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Enterocytozoon hepatopenaei (EHP)Revisited

 

Shrimp News: This is a continuation of the discussion on Enterocytozoon hepatopenaei from The Shrimp List that was posted to my webpage April 13, 2017.

 

Mark Napulan (kram_lewor501@yahoo.com): I constantly update my knowledge on shrimp diseases through various channels, including this List.  I have been travelling in India for quite sometime now and seen the EHP problem in five major shrimp farming states.  Almost all of the shrimp farmers that I interviewed do not know how to deal with EHP.

 

I saw a video of a farm in Thailand that uses 3,000-ppm chlorine to deactivate spores.  Farmers with pond liners in Malaysia are using “hot lime” (below) to get rid of EHP, and it seems to be working.  Last year, in Vietnam, I saw many EHP infected ponds.  I wonder how China is coping with EHP.

 

Since most spores are deactivated by pasteurization or the extremities pH, I would like to gather opinions on the following EHP treatment methods in earthen ponds:

 

1. Bombarding the pond bottom with steam before filling it

2. Covering the pond bottoms with rice hulls and then burning them (cheapest)

3. Torching the pond bottoms with flame throwers

4. Spraying the bottoms with Sodium hydroxide (NaOH) to lower the pH to
    less than 2, followed by treatment with CaO to raise the pH above 14

 

Members of this List seem to be hesitant about sharing their experiences on eradicating EMS spores in shrimp hatcheries and ponds.

 

Bob Rosenberry (bob@shrimpnews.com): Off The List, I asked Mark to define “hot lime”.

 

Mark Napulan (kram_lewor501@yahoo.com): Bob, I was referring to quick lime (CaO).  When it reacts with water at a 1:3 ratio, CaO releases a lot of heat, up to 57 degrees Celsius.  Applying lime increases the pH to more than 12 and deactivates EHP spores.  As an alternative, hydrated lime (Ca(OH)2), which is readily available and inexpensive, can be used.  I’ve seen many farmers applying lime to ponds with dry bottoms.  That’s a mistake.  If you don’t moisten the soil first, it won’t react and won’t increase the pH of the soil.

 

Nelson Gerundo (nelsongerundo@yahoo.com): Mark, the best way to get rid EHP spores on the bottom of you ponds is to use pond liners.  This makes it is easier to acidify the remaining water in your pond.  After flushing, the empty lined shrimp ponds can be easily cleaned with high-pressure washers and left to dry.  EHP spores are resistant to drying, but if the washed black liner is left in the sun, the already acid treated EHP spores will be further inactivated by heat emanating from the liner.

 

Loc Tran (thuuloc@email.arizona.edu): The Shrimp Vet Laboratory in Vietnam has received many EHP positive cases from hatcheries, broodstock feces, live feeds, PLs, sediments and shrimp in growout.

 

Farmers are not paying enough attention to EHP.  If you are a hatchery manager, you need to check everything and implement a program to properly disinfect everything between growout cycles.  If you are a farmer, you need to choose your PL supplier carefully.  Get more than one sample of his PLs.  If any of the samples are EHP positive, choose another supplier.  Also, make sure your ponds are properly cleaned before stocking.

 

An EHP polymerase chain reaction (PCR) test is only $9 at our lab.  It’s worth it.

 

Mark Napulan (kram_lewor501@yahoo.com): Loc, nice hearing from you and thanks for your insights.  Have you tried exposing EHP spores to a pasteurization temperature?  If yes, for how long?  And do you think spraying the soil with NaOH followed by Cao could deactivate the spores in earthen ponds?  Is it worth a try?

 

Nelson, you’re right that at extremely high pH, spores can be deactivated in lined ponds.  However, you have to understand that not all farmers can afford the luxury of lined ponds.  That’s why I emphasize earthen ponds.

 

Nelson Gerundo (nelsongerundo@yahoo.com): Mark, the world is just beginning to learn about EHP.  One thing we know for sure: acidification followed by liming is the way to inactivate the spores.  It might even destroy the spores, but, of course, if you stock those ponds with EHP positive PLs, you’re going to have slow growth.

 

Daniel Gruenberg (daniel@acquestra.com): Nelson, what’s your source of information on the acidification/liming process?  Do you have any data that you can share with us?  Published papers?  Actual experiences?  Or are you just postulating?  We need to know dose levels, the names of the acids, the pH levels and how long the treatments should last.

 

It may very well be a brilliant idea.  I’m not saying it doesn’t make logical sense.  But sometimes nature throws us a curve ball.  In God We Trust—All Others Bring Data!

 

Robert Bauman (hankbauman@gmail.com): Stocking clean PLs to prevent EHP is sound advice, but if your farm is in an area where there are abundant spores in the water, how long will your shrimp remain clean after stocking?

 

Gianluigi Negroni (gigineg@gmail.com): Mark, unfortunately, I have not run any scientific tests, but I use sodium ferrate to deactivate all living organism in my water.  Sodium ferrate (Na2FeO4) is not very stable, so you need a machine to produce it on site.

 

Nelson Gerundo (nelsongerundo@yahoo.com): Now that we have a baseline concept for destroying EHP spores through acidification followed by liming (alkalization), scientists need to determine the amounts and contact times for the treatments.

 

Daniel Gruenberg (daniel@acquestra.com): Loc, do you know anything about the amounts and contact times for the acidification/liming treatment?

 

The information that I have suggests a 25% solution of NaOH followed by acidified chlorine at 200 ppm for hatchery surfaces and six metric tons per hectare of CaO spread on dry soil and then moistened to activate the lime for dirt ponds.  After that, wait five days for the pH to reach 12+.  Is this still the main suggestion?

 

I have not heard that lined ponds do any better than dirt ponds that are properly managed.  What is the latest information on EHP in Vietnam?  Although there are no external signs of EHP, the combination of stress and spore presence does seem to dramatically slow growth in my ponds.

 

Loc Tran (thuuloc@email.arizona.edu): Nelson, at our laboratory, we have done many EHP challenges.  Some patterns are quite interesting.  For example, we discovered that EHP can be transmitted via cohabitation.  With a net, we divided a tank into two sections and stocked EHP positives on one side of the tank and EHP negatives on the other side.  Then we feed the EHP positive shrimp and let them release contaminated feces.  The EHP negative shrimp were infected within 24 hours.  After ten days, they had exponential numbers of spores in their hepatopancreases, a multiplication of 1,000 to 10,000 in less than two weeks.

 

The treatments that we are currently recommending are better sanitation and disinfection.  Many promising chemicals have been proven to work on EHP.  It is also important to prevent EHP from multiplying inside the hepatopancreas.  We are working on that problem at our lab.

 

Here are a few observations/conclusion from my team’s research.  It’s not my research; I have a team of junior researchers at my lab.

 

1. Here are a couple of ideas for preventing bloodworms, a popular broodstock feed, from bringing EHP into your hatchery.  In theory, bloodworms can be pasteurized by steaming them for around five to seven minutes (not cooked).  In reality, however, it’s quite difficult to say if “steamed” bloodworms are safe or not.  We’re not sure if the inside of the worm reaches 60+ degree C.  One alternative, you can “block-freeze” bloodworms without damaging much of their nutritional quality.  Using these techniques (along with a lot of other things), we kept one hatchery EHP-free for over a year.

 

We froze EHP-positive shrimp, defrosted them, but could not infect specific pathogen free shrimp with the frozen, EHP-infected material.

 

2. We don’t know the best method for disinfecting ponds.  We’re dealing with shrimp farmers using earthen ponds that have very limited resources.  We advise them to remove the sludge from previous crops (very, very important), and to treat the pond bottom with CaO at 500-kg/1,000 m2.

 

Spray surfaces with a solution that’s 300-ppm formalin.  Spray the pond bottom with a solution that’s at least 100-ppm formalin.

 

Nelson Gerundo (nelsongerundo@yahoo.com): Shrimp farmers in Thailand, Vietnam, India, Indonesia, Malaysia and China—a reliable and cheap EHP/PCR Test Kit that costs only $9 is available from SHRIMPVET.  You can use it to check postlarvae, adults, pond bottoms and your source water.

 

For those who might want to acquire a distributorship for the SHRIMP VET EHP/PCR TEST KIT in your country, contact Founder and Director Dr. Loc Tran at: SHRIMPVET LABORATORY, 307 Long Lam University Campus, LinhTrung Ward, ThuDuc District, Ho Chi Minh City, Vietnam (Phone +84-913-210-290, Email thuuloc@email.arizona.edu, Webpage http://shrimpvet.com).

 

Bob Rosenberry (bob@shrimpnews.com): Also see To Lime or Not to Lime, a Shrimp List discussion from February 6, 2012.

 

Sources: 1. The Shrimp List (a mailing list for shrimp farmers).  Subject: Enterocytozoon hepatopenaei (EHP).  April 18 to 20, 2017.  2. Bob Rosenberry, Shrimp News International, April 28, 2017.

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