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March 21, 2014
Washington State—AquaInTech—Managing EMS
Shrimp News: This report is an excerpt from an article on Dr. Stephen Newman’s webpage. To get a copy of the full article, click on the link in the source at the end of this report.
There is still much to be learned about EMS/AHPNS and the nature of the disease process. What we have learned so far is that the bacteria that causes EMS appears to be naturally occurring in marine ecosystems and that its mechanism of disease production is tied in with its ability to bind to chitin and the production of a toxin. The data to date suggest that the bacteria forms a biofilm on the shrimp’s stomach lining and the gastric mill, both of which are sloughed off with each molt because they are chitinous and, basically, part of the exoskeleton. The biofilm is a highly evolved mechanisms for toxin production and notoriously difficult to get rid of. It protects the bacteria from the onslaught of a variety of chemicals.
Elimination of the vectors in ponds is simply not feasible as the costs of constantly eliminating rotifers and any other chitinous surfaced, bacterial consuming zooplankton would be prohibitive even if it were possible. Cysts can be moved through air and water and are often buried deep in sediments. While prudent practices can lessen the Vibrio load in the environment (using products such as our PRO4000x) and lower organic loads that Vibrios feed on, there is data that there are wide variations in virulence of the strains that cause EMS. This wide disparity in virulence means that efforts to lessen the impact of Vibrio loads in the environment may not be useful in areas where highly virulent strains are found. On the other hand, in environments where the strains are not as virulent, it may very well be possible to lessen their severity.
Using standard feeding techniques is not a good idea if you want to eliminate EMS. The bacteria are likely transmitted via vectors, which could include any chitin-containing organism like rotifers and algae. Any compound that is delivered via the feed must be in frequent contact with the biofilm in order to be effective. Typically antibiotics don’t work because biofilms are unaffected by them and the bacterium does not appear to cause disease by an internal invasion of the shrimp.
Strategies for Control
The first step is to address the possibility that pond reared broodstock may be carrying the bacteria as a part of its normal intestinal flora. While acute mortality in broodstock is possible, it is not common. The bacteria typically attach to the stomach although they may also be attached to external surfaces, and they have been reported to occur in feces of broodstock. The external surfaces of broodstock should be disinfected. Formalin, iodophors, Chlorion, and other surface disinfectants all have proven anti-Vibrio activity. Spawning in water that contains the normal spawning fluids and material from adults (feces) suggests that the eggs may be contaminated. Eggs should be washed with clean water and surface disinfected with any of a number of compounds such as iodophors (or others mentioned above). Washed eggs should not be returned to the spawning tanks. After the eggs hatch, the nauplii should also be washed.
Traditional postlarvae-rearing systems have not been shown to be a source of high levels of the pathogen. There are reports of low levels occurring in these systems, but no active disease outbreaks. Using our tableted bacterial product, PRO4000x, one tablet per 5 to 20 metric tons of water daily in production tanks has been shown to be an effective tool to lower overall Vibrio loads.
The real problem with EMS occurs when shrimp are stocked. There is a very high probability that the bacterium is a component of what would be the “normal” flora of the pond. Strains of the Vibrio bacterium have been found attached to algae and in filter feeders ranging from rotifers to mussels. It may be impossible to prevent some ingestion of the bacteria in your shrimp regardless of what tools you use. Can we inhibit the bacteria to a sufficient extent to allow the animals to grow and feed normally? That’s the challenge.
Ponds should be prepared by turning the bottom soils to a depth of at least 10-15 centimeters allowing the air to dry them out. Using chlorine or broadly active antimicrobials could make the problem worse, although doing something to lessen the WSSV vector loads would be prudent. For the farm, I would start with a heavy dose of PRO4000x. One kilogram per hectare (approximately 63 tablets) prior to filling the ponds. After that, wait for 18-20 hours, and then add a good source of carbon. DO NOT USE IT EARLIER THAN THIS because it has been shown that the bacterium that causes EMS may thrive on lower molecular weight sugars. If you feed the Vibrios instead of the bacillus they will outgrow the bacillus.
At this time we still do not have a good understanding of where in the environment the pathogenic bacteria is and what the reservoirs are. There are reports that rotifers and other zooplanktonic forms may be a major source of the EMS bacterium. This presents several challenges. The first—how to eliminate them from the ponds. While the use of pesticides and disinfectants toxic to rotifers might be useful, they don’t always work because rotifers produce cysts that survive in the bottom sediments. As long as there are significant reservoirs that contain high levels of the etiologic agent of AHPNS, control will be problematic. If the bacteria caused a systemic inflammation as part of the disease process, then feeding antibiotics might be a useful tool. No one, however, has yet to suggest that this is the case, and there are no reports where the use of antibiotics has helped. Biological control is an option. For, example, certain fish species aggressively feed on rotifers.
EMS is complicated and multifactorial. We have no idea yet how to control it, although many things are being tested in the lab and the field. To date none have worked.
Information: Stephen G. Newman, Ph.D., AquaInTech, Inc., 6722 162nd Place SW, Lynnwood, WA 98037 USA (phone 1-425-787-5218, mobile 1-425-239-7682, fax 1-425-741-0857, email email@example.com, webpage http://www.shrimpaquaculture.com and).
Source: Shrimp Aquaculture.com. Managing EMS/AHPNS. Received March 20, 2014.
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