Aquaculture Business Research Center (ABRC)

Faculty of Fisheries, Kasetsart University, Thailand

The Practical Vol.4 issue 13

 

Since 2011, Shrimp aquaculturists in China, Vietnam, Thai, and Malaysia all suffered from EMS (Early Mortality Syndrome) that academic professor in many countries called “AHPNS” (Acute hepatopancreatic necrosis syndrome). Many farms had to stop their businesses. Furthermore, no one has yet identify the actual causes and come up with the solution on how to prevent this outbreak.

 

In Thailand, Aquaculture Business Research Center (ABRC), Faculty of Fisheries, Kasetsart University found the outbreak of this syndrome since the late August 2011 around Pungrad estuary, Nai-Arm district, Chantaburi province. We found 8-to-15-day-old infected shrimp that were similar to AHPNS found in China and Vietnam. When conducted histopathology study of infected shrimp, we found abmornalities on the base of hepatopancrease. No fat deposit in hepatopancrease. We also found quite a number of bacterial infections in hepatopancrease and no sight of virus infection.

 

At that time, this disease infected around 200 ponds. We found out that shrimp got infected only in a few days after stocked in the pond, and no cross-contamination between the ponds.  Farms that got infected tried to stop this outbreak by using chemicals to kill shrimp and keep water in the ponds until they harvest shrimp in the other ponds then release all the water out. No report on finding the outbreak of this disease ever since.

 

Until January-April 2012, we found EMS outbreak once again in Pra-sair estuary, Klaeng district, Rayong province. The study by Prof. Timothy William Flegel, Center of Excellence for Shrimp Molecular Biology and Biotechnology (Centex Shrimp), Faculty of Science, Mahidol University, was able to isolate bacteria in Berkholdiales species, Commomonadaceae family from the infected shrimp and one species of bacteriophage. The Bacteria we found in Thailand is similar to the shrimp infected with AHPNS from Vietnam.

 

The study from Center of Excellence for Shrimp Molecular Biology and Biotechnology has applied the bacteria and bacteriophage from infected shrimp into the pond that stock normal shrimp and also injected into normal shrimp’s muscle in the laboratory. The tested shrimp did not have any symptom of the disease or die. This shows that the bacteria and bacteriophage solely did not cause the disease.

 

In the farms that we found infected shrimp, the farms only use chemicals to kill carriers during the water treatment period before stocking shrimp, but no one uses effective chemicals to kill pathogens in the water. Moreover, most farms that got infected with 14 days after stocking shrimp use a lot of probiotics to adjust water to low pH because they believe that it will make the shrimp molt more often, grow faster, and control bacteria in water.

 

Aquaculture Business Research Center (ABRC) has informed aquaculturists in the infected area to use the closed system and use effective chemical to kill all kinds of pathogens before stocking shrimp and stop lowering pH, because shrimp is in its weakest condition during the molting stage. We also informed farmers to check the quality of postlarvae before stocking such as pathogens and amount of fat deposit in liver, because we found out that infected shrimp show abnormalities in hepatopancrease and barely contain fat deposit.

 

Since May 2012, many shrimp die throughout Thailand, especially when weather changes, such as raining or plankton drop. We found a large quantity of 25-45 day old shrimp died. Most of the shrimp are molting shrimp. Many farmers believe that all of the shrimp died from EMS, when some of them might infect from WSSV. Most ponds that got infected have pH lower than 7.8 in the morning and alkalinity lower than 100 ppm. Some farms with high acidity for example in Khlung district and Laem Sing district in Chantaburi province find shrimp gradually die from the disease.

 

The amount of feed is also count into consideration. We found out that if shrimp die and farmers continue to feed shrimp as usual, shrimp will die more. When they stop feeding, no more death shrimp is found. The farmers are unsure what to do, since feeding less also lead to less survival rate. Pathology study of these infected shrimp shows the shrink of hepatopancrease and the fall out of epithelial cell of hepatic duct since shrimp do not take feed. Some samples found infection of protozoa, microsporidean species. Some samples found infection of protozoa that is similar to gregarine. Shrimp in its last stage found large amount of Vibrio spp., but when continue to raise these infected shrimp in the laboratory, most of the shrimp did not die and continue alive.

 

These results show that shrimp in many areas die from many reasons. The quality of postlarvae is the biggest concern, since most infected shrimp show symptoms from the disease only within a few days after stocking despite the defectant used. Aquaculture Business Research Center (ABRC) has conducted research on effects of quality of vannamei post larvae towards EMS. In vannamei farm in Klaeng district in Rayong province, researchers collect sample shrimp post larvae from 21 ponds before releasing to the ponds. The researchers use the methods that was adapt from shrimp biotech method (Gunnawit&Kamron, 2003). The method is divided into two parts as following:

1)         Grading  by the health quality using microscope

Test the abnormalities in 10 samples of postlarvae using microscope. Each postlarvae will be determined using 5 topics: quality of hepatopancrease, quantity of fat deposit in hepatopancrease, external parasite, quality of appendage, and Muscle:Gut ratio. These will be recorded into the dashboard for grading, which the qualified shrimp must gain more than 85% of total score.

2)         Determine Vibrio infection

Test the quantity of vibrio among 100 samples of postlarvae. Test vibrio by using selective media, which is Thiosulfate Citrate Bile Salt Sucrose Agar  )TCBS agar). The qualified standard for this test is having yellow colony below 100, green colony below 10, and no fluorescent colony.

 

From our study, we found only 3 ponds that pass the postlarvae grading-test using microscope. Eighteen ponds did not pass the test. However, no ponds passed the vibrio infection test. All of the postlarvae from the 18 ponds that did not pass the postlarvae grading-test using microscope show symptoms of EMS and died in 20 days after stocked.

 

The research point out the importance of postlarvae quality upon EMS outbreak. Further research whether the cause of the outbreak came from the improper method from hatchery or gene in vannamei is needed. Nowadays, gene in vannamei is developed for the highest growth level possible. This newly developed shrimp must culture in the proper environment conditions because these shrimp molt more often and require more energy to grow and balance minerals in their bodies to produce new shells. When the environment condition is not stable, shrimp will not eat, molt less, and most of them will die whereas smaller shrimp that molt less will survive. Some farms did not adjust the amount of feed according to the situation and cause more waste in the ponds. These accumulated waste pollute the water and cause shrimp to die more. The more weather conditions change, the more shrimp die out.

 

In addition, various kinds of protozoa that were found in postlarvae shrimp indicated the failure of biosecurity used in hatchery. This causes the uncertainties whether there is any other disguised disease. All of the information above shows that solving this issue requires understandings and cooperation from every sector. The government sector such as the Fisheries Department must set the standard for hatchery, broodstock management, and facilitate the importation of broodstock to improve the quality of broodstock in Thailand and increase diversity of species.

 

Academic bodies must research the causes and prevention of this disease. Exporting sectors must ensure with international buyers that Thailand is in control of the outbreak and have enough products for exporting. Hatcheries must examine disease contamination in broodstock and find prevention. Developing new species that is able to stand weather conditions in Thailand is also needed.

 

For aquaculturists, ponds must be prepared in their farms before stocking. Close system should work best in area with the outbreak. Aquaculturists must use solutions that kill pathogens in the water before stocking. Do not use chemicals to kill only carriers, because it will kill only crustaceans and exclude diseases in the water.

 

Aquaculturists must control the amount of feed and make sure that the aerators provide enough oxygen. Dissolved Oxygen (DO) should be higher than 5 ppm in the morning to decompose waste in the bottom of the pond. Add lime in the area with low pH in the morning to -8.2 to slow down the molting stage. Alkalinity must higher than 100 ppm and add proper amount of minerals to ease shrimp molt.

 

If shrimp start to die and did not cause from virus, aquaculturists must stop feeding, zero exchanging water, and adjust pH level of water in the morning to 8.0-8.2 until shrimp no longer die. When the rest of the shrimp get healthier (i.e. shrimp in the feed tray react normally), start to give feed that adds sea salt in 50 grams/1 kilogram of feed continuously. Many aquaculturists who use this method during the early stage should be able to continue the process. Unless shrimp start to die again, aquaculturists should stop this cycle, because the outbreak will usually cause all of the shrimp to die out.

 

Reference:

Gunnawit  Rujirawit and KumronWaiyakrata. 2003. Examining shrimp postlarvae using Biotechlab method. Page 271-280. Via SurasakDilokKiat. Leading Thai shrimp to the world leader in sustainable way. K.PL press, Bangkok

 

Nattinee Mankongwongsiri. 2013. The case study of Early Mortality Syndrome in Vannamei in Thailand. Thesis paper for master degree. Kasetsart University.