IHHNV and WSSV, two viral pathogens of interest in Thai aquaculture
Diva January Aldama Cano, PhDPostdoctoral researcher, Centex Shrimp
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
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)
Flegel, 2019
• E H P• W F S
Thai shrimp production over the years
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
• 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)
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
OIE- Diagnostic manual for aquaticanimal disease
Is it appropriate to still keepIHHNV as an OIE listed
disease?
• 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
• 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
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
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
• 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
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
Bird droppings
Roof netting
Example of biosecure farm in the south of Thailand
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
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
WSSV therapeutic measures
Strategies are based onthe viral bindingproteins (VBP) fromshrimp that can be used to study host-pathogen interactionsin vivo
Vaccinations
Delivery of recombinantproteins as a vaccine that can protect against the virus in P. monodon
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
• 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)
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
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