreaks of the disease seemed to be associated with certain types of environment and physical stresses (i.e. extremes in salinity and temperature, operating by cast net, etc.) and possibly with the use of low quality feeds. IMN presents as a disease in L. vannamei with an acute onset of gross signs and elevated mortalities.
Horizontal transmission has been demonstrated via cannibalism. Vertical transmission (direct passage from parents to offspring via eggs or sperm) is likely but not confirmed
Disease Agent &Nature of Infectious
The Infectious myonecrosis is caused by infection with infectious myonecrosis virus (IMNV), a putative totivirus. IMN is a viral disease that causes substantial mortalities in farmed populations of P. vannamei. The principal target tissue for IMNV infection is the striated muscle (skeletal and less often, cardiac muscle). IMNV-infected shrimp present focal to extensive white necrotic areas in striated muscle, especially in the distal abdominal segments and tail fan.
Shrimp in the acute phase of IMN present focal to extensive white necrotic areas in striated (skeletal) muscles.
The signs at the pond level are lethargy, less appetite, large numbers of moribund animals and significant mortalities (up to 70%) during or following stressful conditions. The acute form of the disease produces gross signs and elevated mortalities but disease progresses to a chronic phase with persistent low-level mortalities. Other than the appearance of whitish tail muscle, shrimp become lethargic and have reduced feed consumption.
Gross pathological signs are focal to extensive white necrotic areas in the striated muscle commonly observed in distal abdominal segments, Necrotic and reddened tail fan, lymphoid organs increased to 3 to 4 times normal size, Moribund shrimp with a full stomach feeding affected individuals may continue to feed until death.
Skeletal Muscle Necrosis Diseases
Microscopic pathological signs are: coagulative necrosis of skeletal muscle by haemolytic infiltration and fibrosis. The histopathological examination confirmed that most of the challenged
Shrimp have lesions in skeletal muscle, including multifocal necrosis, fibrocytic inflammation and phagocytosis. IMN is seems to similar diseases to like Infection with Macrobrachium rosenbergii nodavirus (MrNV) and infection with shrimp haemocyte iridescent virus (SHIV).
Penaeid White Tail Disease (PWTD)
Penaeid White Tail Disease (PWTD) causes by the virus in P. vannamei named Penaeus vannamei Nodavirus or PvNV. Viral muscle necrosis is an emerging disease of Penaeus (Litopenaeus) vannamei caused by Penaeus vannamei nodavirus (PvNV). The disease was first reported in Belize in 2004 and caused 50% production losses in affected shrimp ponds.
Muscle necrosis in shrimp often results in the appearance of opaque white lesions in the tail muscle in response to environmental factors such as low dissolved oxygen, sudden changes in temperature or salinity, or other stress. Severe hypoxia and a shortage of oxygen in tissue can occur in the terminal phase of many infectious diseases and result in necrosis of abdominal muscles.
The exhibiting of clinical signs are white, opaque lesions in the tails and histopathology similar to those of shrimps infected by infectious myonecrosis virus (IMNV). Histological examination revealed multifocal necrosis and hemocytic fibrosis in the skeletal muscle. In addition, basophilic, cytoplasmic inclusions were found in striated muscle, lymphoid organ and connective tissues.
Although the virus does not cause serious mortality, the infection impacts survival in grow-out ponds. Sporadic mortality of infected shrimp has been recorded when they are under environmental stress such as crowding (stocking density (> 50 m-2) and high temperature (> 32ºC) and survival decreased to 40% and increased food conversion ratio. The disease has not been reported from India.
Penaeid White Tail Disease (PWTD)
White tail disease in penaeid shrimp can be caused by either IMNV or PvNV. Both viruses target primarily the skeletal muscle and result in very similar gross signs. However, there is a difference in the mortality caused by these viruses. PvNV appeared to be less virulent than IMNV.
Bacterial White Tail Disease (BWTD)
The bacterial diseases especially vibriosis are becoming another important threatening to the sustainable development of the Penaeid shrimp aquaculture industry. In shrimp aquaculture system, the most pathogenic vibriosis are Vibrio harveyi, V. alginolyticus and V.parahaemolyticus.
As opportunistic pathogens may lead to mortality of affected aquatic animals due to stressful events, such as sudden changes in temperature and salinity. Nonluminescent and highly virulent vibrio harveyi strain is associated with Bacterial White Tail Disease (BWTD) in shrimp culture ponds. Sometimes, BWTD could also cause mass mortalities in pond-cultured shrimp.
Gross Signs of BWTD:
The extensive areas of whitish muscle particularly in the distal abdominal segments with or without a red discoloration in the body and appendages. The signs are opaque or whitish appearance of diseased shrimp is due to muscle necrosis.
The BWTD causes to high mortality during sudden changes of water parameters and stressful conditions. Histopathological analysis shows that muscle fibers composing the whitish tail muscle are damaged in different degrees.
Bacterial White Tail Disease (BWTD)
The prevention is always better than cure. Disease prevention largely depends on the implementation of biosecurity measures and best management practices in the farms. The other preventive measures are disinfection, drying, removing the accumulated organic matter, tilling, liming, management of feed, improving water quality parameters. Also proper attention should be on aquatic animal health management and with enhancing the immune system of the shrimp.
Further proper disease management is responsible for resulting in a successful growth, production, enhanced economic status and simultaneously sustainability of culture.