Shrimp farming is a multi-billion dollar industry contributing a major income to several countries in Asia and South America. The rapid growth of shrimp farming led to an economic boom but, unfortunately, the outbreak of viral diseases has increased the economic risks and slowed the industry development (Flegel, 2006). The most important diseases of cultured penaeid shrimp, in terms of economic impact, in Asia, the Indo-Pacific, and the Americas have infectious etiologies. Among the infectious diseases of cultured shrimp, certain virus-caused diseases stand out as the most significant. The pandemics due to the penaeid viruses WSSV (White spot) and TSV (Taura Syndrome), and to a lesser extent to IHHNV (Infectious Hypodermal and Hematopoietic Necrosis virus) and YHV (Yellow Head), have cost the penaeid shrimp industry billions of dollars in lost crops, jobs and export revenue. The social and economic impacts of the pandemics caused by these pathogens have been profound in countries in which shrimp farming constitutes a significant industry. In the present study some of the common disease in L. vannamei was recorded all along the state of Andrapradesh, Orissa and Tamilnadu and described in detail.
 

During the study period (Jan 2011 to July 2012) diseased L. vannamei were collected from different shrimp farms located in the state of Andrapradesh, Orissa and Tamilnadu. Whole shrimps were immediately immersed in 95% ethanol in plastic containers. Samples were maintained at 4℃ until processing.
 

In the present study black gill disease observed at shrimp farms in Andrapradesh. The black gills disease is very common in penaeid shrimps. It often occurs when there is too much plankton in the water or too much mud on the pond bottom. It is especially a problem in ponds with high population density (over 60 shrimp/m²), not enough aeration or when the water is not changed frequently enough or irregular probiotic application. Before the shrimp appear diseased or start to die its gills turn black.
 

Black gill disease is reasonably easy to cure. If farmer noticed dark-colored gills in some shrimp it is suggested to change the water more often and also provide more aerators. This will usually solve the problem quickly. Addition of lime according to the pH concentration is also recommended. Similar disease was already reported by Lightner and Redman (1998), MPEDA/NACA (2003), Doughtie and Rao (1983) and Johnson (1995) (Figure 1, 2).

In the present study IHHNV (Infectious Hypodermal and Hematopoietic Necrosis Virus) disease observed at shrimp farms in Andrapradesh and Orissa. IHHNV is very common in grow out ponds, especially when the postlarvae brought from broodstock that were raised in earthen ponds in Thailand. The white leg shrimps with IHHNV are easy to observe because they display a typical clinical signs of a deformed or stunted rostrum (Figure 3). It might be bent to one side and the body of the shrimp might be crooked. These clinical signs will be apparent after the shrimp have been in growing out pond for 30 days.

 

Figure 3 IHHNV (Infectious Hypodermal and Hematopoietic Necrosis Virus) affected shrimps

 

In general the deformed shrimp will be seen in only 10-20% of the shrimp population, but if it is very severe, 70-80% will be affected. The diseased shrimp will grow slowly and will have a low survival rate, so the overall yield will be lower. However, the diseased shrimp will not usually die, they will just be weakened. The occurrence of IHHNV is likely to rise in the future as more farmers use post larvae produced from brood stock from local shrimp farms than imported ones. Outbreaks of IHHNV can occur at any time of the year. The main risk factor is the source of the shrimp larvae. It is very unlikely that larvae produced from specific pathogen-free parents will get IHHNV.
 

The virus which causes infectious hypodermal and haematopoietic necrosis infects many species of shrimps including P. monodon, P. japonicus, P. stylirostris, P. vannamei, P. semisulcatus, P. occidentalis and P. californiensis. IHHN has reported in central and South America, India, Indonesia, China, Taiwan, Thailand, Malaysia, Philippines, Hawaii, Guam, French Polynesia, New Caledonia and Israel (Bondad-Reantaso et al., 2001). The disease IHHN, and later its causative agent, IHHNV, was first described as the cause of acute epizootics and mass mortalities (> 90%) in juvenile and subadult of L. stylirostris farmed in super-intensive raceway systems in Hawaii (Brock et al., 1983; Lightner et al., 1983, 1983a; Brock and Lightner, 1990). Shortly after its discovery in L. stylirostris, the virus was found in L. vannamei being cultured at the same facility in Hawaii and these L. vannamei were shown to be asymptomatic carriers of the virus (Lightner et al., 1983a; Bell and Lightner, 1984). Hence, the economic and production impacts of IHHNV infection in L. vannamei are due to reduced and irregular growth and small sized shrimp at harvest and not to elevated mortality. To mitigate this effect, several strategies have been used. With one strategy, selected lines of L. stylirostris, which were not only resistant to IHHN disease, but are also refractory to infection, were developed (Dhar et al., 2001). IHHNV-free lines of L. vannamei were also developed as SPF (specific pathogen-free) lines and these stocks were the first developed in the SPF stock development program (Pruder et al., 1995).

 

In the present study WMD (White muscle disease) was observed at shrimp farms in Tamilnadu and Andrapradesh. The epizootic of WMD was first recorded in November 2001 in few hatchery reared PL of M. rosenbergii in Andrapradesh and Tamilnadu states. More than 18 cases of WMD in freshwater prawn hatcheries with PL mortalities ranging from 30% to 100% were recorded from November 2001 to December 2002. The first sign related to this disease was the poor feeding (mineral deficiency in water, poor quality feed) and lethargy of the prawns especially during the first 5 days of PL settlement resulting in slow mortality (Figure 4 & 5).

As the morphological and clinical signs of the disease reported by Chen et al. (2001) were similar to the WMD recorded in this study. It is likely that the involvement of the Gram-positive cocci, Lactococcus garvieae in WMDfound by those workers could possibly have been a secondary infection. The disease progressively destroyed the abdominal muscular organization of the prawns especially the striated muscles finally leading to mortality. Several authors reported white muscle disease in shrimps and prawns (Chen et al., 1992; Sudhakaran et al, 2006; Ravi et al., 2009). Besides the above, this white muscle disease also recorded in L. vannamei in the present study.

In the present study white gut disease observed in L. vannamei shrimp farms from Tamilnadu and Andrapradesh. Vibriosis is one of the major disease problems in shellfish and finfish aquaculture. Vibriosis is a bacterial disease responsible for mortality of cultured shrimp worldwide (Lightner and Lewis, 1975; Adams, 1991; Lightner et al., 1992; Lavilla-Pitogo et al., 1996; Lavilla-Pitogo et al., 1998; Chen et al., 2000). Vibrio species are widely distributed in culture facilities throughout the world. Vibrio-related infections frequently occur in hatcheries, but epizootics also commonly occur in pond reared shrimp species. Vibriosis is caused by gram-negative bacteria in the family Vibrionaceae. Outbreaks may occur when environmental factors trigger the rapid multiplication of bacteria already tolerated at low levels within shrimp blood (Sizemore and Davis, 1985), or by bacterial penetration of host barriers. The exoskeleton provides an effective physical barrier to pathogens trying to penetrate the external surface of crustaceans, as well as the foregut and hindgut (Figure6A & B).

 

Figure 6 A & B White gut disease in L. vannamei

                                                                          

The occurrence of five types of diseases: tail necrosis, shell disease, red disease, loose shell syndrome (LSS) and white gut disease (WGD) is by Vibrio spp. in P. monodon from culture ponds of coastal Andhra Pradesh (Jayasree et al., 2006). Among these, LSS, WGD, and red disease caused mass mortalities in shrimp culture ponds. Six species of Vibrio—V. harveyi, V. parahaemolyticus, V. alginolyticus, V. anguillarum, V. vulnificus and V. splendidus—are associated with the diseased shrimp. The distribution and species composition of luminous bacteria in commercial penaeid shrimp hatcheries were studied by Jawahar Abraham and Palaniappan (2004). Theobservation on the presence of V. harveyi (97.30%) and V. orientalis (2.70%) in shrimp gut contents evinced that the primary source of these bacteria in a shrimp hatchery was the faecal matter from brood stock, possibly at the time of spawning. Mortalities due to vibriosis occur when shrimps are stressed by factors such as: poor water quality, crowding, high water temperature, low DO and low water exchange (Lewis, 1973; Lightner and Lewis, 1975; Brock and Lightner, 1990).

 

2.5 Muscle cramp syndrome (MCS)

In the present study muscle cramp disease observed in L. vannamei shrimp farms of Tamilnadu and Andrapradesh. The temperature fluctuations affect the muscle of the shrimps. Muscle cramp disease is very common in grow out ponds (Figure 7).

It is attributed that less amount of dissolved oxygen leads to mass mortality in ponds (Figure 8).This is very common in high stocking ponds. Once dissolved O₂ level was reduced leading to create stress among the shrimps, finally they will die especially inside and sides of the culture ponds.

 

Figure 8 Mass mortality in the check tray due to dissolved oxygen depletion

 

The six diseases mentioned above are serious problems for raising white leg shrimp in intensive culture systems. The farmers should have basic knowledge about these diseases and their prevention. If the farmers stock health disease free seed and maintain good water quality and provide nutritious feed to get higher survival.