ihhnv and wssv, two viral pathogens of interest in thai...
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IHHNV and WSSV, two viral pathogens of interest in Thai aquaculture
Diva January Aldama Cano, PhDPostdoctoral researcher, Centex Shrimp
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Talk overview• General introduction
• Infectious hypodermal and hematopoietic necrosis virus (IHHNV)• Background
• Current situation
• White-spot syndrome virus (WSSV)• Background
• Detection and preventive measures
• Guidelines for WSSV control based on our research findings
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Viral diseases in Thai shrimp farming
• Major source of economic loss
• YHV (1995)• 40 million USD (lost in export
revenue)
• WSSV (1996-1997)• 1 billion USD (estimated)
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Flegel, 2019
• E H P• W F S
Thai shrimp production over the years
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Infectious hypodermal and hematopoietic necrosis virus (IHHNV)
• First discovered in blue shrimp P. stylirostrisand white shrimp P. vannamei in the Americas in the early 1980's
• Believed to have been introduced by live experimental stock of P. monodon from Asia
• Densovirus with non-enveloped icosahedral virions averaging 22–23 nm in diameter and containing linear ssDNA of 4.1 kb
Picture: Bonami et al.,1990
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• IHHNV has been reported in several species
• Known to cause mortalities in P. stylirostris juveniles and sub-adults
• In P. vannamei• No mortality
• Reduced growth
• Cuticular deformity known as “runt-deformity syndrome” (RDS)
• Can be passed by vertical and horizontal transmission
• P. monodon• Seems to be unnafected
• P. merguiensis and P. indicus• Refractory to infection (resistant)
(Brock and Main, 1994; Lightner, 1993, 1996; Flegel, 2009)
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Actual status of IHHNV in Asia• This disease is of little concern to the shrimp industry
• No significant loss related to IHHNV episodes in Thailand or in Asia in general in the past 10 years or so after the development and use of SPF shrimp
• Some shrimp carry IHHNV viral fragments inserted into theirgenome (P. stylirostris, P. monodon and P. vannamei)
• Possibility of false positives by following OIE detection methods
Flegel, 2009; Cowley et al ,2018; Tang et al,2006
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OIE- Diagnostic manual for aquaticanimal disease
Is it appropriate to still keepIHHNV as an OIE listed
disease?
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• IHHNV has no longer any significant negative economic impact on aquaculture after the introduction and development of SPF resistant shrimp lines.
• Should it be removed from the OIE list of reportable diseases for the same reason as other disease that are no longer a threat?
As long as they remain on the list they can be used as unjustified barriers to international trade in shrimp and shrimp products
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• Trading difficulties and bannedexports due to presence of disease
• Problems are not based on the detection of the disease itself buta problem with trading, politicaland economical underlines
• No risk of disease transmission to shrimp farms from shrimp thathave been prepared for human consumption
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White spot syndrome virus (WSSV)
•#1 viral treat and major source of concern forshrimp industry worldwide
• It has been extensely reviewed
•Wide host range (>100 species)
•Major threat for P. monodon and P. vannamei
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Pathology• Lead to mortality rate of 100% within days
• Clinical: reduction in food consumption, lethargy, loose cuticle and reddish discolouration, white spots on the inside surface
• The most convenient tissue for histological diagnosis is the subcuticular epithelium
Distinctive white
inclusions INthe cuticle;cannot be
scraped off with
thumbnail
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• infects a wide variety of cells from ectodermal and mesodermal origin
•Gill epithelium, nervous tissue, connective tissueand intestinal epithelial tissue
•Pathogenesis involves widespread tissue necrosis and disintegration
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Transmission• Principle transmission mode is vertical from grossly normal broodstock
carriers
• Most viral particles in supporting tissues of ovary and not in eggs
• Washing of eggs and nauplii: effective in reducing transmission probability
• Screening of broodstock recommended so infected carriers can be eliminated
Broodstock → Nauplii → Post-larvae
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Bird droppings
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Roof netting
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Example of biosecure farm in the south of Thailand
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WSSV detection• The OIE recommended method for diagnosis of WSD is by the use of a
nested PCR method
• A Recent modification has been proposed to OIE recommended method which is a single step method and has equal sensitivity and specificity
WSSVShrimp DNA
Lo et al., 1996
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IQ2000™ WSSV
• Yun-Long Tsai , Yu-Chan Lin , Pin-HsingChou , Ping-Hua Teng , Pei-Yu Lee.Detection of white spot syndrome virus by polymerase chain reaction performed under insulated isothermal conditions
• Journal of Virological Methods Volume 181, Issue 1 2012 134 - 137
• http://dx.doi.org/10.1016/j.jviromet.2012.01.017
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WSSV therapeutic measures
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Strategies are based onthe viral bindingproteins (VBP) fromshrimp that can be used to study host-pathogen interactionsin vivo
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Vaccinations
Delivery of recombinantproteins as a vaccine that can protect against the virus in P. monodon
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Control guidelines for WSSV
•Based on preventive measures• Adequate monitoring of broodstock and PL • Use of domesticated genetically improved SPF
stock• Elimination of potential carriers in the farm• Appropriate treatment of the pond, equipment
and pond water before cultivation
•Biosecurity is still a major problems when itcomes to WSSV outbreaks and control
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• Most of therapeutic measures are successful at laboratory level but hard to implement in a farming setting due to the high cost
• Practical measures• A lot of farmers have opted to avoid shrimp culture during Thai winter time
(oct-feb)
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Acknowledgements
Dr. Kallaya Sritunyalucksana
Dr. Ornchuma Itsathitphaisarn
Prof. Dr. Tim W. Flegel
Centex Shrimp, Mahidol University
Aquatic Animal Health Research Team, BIOTEC
Aqua Expo 2019