EMS (early mortality syndrome) or AHPND (acute hepatopancreatic necrotic disease) first manifested itself in south eastern China in 2009, which subsequently spread to Vietnam in 2010, Malaysia in 2011, Thailand in 2012 and Mexico in August 2013.
In 2013, Dr. Lightner’s team discovered that AHPND was caused by a certain strain of Vibrio parahaemolyticus with virulent genes affecting the hepatopancreas of shrimp. V. parahaemolyticus is a fast growing opportunistic pathogenic bacterium that quickly develops a biofilm which is capable of attaching to chitin surfaces. The bacterium can develop a resting form and becomes dormant for long periods in dry condition. Just to give an idea, Vibrio cholerae can survive in dry form for up to 60 years!
Victoria Alday-Sanz presented a detailed observation on EMS in her presentation at the Biofloc Technology and Shrimp Diseases Workshop Vietnam in December 2013. Her observations indicated that in both the semi-intensive ponds of Mexico and the intensive ponds of Asia, the pond bottom, whether lined or earthen appeared to be the main risk factor. This is validated by the absence of the disease in shrimp placed in a suspended net not touching the AHPND affected pond bottom. P. vannamei, P. monodon and P. chinensis are all susceptible to AHPND. This may imply that the risk factor is not inbreeding, as once believed to be. Similar to infectious myonecrosis virus-IMNV, AHPND is more severe at high temperatures. Fasting improves the shrimp health. Fermentation should be avoided in the sludge area. There is no mortality in 104 CFU/ml pathogen bacteria density.
Some of the possible reasons for Indonesia has remained free of the disease as of today are discussed below:
First of all, after its very bitter experience of contracting IMNV in 2006, the shrimp farming communities in Indonesia have been very careful about trans-border importation of shrimp. The genetic sequence of the Indonesian IMNV is 99.6% similar to that of the Brazilian IMNV, kept in the GenBank. An Indonesian Ministerial Decree 17/2006 had allowed the drawing up of a National Fish Quarantine regulation to protect the country from further introduction of exotic diseases through trans-border movement. The measure has restricted the previously haphazard importation of brood stocks and post larvae.
Secondly, Indonesian farms have very hygienic pond bottoms. More than 90% of the ponds in Indonesia have central discharge systems. Sludge from dead plankton, shrimp feces and uneaten feed accumulated in the center of a pond can easily and regularly be discharged to the outlet canal by simply pulling up the vertical pipes placed on an elbow. The frequency of discharge can be up to 5 or 6 times a day.
Accumulated sludge in the pond bottom is a great liability to the pond water quality. The sludge not only consumes a large portion of oxygen, it ferments, and produces heat and the excessive nutrients together with the warmth from fermentation allows pathogenic bacteria to proliferate. Feed particles either dropped or swept to these areas will be contaminated by the bacteria and end up being eaten by the shrimp. Indonesia has until now been free of AHPND. Its EMS/AHPND-free status is not a coincidence.
Source: Poh Yong Thong 2014, AQUA Culture Asia Pacific Magazine.