Download Viral Diseases of Farmed Shrimp—Present Status and Future

Viral Diseases of Farmed Shrimp—Present Status and
Future Research
Siddhi Booyaratpalin*
Types of Viruses
SEMBV, or white spot virus, is more common in
15–90 day old postlarvae. Affected shrimp often have
a red body and haematoxylin and eosin (H&E) staining
usually shows swollen nuclei. Viruses that appear to be
very similar to, and possibly the same as SEMBV, are
present in China, Japan, India, Malaysia and Vietnam.
This virus also can infect many other species of crustaceans.
The viruses that infect shrimp can be subdivided into
two groups: those that infect the ectoderm and mesoderm; and those that infect the endoderm and hepatopancreas. The viruses of the ectoderm and mesoderm
include:
• IHHNV (infectious hypodermal and hematopoietic
necrosis virus) which has caused severe problems
in the United States of America;
• YHV (yellow head virus) which has caused high
mortalities in Thailand since 1992;
• SEMBV (systemic ectodermal and mesodermal
baculovirus), also known as WSBV (white spot
baculovirus), which is the most serious pathogen in
Asia since 1994; and
• TSV (Taura syndrome virus) which has caused
high mortalities in Central America (Ecuador).
The viruses of the endoderm and hepatopancreas
include:
• BP (Baculovirus penaei type);
• BMNV (baculoviral midgut gland necrosis type
virus);
• HPV (hepatopancreatic parvovirus);
• MBV (Penaeus monodon-type baculovirus); and
• TCBV (type C baculovirus of P. monodon).
In Thailand, YHV and SEMBV cause the most serious problems, while some viruses, such as HPV, cause
no significant losses to farmers. Traditional techniques
for detecting viruses in diseased animals have been by
light microscopy (i.e. stained, squashed cells of gills
etc.) or by transmission electron microscopy (TEM).
Some viruses are associated with occlusion bodies.
Research Strategies
Strategies for carrying out research into these viruses
is based on five approaches: diagnosis; transmission;
carriers and reservoirs; prevention and control; and
treatment.
Diagnosis
As mentioned above, light microscopy, TEM, histopathology and bioassays are the main tools for diagnosis. However, genetic methods (i.e. DNA probes and
PCR—polymerase chain reaction) are recent innovations that have been applied to some viruses. These
appear to be successful approaches for SEMBV but the
methods are in the developmental stages for YHV.
Transmission
Horizontal transmission occurs with YHV and
SEMBV, but the evidence for vertical transmission is
not conclusive.
Carriers and reservoirs
The carriers and reservoirs are mostly crustaceans,
but there is uncertainty regarding shellfish, insects and
plankton.
* National Institute of Coastal Aquaculture (NICA), Songkhla, Thailand.
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Treatment
Unfortunately there is no treatment that has been
found which is effective at curing infected animals.
This is an area that contrasts with other forms of agriculture and animal husbandry.
The main areas which should be priorities for
research are: improved tools for diagnosis; determining possible modes of vertical transmission; development of vaccines and immunostimulants; investigating
forms of viral treatments; and developing viable methods for domesticating broodstock.
Prevention and control
The protocol for minimising and reducing the occurrence of diseases at farms are as follows:
• select healthy postlarvae;
• use a low stocking density;
• ensure the farm is located where it can obtain good
quality water;
• treat the water for 3–4 days before it is used;
• prevent entry of carriers and eliminate carriers;
• disinfect water before it is discharged; and
• use a closed, semi-closed or recycle system.
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