Marine Viruses 4.7 Marine Viruses and Shrimp Aquaculture 45
Part A | 4.7
often otherwise undetectable amounts of DNA can be
amplified by PCR to produce detectable quantities of
the target DNA. This is accomplished using specific
oligonucleotide primers designed for the target DNA
sequence [4.68]. Additional PCR-based methods are
being applied for the development of efficient diagnostic methodologies. Multiple viral diseases can be
simultaneously detected in a single reaction using more
than one set of primers. Real-time PCR is especially attractive as it allows real-time analysis, high specificity,
quantitative results, and the detection of low copy numbers (even as low as a single copy of a viral genome).
4.7.6 Factors Responsible
for Shrimp Viral Diseases
Poor management in the shrimp farming industry leads
to severe pollution in shrimp culture ponds, thereby
creating a suitable environment for the development
of viral diseases. The emergence of disease in shrimp
aquaculture has been attributed to three major factors:
1. High density of cultured shrimp in aquaculture
ponds
2. Importation or transportation of brood stock of
shrimp from one place to another for culture
3. The introduction of wild brood stock into culture
ponds.
The introduction of wild brooders carrying
pathogens and inadequate information about disease
symptoms have been responsible for the emergence
of new diseases in the farming system. For example,
HPV originated in the Indo-Pacific region but was
later spread to America via importation of live Asian
shrimp for aquaculture [4.69]. Similarly, TSV was
found in Ecuador and spread to America and Southeast
Asia [4.70, 71].
Environmental factors and hosts also play important roles in the emergence of shrimp diseases. Poor
sanitation conditions, such as high salinity, pH, and
nitrogen levels, in culture ponds can cause stress in
shrimp, leading to an increased susceptibility to disease.
For example, some viruses such as WSSV are highly
lethal stress-related viruses. The temperature also plays
an important role in virus infection; e.g., WSSV enters
the target cell and replicates at 22
ı C and thus leads to
100% mortality within 3 days. However, at a reduced
temperature of 1620
ı C, 2035 days were required to
reach 100% mortality in experimentally infected crayfish [4.72]. However, high water temperatures from
2733
ı C inhibit the effects of WSSV in shrimp (L.
vannamei) at the acute infection stage. However, at
the chronic stage, increases in water temperature result
in rapid disease progression and mortality in WSSVinfected shrimp [4.73].
4.7.7 Control of Shrimp Viral Disease
The most effective means of control or prevention of
viral disease is to destroy the infected animals, decontaminate the ponds, and start again with virus-free
brood stocks. Viruses, bacteria, protozoa, and fungi
have emerged as major causes of disease in farmed
shrimp. Bacterial, fungal, and protozoan diseases are
manageable by improving culture practices, routine
sanitation, and using probiotics and chemotherapeutic
agents. However, management of viral diseases is problematic and has been responsible for the most costly
epizootic outbreaks reported to date.
Probiotics
Live bacterial cells, referred to as probiotics, have been
applied in aquaculture farms to improve water quality
or prevent disease. The potential benefits of probiotics
in aquaculture ponds include enhanced decomposition
of organic matter, reduction of nitrogen and phosphorus concentrations, improved control of algal growth,
greater availability of dissolved oxygen, less cyanobacteria (blue-green algae), control of ammonia, nitrite,
and hydrogen sulfide, lower incidence of disease and
greater survival, and improved levels of shrimp and fish
production [4.74]. Few detailed studies on disease control using probiotics have been performed [4.75, 76].
However, properly controlled field tests on probiotics
revealed no significant effect on measured water quality
parameters [4.74]. The use of probiotics in commercial shrimp farming would be beneficial for control of
disease only if positive evidence of efficacy with cost
benefit analysis is acquired.
Shrimp–Virus Interactions
Shrimp–virus interactions at the molecular and genetic
levels are interesting phenomena that may be useful in
disease control. Many new shrimp genes have been discovered, some of which may lead to new products for
disease control. A study performed in Japan indicated
that Kuruma shrimp (P. japonicus) survived in a pond
after WSSV injection; it was not protected from infection, but showed resistance to disease. This mechanism
was designated as a quasi-immune response (immunelike system). Recently, a factor was found in shrimp
hemolymph that could prevent shrimp from dying upon
Part A | 4.7
often otherwise undetectable amounts of DNA can be
amplified by PCR to produce detectable quantities of
the target DNA. This is accomplished using specific
oligonucleotide primers designed for the target DNA
sequence [4.68]. Additional PCR-based methods are
being applied for the development of efficient diagnostic methodologies. Multiple viral diseases can be
simultaneously detected in a single reaction using more
than one set of primers. Real-time PCR is especially attractive as it allows real-time analysis, high specificity,
quantitative results, and the detection of low copy numbers (even as low as a single copy of a viral genome).
4.7.6 Factors Responsible
for Shrimp Viral Diseases
Poor management in the shrimp farming industry leads
to severe pollution in shrimp culture ponds, thereby
creating a suitable environment for the development
of viral diseases. The emergence of disease in shrimp
aquaculture has been attributed to three major factors:
1. High density of cultured shrimp in aquaculture
ponds
2. Importation or transportation of brood stock of
shrimp from one place to another for culture
3. The introduction of wild brood stock into culture
ponds.
The introduction of wild brooders carrying
pathogens and inadequate information about disease
symptoms have been responsible for the emergence
of new diseases in the farming system. For example,
HPV originated in the Indo-Pacific region but was
later spread to America via importation of live Asian
shrimp for aquaculture [4.69]. Similarly, TSV was
found in Ecuador and spread to America and Southeast
Asia [4.70, 71].
Environmental factors and hosts also play important roles in the emergence of shrimp diseases. Poor
sanitation conditions, such as high salinity, pH, and
nitrogen levels, in culture ponds can cause stress in
shrimp, leading to an increased susceptibility to disease.
For example, some viruses such as WSSV are highly
lethal stress-related viruses. The temperature also plays
an important role in virus infection; e.g., WSSV enters
the target cell and replicates at 22
ı C and thus leads to
100% mortality within 3 days. However, at a reduced
temperature of 1620
ı C, 2035 days were required to
reach 100% mortality in experimentally infected crayfish [4.72]. However, high water temperatures from
2733
ı C inhibit the effects of WSSV in shrimp (L.
vannamei) at the acute infection stage. However, at
the chronic stage, increases in water temperature result
in rapid disease progression and mortality in WSSVinfected shrimp [4.73].
4.7.7 Control of Shrimp Viral Disease
The most effective means of control or prevention of
viral disease is to destroy the infected animals, decontaminate the ponds, and start again with virus-free
brood stocks. Viruses, bacteria, protozoa, and fungi
have emerged as major causes of disease in farmed
shrimp. Bacterial, fungal, and protozoan diseases are
manageable by improving culture practices, routine
sanitation, and using probiotics and chemotherapeutic
agents. However, management of viral diseases is problematic and has been responsible for the most costly
epizootic outbreaks reported to date.
Probiotics
Live bacterial cells, referred to as probiotics, have been
applied in aquaculture farms to improve water quality
or prevent disease. The potential benefits of probiotics
in aquaculture ponds include enhanced decomposition
of organic matter, reduction of nitrogen and phosphorus concentrations, improved control of algal growth,
greater availability of dissolved oxygen, less cyanobacteria (blue-green algae), control of ammonia, nitrite,
and hydrogen sulfide, lower incidence of disease and
greater survival, and improved levels of shrimp and fish
production [4.74]. Few detailed studies on disease control using probiotics have been performed [4.75, 76].
However, properly controlled field tests on probiotics
revealed no significant effect on measured water quality
parameters [4.74]. The use of probiotics in commercial shrimp farming would be beneficial for control of
disease only if positive evidence of efficacy with cost
benefit analysis is acquired.
Shrimp–Virus Interactions
Shrimp–virus interactions at the molecular and genetic
levels are interesting phenomena that may be useful in
disease control. Many new shrimp genes have been discovered, some of which may lead to new products for
disease control. A study performed in Japan indicated
that Kuruma shrimp (P. japonicus) survived in a pond
after WSSV injection; it was not protected from infection, but showed resistance to disease. This mechanism
was designated as a quasi-immune response (immunelike system). Recently, a factor was found in shrimp
hemolymph that could prevent shrimp from dying upon