|Culprit behind massive shrimp die-offs in Asia unmasked|
|Friday, 10 May 2013 17:21|
Bacterium responsible for Early Mortality Syndrome of shrimp – crucial first step in finding effective ways to combat the disease
In what has been deemed a "major breakthrough", researchers at the University of Arizona have identified the causative agent behind a mysterious disease that has been decimating shrimp farms in Asia, reports the United Nations Food and Agriculture Organisation (FAO).The disease, known as Shrimp Early Mortality Syndrome (EMS) or Acute Hepatopancreatic Necrosis Syndrome (AHPNS), has over the past two years caused large-scale die-offs of cultivated shrimp in several countries in Asia, where one million people depend on shrimp aquaculture for their livelihoods.
In 2011, the Asian region produced three million tonnes of shrimp with a production value of USD13.3 billion.
Infected shrimp ponds experience extremely high levels of mortality early in their growing cycle — as high as 100 per cent death rates in some cases.
So far, the cause of the illness has baffled scientists, animal health authorities and farmers, making prevention and treatment difficult.
But now the identity of the culprit has been found: a strain of a bacterium commonly found in brackish coastal waters around the globe, Vibrio parahaemolyticus.
A team of researchers at the University of Arizona have managed to isolate the strain and use it to infect healthy shrimp with EMS/AHPNS — a scientific method known as Koch's Postulate.
"We succeeded in isolating a pure culture of the V. parahaemolyticus strain and reproduced the EMS/AHPNS pathology in our laboratory," said Professor Donald V Lightner of the Aquaculture Pathology Laboratory at the University of Arizona (UA).
"The high virulence of this agent to shrimp may be due to a phage which affects this particular strain of V. parahaemolyticus," he added.
The effort to study EMS, identify its pathology and respond to EMS was supported by a coalition of partners including UA; FAO; the World Organisation for Animal Health (OIE); the World Bank; the Network of Aquaculture Centres in Asia-Pacific (NACA); the Global Aquaculture Alliance (GAA); the Ministry of Agriculture and Rural Development of Viet Nam; CP Foods; the Minh Phu Seafood Corporation; Grobest Inc. and the Uni-President Feed Company.
This breakthrough finding by UA of a bacterial aetiology is a crucial first step in finding effective ways to combat EMS.
EMS/AHPNS initially surfaced in 2009. By 2010, outbreaks had become serious. In China in 2011, farms in Hainan, Guangdong, Fujian and Guangxi suffered almost 80 per cent losses. In Thailand, shrimp production for 2013 is predicted to be down 30 per cent from last year due to EMS. Production on some farms in eastern parts of the country has been cut by 60 per cent.
FAO first fielded a mission to Viet Nam through its Crisis Management Centre for Animal Health to investigate the disease in 2011 which pointed to an infectious agent and since 2012 is implementing an emergency technical assistance project in Viet Nam.
No risk to human health
Just 1-2 per cent of wild V. parahaemolyticus strains worldwide contain these two genes — and the strain identified by Lightner and his team as responsible for EMS is not among them.
"The strain of V. parahaemolyticus we isolated appears not to have the genes that confer virulence to human infections," said Lightner.
"There have been no reports of human illness being associated with EMS, and these new findings would tend to confirm that EMS-infected shrimp do not pose a health risk to people," added Iddya Karunasagar, a seafood safety expert at FAO.
Only shrimp vulnerable
Clinical signs of the disease include lethargy, slow growth, an empty stomach and midgut and a pale and atrophied hepatopancreas (an internal digestive organ that serves the function of a liver), often with black streaks. Within 30 days of a pond being stocked large-scale die-offs begin.
So far countries officially reporting EMS include China, Malaysia, Thailand and Viet Nam, but any place where P. monodon and P. vannamei are cultivated is potentially at risk. This includes most of Asia and much of Latin America, where shrimp farming is also important, as well African countries where shrimp are cultivated (Madagascar, Egypt, Mozambique and Tanzania).
Disease spread would appear to be linked to proximity to already-infected farms or the movement of infected live shrimp, usually juveniles used to stock ponds.
Lightner's team was unable to reproduce EMS using frozen and thawed shrimp samples, suggesting freezing kills the responsible bacterium. Since international shrimp trade is mostly in frozen form, there is apparently no or very low risk of disease transmission from these products.
Dealing with EMS
At the same time, FAO is engaging with partners to organize a concerted, inter-regional effort to address the disease.
For shrimp farmers, reliance on already-established aquaculture and biosecurity best practices will help prevent EMS-related problems. These include:
Off farm, any movement of live or unfrozen shrimp products should also comply with established best practices.
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