The researchers found that EMS is caused by a bacterial agent, which is transmitted orally, colonises the shrimp gastrointestinal tract and produces a toxin that causes tissue destruction and dysfunction of the shrimp digestive organ known as the hepatopancreas. It does not affect humans.
Lightner’s team identified the EMS/AHPNS pathogen as a unique strain of a relatively common bacterium, Vibrio parahaemolyticus, that is infected by a virus known as a phage, which causes it to release a potent toxin. A similar phenomenon occurs in the human disease cholera, where a phage makes the Vibrio cholerae bacterium capable of producing a toxin that causes cholera’s life-threatening diarrhea.
Research continues on the development of diagnostic tests for rapid detection of the EMS/AHPNS pathogen that will enable improved management of hatcheries and ponds, and help lead to a long-term solution for the disease. It will also enable a better evaluation of risks associated with importation of frozen shrimp or other products from countries affected by EMS.
Some countries have implemented policies that restrict the importation of frozen shrimp or other products from EMS-affected countries. Lightner said frozen shrimp likely pose a low risk for contamination of wild shrimp or the environment because EMS-infected shrimp are typically very small and do not enter international commerce. Also, his repeated attempts to transmit the disease using frozen tissue were unsuccessful.
Since EMS was first reported in China in 2009, it has spread to Viet Nam, Malaysia and Thailand, and now causes annual losses more than $1 billion. EMS outbreaks typically occur within the first 30 days after stocking a newly prepared shrimp pond, and mortality can exceed 70 per cent.
In an effort to learn from past epidemics and improve future policy, the World Bank and the Responsible Aquaculture Foundation, a charitable education and training organisation founded by the Global Aquaculture Alliance, initiated a case study on EMS in Viet Nam in July 2012. Its purpose was to investigate the introduction, transmission and impacts of EMS, and recommend management measures for the public and private sectors.
The study team included Lightner, who with University of Arizona co-workers recently identified the EMS/AHPNS pathogen. At a panel discussion on EMS at the Global Aquaculture Alliance’s GOAL 2012 meeting in October 2012, Lightner and Timothy Flegel speculated that the elusive nature of the disease might be explained by a bacteriophage.
EMS is among the topics that will be addressed at GAA’s GOAL 2013 conference in Paris, France, from 7 to 10 October.