Transcript Marine Shrimp Conference

Center of Excellence for Shrimp Molecular Biology and Biotechnology
Problematic transfer of viruses
amongst penaeid shrimp
Tim Flegel
Centex Shrimp, Chalermprakiat Building, Fac. Science
Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
National Center for Genetic Engineering and Biotechnology
Overall viewpoint
 Live aquatic animals present far the highest
risk for viral disease transfer
 All well studied cases of shrimp viral disease
transfer have occurred by this route
 Most cases have involved shrimp for
aquaculture
 No 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
Cause of import
No. of %
records
Known and intentional
Aquaculture
Fisheries
Ornamental
Research
Oher reasons
Total intentional
1386 39
299 8
263 7
104 3
272 8
2324 65
Known but unintentional
Angling/sport
Diffused from other countries
Accidental
Bait
Total unintentional
Unknown
283 8
139 4
267 8
14 >1
703 20
552 15
Source: FAO website
Importations by group
Species group
Fishes
Molluscs
Crustaceans
Algae and plants
Other invertebrates
Other vertebrates
Source: FAO website
# records %
2574 82
294 9
191 6
35 1
29 1
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 infections
 Dual, triple and multiple viral infections are
often seen in shrimp, but rarely reported
 We rarely examine “healthy” shrimp
 This applies also to other crustaceans and
arthropods such as insects
 Viruses 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
Infection status # Shrimp % Total
14
5.9
Uninfected
223
94.1
Infected
237
100.0
Total tested
50
21.1
Single infections
MBV
8
3.4
HPV
13
5.5
WSSV
19
8.0
IHHNV
10
4.2
69
29.1
Dual infections
80
33.8
Triple infections
24
10.1
Quadruple
173
73.0
Dual to quadruple
Grossly healthy
shrimp
A study of 6 ponds in
Thailand
(Flegel et al. 2001)
Triple viral infection
Example of dangerous exchanges
P. monodon
YHV
WSSV
IHHNV
MOV?
LOVV?
TSV?
BP?
Reo-like
virus?
P. vannamei
LOVV-like virus in P. monodon
 Dr. 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 shrimp
 Result 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 needed
 For a non-destructive detection method to
screen broodstock and PL
 To identify possible carriers
 We 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 transfer
SOURCE
ASSESSED RECOMMENDATION
RISK
Live stocks
Very high
No import from
infected areas
Green shrimp
High
No import from
for bait
infected areas
Green shrimp
Low
Proper disposal of
for processing
waste and water
Green shrimp
Very low
Free trade
for human food (acceptable)
Cooked shrimp
No risk
Free trade
Shrimp head
meal
Ballast water
No risk
Free trade
?
Ballast water
exchange
Important lessons
 Crustaceans can carry unknown viral
pathogens as innocuous, active infections
 Many carriers (including PL) are grossly
normal, so PCR needed for detection
 These 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