Part A | 4.8
46 Part A Marine Flora and Fauna
injection of WSSV [4.77]. Another phenomenon has
been reported that describes protection against WSD
(white spot disease) by persistent IHHNV infection in
P. stylirostris. However, infection was not prevented,
although disease severity was reduced [4.78]. We still
know very little about the interaction of shrimp with
viral pathogens, and a better understanding of shrimp–
virus interactions may lead to the development of better
methods for viral disease control.
Shrimp Viral Vaccines
The term vaccine is applicable to vertebrates because
the vertebrate process involves antibodies. However,
antibodies do not occur in shrimp. In addition, vaccinated shrimp generally become infected but do not
develop disease as a result. Therefore, the term tolerines
has been recommended to describe agents that could be
used in the same way as vaccines in shrimp [4.79].
Two types of tolerine have been studied in shrimp;
the first type was developed in Thailand in the mid1990s and is still commercially available under the
brand name SEMBVAC, while the second consists of
inactivated whole particles of WSSV. Shrimp acquires
some degree of tolerance to WSSV and suffer less
from the disease after infection due to ingestion of
these products [4.80]. Other types of tolerance have
also been reported consisting of individual or mixed
protein subunits of viral particles that are administered either by injection or by mixing with shrimp
feed [4.81, 82].
RNA Interference (RNAi)
RNA interference (RNAi) is a gene silencing technology and the process by which a gene is post transcriptionally suppressed using dsRNA with sequences
that match those of viral genes to destroy their homologous mRNA in a sequence-specific manner [4.83].
This technology was recently used in the laboratory
to protect shrimp from viral diseases [4.84, 85]. RNAi
was used to suppress the replication of YHV and
PmDNV [4.83, 86]. However, application of this technology has disadvantages associated with issues of cost,
safety, and public acceptance of genetic engineering
techniques.
4.8 Conclusion
Viruses, one of the most rapidly evolving genetic agents
among all biological entities are involved noncyclic
changes in their genetic characteristics. Furthermore,
the abundance of both DNA and RNA viruses in the
marine environment, viral diversity, and the interaction
between their hosts are ecologically important. A disease caused by marine viruses can cause a huge impact
on aquaculture practice and aquatic organisms. In some
circumstances, the environment pressure or high density of cultured organisms also allows for stress to the
organisms, which facilitate virus infections. Moreover,
it is challenging to control virus infection in wild marine organisms and viral host shift events in the ocean.
Molecular biology-based methods are rapidly advancing the study of virus genomes and their molecular
mechanisms. These advanced studies enable the development of rapid diagnostic tools, which could be
useful for identifying viral pathogens in marine organisms and marine water samples. The advantages of
molecular diagnostic methods are that they are sensitive and beneficial to preventing the spread of viral
diseases to other organisms, due to the diagnosis at an
early stage of infections in farmed aquaculture systems.
Furthermore, therapeutic studies such as virus–host interactions, phage therapy, and RNAi are tools to control
viral diseases. Although it is possible to control viral
diseases in farmed aquaculture systems by using modern technology, it is still a big question as to how to
control viral disease in the ocean. In addition, we must
consider the cost of applying new technologies. Despite the fact that the diagnosis and control of known
viral pathogens are quite possible, it is an unresolved
problem to diagnose an unidentified or unknown virus
population in the marine ecosystem. Even though there
are a few drawbacks of viral diagnosis or control of
virus impacts in aquaculture organisms, it is necessary
to characterize the viruses associated with marine organisms for further studies in the future. Moreover,
only a tiny fraction of viruses has been discovered and
the potential discovery still seems as vast as the ocean
itself. Therefore, further research is still needed to analyze the presence of viruses in the marine environment
and the new potential hosts of marine viruses yet to be
discovered.
46 Part A Marine Flora and Fauna
injection of WSSV [4.77]. Another phenomenon has
been reported that describes protection against WSD
(white spot disease) by persistent IHHNV infection in
P. stylirostris. However, infection was not prevented,
although disease severity was reduced [4.78]. We still
know very little about the interaction of shrimp with
viral pathogens, and a better understanding of shrimp–
virus interactions may lead to the development of better
methods for viral disease control.
Shrimp Viral Vaccines
The term vaccine is applicable to vertebrates because
the vertebrate process involves antibodies. However,
antibodies do not occur in shrimp. In addition, vaccinated shrimp generally become infected but do not
develop disease as a result. Therefore, the term tolerines
has been recommended to describe agents that could be
used in the same way as vaccines in shrimp [4.79].
Two types of tolerine have been studied in shrimp;
the first type was developed in Thailand in the mid1990s and is still commercially available under the
brand name SEMBVAC, while the second consists of
inactivated whole particles of WSSV. Shrimp acquires
some degree of tolerance to WSSV and suffer less
from the disease after infection due to ingestion of
these products [4.80]. Other types of tolerance have
also been reported consisting of individual or mixed
protein subunits of viral particles that are administered either by injection or by mixing with shrimp
feed [4.81, 82].
RNA Interference (RNAi)
RNA interference (RNAi) is a gene silencing technology and the process by which a gene is post transcriptionally suppressed using dsRNA with sequences
that match those of viral genes to destroy their homologous mRNA in a sequence-specific manner [4.83].
This technology was recently used in the laboratory
to protect shrimp from viral diseases [4.84, 85]. RNAi
was used to suppress the replication of YHV and
PmDNV [4.83, 86]. However, application of this technology has disadvantages associated with issues of cost,
safety, and public acceptance of genetic engineering
techniques.
4.8 Conclusion
Viruses, one of the most rapidly evolving genetic agents
among all biological entities are involved noncyclic
changes in their genetic characteristics. Furthermore,
the abundance of both DNA and RNA viruses in the
marine environment, viral diversity, and the interaction
between their hosts are ecologically important. A disease caused by marine viruses can cause a huge impact
on aquaculture practice and aquatic organisms. In some
circumstances, the environment pressure or high density of cultured organisms also allows for stress to the
organisms, which facilitate virus infections. Moreover,
it is challenging to control virus infection in wild marine organisms and viral host shift events in the ocean.
Molecular biology-based methods are rapidly advancing the study of virus genomes and their molecular
mechanisms. These advanced studies enable the development of rapid diagnostic tools, which could be
useful for identifying viral pathogens in marine organisms and marine water samples. The advantages of
molecular diagnostic methods are that they are sensitive and beneficial to preventing the spread of viral
diseases to other organisms, due to the diagnosis at an
early stage of infections in farmed aquaculture systems.
Furthermore, therapeutic studies such as virus–host interactions, phage therapy, and RNAi are tools to control
viral diseases. Although it is possible to control viral
diseases in farmed aquaculture systems by using modern technology, it is still a big question as to how to
control viral disease in the ocean. In addition, we must
consider the cost of applying new technologies. Despite the fact that the diagnosis and control of known
viral pathogens are quite possible, it is an unresolved
problem to diagnose an unidentified or unknown virus
population in the marine ecosystem. Even though there
are a few drawbacks of viral diagnosis or control of
virus impacts in aquaculture organisms, it is necessary
to characterize the viruses associated with marine organisms for further studies in the future. Moreover,
only a tiny fraction of viruses has been discovered and
the potential discovery still seems as vast as the ocean
itself. Therefore, further research is still needed to analyze the presence of viruses in the marine environment
and the new potential hosts of marine viruses yet to be
discovered.