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November 17, 2015
Massachusetts—John Sackton on EHP
At the Global Aquaculture Alliance’s GOAL shrimp session in Vancouver, British Columbia, Canada (October 2015), a panel of shrimp experts talked about the microsporidian Enterocytozoon hepatopenaei, a spore-forming fungal parasite, and how it might affect the global shrimp supply. John Sackton, Editor and Publisher at Seafood.com (an online, subscription-based, fisheries news service) prepared the following report based on interviews with a number of the shrimp experts, who spoke on the condition that they would not be quoted by name.
When a spore finds a suitable host in a shrimp, it reduces the shrimp’s growth rate and increases size variation in the harvest. In EHP infected ponds, the harvest might have five size categories and many small shrimp.
An expert from Thailand said: “EHP first effected Thailand in 2011, before EMS, and we saw approximately a 10 percent decline in production.... I think...this infection will cause a decline of from 10-15 percent when compared to a time before the disease.”
However, there are some mitigating factors. First, and most important is density. The lower the density, the less impact from EHP. In India, for example, farms that stock at 20 postlarvae per square meter get better growth and produce larger shrimp than farms that stock at 50/PLs/m2.
A second factor is salinity. Low salinity favors less impact and better growth; high salinity appears to correlate with poor growth and greater impact.
The severity of EHP is directly related to the number of spores in the hepatopancreas: the higher the number of spores, the greater the impact on growth. The spore count generally increases with the amount of days the shrimp are in the ponds; after 40 days there is a higher load than initially.
Once the infection occurs, there is no known treatment. The spores are nearly indestructible. Some industry experts believe that the spores can withstand 50 years of drying, or 200 parts per million of chlorine disinfectant. So if a hatchery or farm is infected, drastic decontamination measures must be taken before restocking.
The infection starts with broodstock. If the broodstock carries the spores, it will pass them on to its nauplii and postlarvae. If many farms in an area become infected, the waterways will carry the spores and pass them on to uninfected farms, hatcheries and broodstock.
Stopping the infection is made more difficult because it is not as simple as with other diseases. Polymerase chain reaction (PCR) is the simplest test to identify the presence of EHP. However, PCR requires a certain quantity of DNA to detect it. If an animal only carries a few spores; there will not be enough DNA to detect it.
When EHP is left untreated, bad things happen. It has been especially prevalent in China for a number of years. The spores build up in the environment, resulting in slower and slower growth. Farmers might produce 20-gram shrimp the first year, 15-gram shrimp the second year and 10-gram shrimp in the third year. At the moment, this is largely the story for China shrimp farming. Farmers have been unable to produce large shrimp, and they are turning to antibiotic use to treat the disease. As a result, China has had to import shrimp to get the sizes it needs for exports, and to guarantee antibiotic free shrimp to ship to the USA and elsewhere.
The spike in rejections of shrimp from Malaysia due to high levels of antibiotic residue can be traced to this problem. China transshipped shrimp to Malaysia, and rejections of Malaysian shrimp by the USA/FDA spiked. Malaysia has cracked down and successfully prevented this practice, and FDA rejections have fallen sharply.
The current understanding of EHP is that with low stocking densities, the slow growth problem is not as severe. So one impact may be plentiful large shrimp in the global market, which is counter intuitive for a disease that restricts growth.
It does seem clear that the newer production areas where there has been a spectacular growth of output, such as India, will see a leveling off as ponds become less productive in their third or fourth year.
Source: Seafood.com (an online, subscription-based, fisheries news service). Editor and Publisher, John Sackton (phone 1-781-861-1441, email email@example.com). EHP Disease in Shrimp May be Hard to Control, Likely Will Increase Volatility in Market. John Sackton. November 17, 2015.
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