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