Though contained for the most part to the east of the country, outbreaks of EMS have begun to show up in Thailand’s southern provinces as well. So far, no one has conducted an extensive economic study of EMS’s impact on the Asian prawn fishery, but it numbers in the millions of US dollars, if not billions.
With so much money on the line, both shrimp farmers and academic researchers have entered a race to find the phenomenon’s cause, but so far, it remains an enigma.
Authorities can’t even agree on whether or not it’s a disease. Dr. Putth hesitated to call EMS an epidemic because, in order for something to earn that status, its cause must be known. In the case of EMS, all scientists and farmers have to go off of are its effects.
EMS causes a prawn’s hepatopancreas (essentially a liver and pancreas combined) to malfunction. This in turn interferes with the animal’s ability to digest food, thereby weakening it and leaving it susceptible to disease. (The technical name for EMS is Acute Hepatopancreatic Necrosis Syndrome (AHPNS), though EMS has entered more common usage, owing at least in part to its brevity.)
Prawns typically run afoul of EMS while in the post-larval phase, which in general lasts for the first 35 days after they are planted in a shrimp pond. Though academics studying the phenomenon remain skeptical as to whether or not EMS can also affect mature shrimp, farmers such as Gruenberg insist that it can and does.
After at first assuming that a virus had caused EMS, researchers soon widened their range of possible culprits to include genetic abnormalities, bacterial infections, toxins and shifting environmental conditions.
A March 2013 disease advisory for EMS released by the Network of Aquaculture Centres in Asia-Pacific summarized the situation bluntly: “So far, no causative agent has yet been found and the disease is still considered idiopathic.”
Gruenberg believes that he may have found the cause, but his experiments are still in the initial stages and he prefaces his description of them with a caveat that they at best constitute a “working hypothesis.”
He believes that selective breeding in one of the most widely farmed specie of shrimp – L. vannamei – has led to the current catastrophe. By selecting shrimp for fast growth, Gruenberg contends that breeders have inadvertently selected for weak immune systems as well.
With inadequate immune systems (“like an AIDS patient,” says Gruenberg) these shrimp become susceptible to infection from a parasite known as Gregarine, which acts somewhat like a crustacean version of malaria.
Gruenberg and his team have set up a series of experimental shrimp ponds in which they are trying to save shrimp from EMS by feeding them copepods – a nutrient-dense zooplankton that can help compensate for the prawns’ ravaged digestive capabilities and weakened immune systems.
So far, Gruenberg’s experiments have turned out well, but he has yet to apply them on a scale large enough for the victory to qualify as decisive.
“Normally if a pond starts getting EMS, within a few weeks you’re going to be getting 100% mortality,” he says, “but we were able to save about 50% of the prawns at [the experimental] pond. So that opened our minds to nutrition as one possible solution.”
In the mean time, Southeast Asia’s prawn farmers have little choice but to sit by and anxiously await a cure.
When asked about how bad things could get if the EMS outbreak continues, Chakarin, Chairman of Chanthaburi Shrimp Farmers Club, responds simply, “I dare not say.”
