The Pacific white shrimp, Litopenaeus vannamei, is now the species of choice of shrimp farming industry in India. This is because of its inherent advantage viz., high tolerance for salinity, perennial high health, seed availability; low protein requirement and high yield in a short grow out period. Considering these advantages, commercial farming of Litopenaeus vannamei was permitted in India in the year 2009 by coastal Aquaculture Authority (CAA) and in order to prevent entry of exotic pathogens into the India aqua culture system, the imported specific pathogen free (SPC) brood stock from approved suppliers are screened for all OIE (World Organization for Animal Health) listed shrimp viruses at Rajiv Gandhi Centre for Aquaculture (RGCA) quarantine facility and then supplied to CAA registered hatcheries. Although, the imported bloodstock has been certified as specific pathogen free (SPC) by bloodstock supplier and ratified by Quarantine facility in Tamil Nadu that the bloodstock is Hi-health, infection by endemic viral pathogens like white spot syndrome virus (WSSV) and Infectious hypodermal hematopoietic necrosis virus (IHHNV) have increased as a result of continuous expansion and intensification of farming operations without implementing proper management practices.
However, so far there are no reported incidents of listed exotic viral infections of L. vannamei such as Infectious Myonecrosis virus (IMNV), Taura syndrome virus (TSV) and Yellow head virus (YHV). Since March 2011; a new syndrome has struck the industry. The syndrome was such that, the farmed shrimp in the affected ponds show different mortality patterns which are the result of unusual symptoms with no co-relation to any other reported diseases. Some farmers have lost up to four crops, with mortality percentage reaching 70% in the most the cases. As a result of this several farmers have closed down their operations. This condition is called as “Running mortality Syndrome” (RMS).
Running mortality syndrome is such type of syndrome associated with morbidity and mortality which leads to less productivity. According to the examination of a study regarding the hemolymph and hepatopancreas of the infected shrimps. The results revealed that, the affected shrimp showed the higher percentage of Vibrio species such as Vibrio parahaemolyticus and Vibrio azureus. Histopathological studies also revealed that, high B cells in the hepatopancreas in the early stage of disease indicate the over feeding of shrimps. Running Mortality Syndrome is depicted be pond management associated syndrome rather than infectious in nature and thus can be overcome through best management practices.
Since 2011, RMS has been widely prevalent in the shrimp farms in Andhra Pradesh (AP) and Tamil Nadu (TN)
In the early stages of disease Litopenaeus vannamei, apparently started showing gross signs, which were characterized by the antennae cut and Uropods turned normal to red in color. Later on, Hepatopancreas turns reddish yellow (Figure1) and finally entire body turns dark red in color (Figure 2). Continuous internal mortality is noticed throughout the day (Figure 3).
The dead shrimps are settled at the bottom of the tank and will not come to sides or surface. Farmers will come to know about this only when dead and cannibalized shrimp come to check tray. The mortality is noticed irrespective of seed source. The occurrence of mortality is directly proportional to stocking density. Higher the stocking density early the mortality sets in. Mortality rate is relatively more in low saline ponds. White or yellow fecal matter is noticed in the gut. Mortality noticed only during inter-moult stage. Very poor survival and high feed conversion ratios were reported at the time of harvest
The shrimps with the characteristic symptoms were fixed in Davidson’s fixative and tissue sections were stained with Harries hematoxylin and eosin. During early stages there are relatively more B and R cells in the tubules of Hepatopancreas (Figure 4). Later stages of disease show sloughing of hepatocytes (Figure 5) and light hemocytic inflammation with no symptoms of known enteric viruses. Few cells are exhibiting karyomegaly. High B cells in the hepatopancreas in the early stage of disease indicate the over feeding of shrimps.
In the beginning days, farmers managed this disease by regularly removing the dead shrimp from the pond. Reducing the stocking density by partial harvesting reduced the mortality. Reduced feed quantity or suspending feed for few days reduce the mortality. Some farmers opted for extensive culture and were able to reach the production up to 30 counts without RMS. The culture practices like stocking bigger post larvae, reducing stocking density, nursery management, partial harvest, strict feed management and Polyculture with has reduced the incidence of RMS
In conclusion, the intensive aquaculture practices have caused to increase the various sources of anthropogenic stress factors to aquatic ecosystems have led to the emergence of many new diseases in shrimp industry. High B cells in the hepatopancreas in the early stage of disease indicate over feeding
Delayed Hemolymph clotting time and expansion of chromatophores indicates stress. Water exchange is reduced on biosecurity grounds. Recently there is a change in the feeding program. Over feeding is being practiced right from the day one of stocking in order to get maximum growth. Check tray feeding percentage has come down from 1% of the feed to 0.2%. Over feeding always causes water quality problems and causes stress to the cultured organisms. Stressed shrimp will exhibit poor immune response. On the other hand, Shrimp breeding program focus on faster growth and these lines are less tolerant to stress. Opportunistic pathogens like vibrio naturally proliferate during stressful conditions aggravating the problem
In the present study, from the Histopathological diagnosis and microscopic observations, we can conclude that any known viruses did not cause the IMS. Reduction of mortality on reduction of feed and stoking density indicate water quality and carrying capacity issues. Some opportunistic pathogens like Vibrio sp. naturally proliferate during stress conditions to aggravate the problem. There are incidences of RMS and WSSV causing severe mortalities.
Basing on the present study observations and histopathological examination of the hepatopancreas of L. vannamei, we are intended to deliver some conclusive remarks as follows. For the sustainable aquaculture operations, it is essential to procure high quality seed, followed by better pond management practices, biosecurity measures and health management of the shrimps during the entire culture period.