Problematic transfer of viruses amongst penaeid shrimp
Center of Excellence for Shrimp Molecular Biology and Biotechnology
National Center for Genetic Engineering and Biotechnology
Tim FlegelCentex Shrimp, Chalermprakiat Building, Fac. Science
Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
Overall viewpoint Live aquatic animals present far the highest
risk for viral disease transferAll well studied cases of shrimp viral disease
transfer have occurred by this routeMost cases have involved shrimp for
aquacultureNo proven case of transfer by importation
and use of fresh frozen shrimp for human consumption
Disease imported with crustaceans in ballast and on hulls has not been properly assessed
Intentional import of live shrimp
All major cases of shrimp viral transfer have occurred by importation of brooders and PL
This includes importation of exotic species and resident species from other countries
Three well documented cases:– Infectious hypodermal and hematopoeitic
necrosis virus (IHHNV) from Asia to the Americas– White spot syndrome virus (WSSV) initially from
China to Japan but then to Asia and the Americas
– Taura syndrome virus (TSV) initially around the Americas but then to Taiwan and Asia
Most exotic introductions intentional
Source: FAO website
Importations by group
Source: FAO website
Species group # records % Fishes 2574 82 Molluscs 294 9 Crustaceans 191 6 Algae and plants 35 1 Other invertebrates 29 1 Other vertebrates 18 1
Shrimp viruses a particular danger
Viruses alone are responsible for the most severe losses in shrimp aquaculture
We know that shrimp react to viruses in a way different from vertebrates (from fish to man)
Shrimp and other crustaceans are characterized by persistent viral infections
These infections often produce no gross signs of disease and no mortality
Many of these “hidden” or “cryptic” viruses are still unknown
Dual and multiple viral infectionsDual, triple and multiple viral infections
are often seen in shrimp, but rarely reported
We rarely examine “healthy” shrimpThis applies also to other crustaceans and
arthropods such as insectsViruses with the same name may be quite
different genetically (e.g., HPV variants) A cryptic virus tolerated in one host or
location may not be tolerated in another host or location
Crustaceans a particular threat
Grossly Grossly healthy healthy shrimpshrimpA study of 6 ponds in Thailand
(Flegel et al. 2001)
Triple viral infection
Example of dangerous exchanges
YHVWSSVIHHNVMOV?
LOVV?TSV?BP?Reo-like
virus?
P. monodon P. vannamei
LOVV-like virus in P. monodonDr. Lightner found this in LO of P. monodon
brooders from the Andaman Sea in late 2002
Virions 25 nm
Test with P. monodon
Aj. Boonsirm took slow growing P. monodon
Homogenized LO and filtered out bacteria Injected into SPF P. monodon and cultured
with un-injected control shrimpResult after 2 months
– Un-injected shrimp………. 8 g– LO injected shrimp………. 4 g
LO samples sent for microscopy
Light microscopy of LO
Electron microscopy of LO
Virions 25 nm
Probe urgently neededFor a non-destructive detection method to
screen broodstock and PLTo identify possible carriersWe now have a tentative probe that gives
positive results with test shrimp and negative with control shrimp
Its sensitivity, specificity and applicability are being tested
Risks of viral transferSOURCE ASSESSED
RISK RECOMMENDATION
Live stocks Very high No import from infected areas
Green shrimp for bait
High No import from infected areas
Green shrimp for processing
Low Proper disposal of waste and water
Green shrimp for human food
Very low (acceptable)
Free trade
Cooked shrimp No risk Free trade
Shrimp head meal
No risk Free trade
Ballast water ? Ballast water exchange
Important lessonsCrustaceans can carry unknown viral
pathogens as innocuous, active infectionsMany carriers (including PL) are grossly
normal, so PCR needed for detectionThese viruses may be deadly to other
species or the same species at distant locations
Greater geographical separation = greater danger
By far, the most common route of transfer is with live broodstock or PL for aquaculture
No proven transfer from processing plants or from shrimp for human consumption
Conclusions Careless movement of live stocks has been
the cause of most shrimp disease transfers Live introductions from ballast water and hull
fouling may also be a source of disease The evidence in hand suggests that frozen
shrimp for human consumption does not pose a significant transfer risk
Any risks from frozen shrimp can be decreased substantially by simple risk reduction measures
PCR is recommended for screening live shrimp but is inappropriate for frozen shrimp