Current Status of Shrimp Diseases in SE Asia

Presented byStephen G. Newman Ph.D.

PresidentAquaintech Inc.

sgnewm@aqua-in-tech.com

Presentation

• Overview of global shrimp production

• Diseases– AHPNS (EMS)

– EHP

– CMNV

– White feces

– Many others known and no doubt many that remain to be identified

• Conclusions

SE Asia and China global shrimp production(2015 estimated)

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100

200

300

400

500

600

700

800

2006 2007 2008 2009 2010 2011 2012 2013 2014 2015

Thailand China India Vietnam Indonesia

America’s, Mexico and Ecuador shrimp production(2015 estimated)

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50

100

150

200

250

300

350

2008 2009 2010 2011 2012 2013 2014 2015

America's Mexico Ecuador

Global shrimp production

• Larger producers facing long term problems.• Thailand and China will not recover quickly (recover

means returning to 2010 levels). • Vietnam is significantly affected although their export

numbers do not show that this is the case. Moving into new areas? Continued spread of P. vannamei culture. High Chinese taxes mean more than half of Ecuadorian shrimp is landed in Vietnam and then trucked over the border into China.

• Expansion occurring in unaffected areas.• Unaffected areas will expand as long as $$$ incentive

continues.

New ponds in Vietnam

Disease

Disease is the result of interacting factors

Pathogen

Environment

HostBiosecurity

Disease

Can it produce disease?

Stressors inherent in production paradigm

Host Susceptibility

Where can the process be impacted?

The consequence

Applies to any disease process specifics might differ Point is that diseases are not usually simple

CMNV

Covert Mortality Sydr.

Shrimp

Bad management

Highly virulent Vibrio

From Jie

Tip of the iceberg

A good analogyWhat is affecting production is not always obvious. Many factors influence disease processesSome we know (what we see)Others we have some idea about andOthers we have no idea what they are

These can be things such as sub-lethal levels of hydrogen sulfide as an example or some as of yet uncharacterized pathogen(s). Often times we do not know what is operating here and it is common to blame problems in feed, seed, etc.

• Focusing on EMS, EHP and CMNV

• Others are problematic as well

• Serious problems a result of poor biosecurity and failure to follow common sense principals.

No biosecurity is the norm in many parts of SE Asia or even worse SCAM based biosecurity

Flegel et al. Examined 200 Thai ponds at random for prevalence GOAL 2014

Infected with the E. hepatopenaei (EHP) 72/148 = 49%

Mortality <35 days, i.e. the classic definition of EMS 21/148 = 14%

Infected with white spot syndrome virus (WSSV) 8/148 = 5%

Covert mortality nodavirus (CMNV) 64/148 = 43%

Early mortality with no HP lesions 5/148 = 3%

EMS

• What we know

• What we don’t know

• What are the options for control?

Bacterial in origin

Due to specific strains of Vibrio parahaemolyticus (Vibrio harveyi also, others? 125 species of vibrio and counting)

Nothing in colonial morphology that stands outGreen colonies on TCBSMost strains of V.P are not shrimp pathogensNot an obligate pathogen

can be present without disease

Disease occurs in small shrimp typically within a few weeks of stocking in ponds thus the name.

Toxin produces pathology. The vibrio is the means by which the genes are moved. No genes no disease.

From Jie 2014

AHPNS (EMS) Acute hepatopancreatic necrosis syndrome or disease

• Plasmid mediated toxin production

– PIR toxins (2) punch holes in HP cells eventually disrupting the organ resulting in a weakened animal that dies from a massive secondary bacterial infection.

– EMS bacteria is not thought to be the culprit. Animals succumb to massive secondary bacterial infections.

Plasmids

Toxin genes can be spread through a common mechanism of gene movement-a small circular piece of DNA that bacteria readily exchange. Means by which antibiotic resistance and many other gene products are spread between bacterial species.

• Toxin is a protein. Action of two proteins that must act together for virulence.

• Broodstock feces are contaminated connection to the use of live feeds (polychaetes, barnacles, in filter feeders, on zooplankton) and even in sludge.

• One working theory is that the bacteria rapidly colonizes uneaten feed (generation time of typical vibrios is 10 minutes or so)

Biofilm formation on gastric mill and stomach may be involved in virulence.

What do we know from the field?

• Lined ponds lower risk

• Producing P. monodon lower risk

• Using well water lowers the risk

• Higher water exchange rates lower the risk

• Low salinities lower the risk

• Not an obligate pathogen. >10000 CFU/ml

• Many areas with positive shrimp with no acute disease

• Low to no feed consumption lower risk

What we don’t know

• Not clear to what extent false vertical transmission is involved in the development of the acute disease. Possibly significant to start.

• The presence of vectors. What role? What are they? Found in filter feeders in ponds (rotifers and copepods). Possibly attached to algae and other surfaces.

• The role of the microbial ecology in impacting disease process.

• How to effectively prevent the problem.

Suggested approaches to management

• Eliminate false vertical transmission– Don’t use live feeds from sources that can be

carrying the bacteria (China?) See following table.

– Minimize mass spawning to avoid contamination of eggs and nauplii by feces.

– Use appropriate biosecurity protocols• Prophylactic use of antibiotics in broodstock

• Copious washing of eggs and nauplii with disinfectants and clean water

• Do not stock PLs carrying the bacteria into ponds

Flegel et al. GOAL 2014

Suggested approaches to management

• Change production paradigm if possible.

• If not, use tools for lowering levels of accumulated organic matter in ponds and competing against vibrios.

• Lessen avoidable stress.

Enterocytozoon hepatopenaei (EHP)

• Microsporidian >130 genera identified » > 25 affect crustaceans

• Parasites with complex life cycle

• Wide spread throughout SE Asia (reported in late 1990’s in Malaysia?)

• Possibly associated with white feces– i.e. EHP positive with white feces

• EHP negative no reports of white feces

• Not clear if there is any linkage publications suggest not

EHP

Individual spores (from Tourtip et al., J. Inv Pathology 102 (2009) 21-29

What do we know?

• Tissue specific for HP unlike most other microsporidians which affect muscle

• Transmitted orally between shrimp with no need for an intermediate vector very unusual and very worrisome

• Very difficult to visualize with normal histology

• Requires practice and patience

• Once spores are in ponds very difficult to eradicate

HP with EHP spores

Spores

What we do not know?

• Where it originated?

• What, if any vectors and carriers are there?

• How to eradicate it.

• How widespread it is.

Control of EHP

• Two fold approach to possible control

– Broodstock carriers shed spores in feces which contaminate egg and nauplii surfaces.

– Farmed animals shed spores into ponds which in turn are available to re-infect foraging shrimp feeding on dying shrimp and spreading spores.

– Reduce organic matter in ponds and use caustic treatments before stocking.

Covert Mortality Nodavirus (CMNV)

• Shrimp from ponds showing the characteristic pathology.

Courtesy of Jie Huang

What we know

• Not new. Chinese coined the term “bottom death”. It has been around for more than a decade.

• Shrimp die on the bottom.

• Kills at temps above 28 C.

• The mortality appears at about 1 month post-stocking and increases after 60–80 days post-stocking, may be accompanied by an increase of nitrite nitrogen with a cumulative mortality up to 80 %.

What we know continued

• Found in polychaetes used to feed broodstock

• Likely wide spread.

• Expect it to be present in animals that have been fed Chinese polychaetes any time in the last ??? years.

• Has been confirmed in the America’s (Mexico).

Virions of CMNVSole publication shows that there may

be two different viruses involved.

Interesting aside is that this virus was first characterized at the same time that EMS was identified. Relationship?

Virulence testing of CMNV

Disease readily transmitted orally.

What we do not know

• Vectors? what they are. Found in multiple shrimp species

• Role of vectors and vertical transmission in the disease process

• Relationship to other pathogens such as EMS or EHP possible that this is a critical cofactor?

• How virulent is it in the field and what role does it play in AHPNS mortalities?

Other diseases

• IMNV Indonesia

• YHV Everywhere

• MrNV Indonesia

• WSSV Everywhere

• Bacterial White Tail Disease China?

• What else? Remember the iceberg analogy.

IMNV (Infectious myonecrosis virus)

From a lab transmission study with negative control

from a natural outbreak

IMNV

• Focal to extensive white necrotic areas in muscles.

• Especially in the abdominal segments and tail fan, which can become necrotic and reddened in some individual shrimp.

• These signs may have a sudden onset following stresses (e.g. capture by cast-net, feeding, sudden changes in temperature or salinity).

• Limited in its range -not really clear why may have some cofactors involved in disease process

Shrimp affected by YHV

YHV/GAV

• Six known variants; only one causes classic disease

• One other variant causes GAV and others are benign (present in healthy animals)

• Wide spread although not considered to be a serious problem though the potential does exist for this to change

Macrobrachium rosenbergii Nodavirus (MrNV) and XVC

Called White tail disease or White muscle disease

White muscle disease originally isolated from freshwater shrimp has been found to cause disease in various Penaeus species. Typically affects PLs.

Environmental stressors particularly low salinities and low temperatures weaken the animals and make them susceptible.

Exact range not known at this time.

WSSV

This virus is established in almost every country where shrimp are farmed.

Still a serious problem when animals are stressed.

Role of vibriosis in mortality.

Vibrio mortality aside from EMS

Bacterial white tail disease caused by non-luminescent Vibrio harveyi

A-natural pond infection B-lab infection in B bottom shrimp is control

Vibrio

• Reported in China

• Mortality is temperature dependent and geographical

• Likely will be moved into other shrimp production areas

• Has it already spread?

Biosecurity

• Implement tighter biosecurity in broodstock management larval production

• Biosecurity failures that contributed to spread of EMS also could have resulted in EHD and CMNV infections and who knows what else?

Conclusions

• Widespread failure of common sense biosecurity practices has resulted in many diseases spreading into areas where they would have been kept out of

• Implement proactive disease management strategies– Stress reduction– Environmental improvement– Immune stimulation– Elimination of carriers, vectors, etc.– No shipments of shrimp from SE Asia to the America’s

Conclusions

• Widely ignored biosecurity and over-reliance on SPF broodstock (assuming that SPF equates with no pathogens being present) will continue to ensure that these diseases will spread and that as others find niches in farmed shrimp that they will continue to spread.

• Farmers need to be much more proactive.