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Disease, Prevention
and Treatment
Cultured shrimps suffer from various diseases due to infectious and non-infectious
causes. Infectious diseases are caused by viruses, bacteria, fungi and certain parasites.
Treatment cannot be carried out effectively when shrimp diseases occur in a pond.
The best way to get rid of diseases is by practicing good farm management or prevention.
In this regard, information on various kinds of diseases and their prevention procedures
are useful.
Infectious Diseases
A. Virus Infection
(i) Monodon Baculovirus Disease
(MBV)
Etiological Agent: MBV-type or PmSNPV is a type A occluded monodon
baculovirus.
Clinical Signs: Lethargy, anorexia, poor feeding, dark colouration
and reduced growth rate. Infected shrimps are often associated with fouling of gills
and appendages by ciliates such as Zoothamnium spp. and Vorticella
spp. Acute infection leads to loss of epithelial cells of hepatopancreas.
Treatment: No treatment available for MBV infection
Prevention and Control: There is little information on prevention
and control of the MBV infection in shrimp pond culture. The prevention method for
the MBV infection is possibly through avoidance by screening the PL's before stocking
shrimp in the pond.
(ii) Hepatopancreatic Parvo-like Virus (HPV) Disease
Etiological Agent: HPV is caused by a small parvo-like virus, 22-24
nm in diameter.
Clinical Signs: Reduced feeding, poor growth rate, body surface
and gill fouling with ciliates and occassional opacity of abdominal muscles. Severe
infections may include a whitish and atrophied hepatopancrease, anorexia and reduced
preening activity. Losses may be occur due to the increased occurance of surface
and gill fouling organisms and secondary infections by the opportunistic Vibrio
spp.
Treatment: No treatment available for HPV infection.
Prevention and Control: No information is available on the prevention
and control procedures for HPV infection. However, screening the PLs before stocking
shrimp by routine histology or the Giemsa-impression smear method is recommended.
Juvenile shrimp with Necrotising Hepatopancreatitis
(NHP)
(Source: FAO technical paper)
(iii) Yellow-head Disease (YHD)
Etiological Agent: Yellow-headed virus (YHV) is a ssRNA, rod shaped,
enveloped virus with two rounded ends.
Clinical Signs: The affected shrimp shows a marked reduction in
food consumption. Following this, a few moribund shrimp will appear swimming slowly
near the surface of the pond dike and remain motionless. The animals have pale bodies,
a swollen cephalothorax with a light yellow to yellowish hepatopancreas and gills.
A high mortality rate may reach 100% of affected populations within 3-5 days from
the onset of disease.
Treatment: No treatment is available for YHV infection
Prevention and Control: The reliable method to prevent the occurrence
of YHD is possibly through avoidance, such as careful selection of post larvae,
reduction or elimination of horizontal transmission including carriers, disinfection
of contaminated ponds or equipment with 30 ppm; and chlorine, providing shrimp with
good waterquality and proper nutrition.
Yellow head disease (YHD) seen in three shrimps
on the left
(iv) White Spot Disease (WSD)
Etiological Agent: The disease is caused by the dsDNA virus, Systemic
Ectodermal and Mesodernal Baculovirus (SEMBV).
Clinical Signs: Clinically affected shrimp were first seen to swim
to the water surface and congregate at the pond dikes. Typical clinical signs include
white spots or patches, 1-2 mm in diameter, on the inside of the shell and carapace,
accompanied by reddish discoloration of the body. SEMBV is able to cause acute epizootics
of 5-10 days duration with mortality rate from 40% to 100%.
Diagnosis Procedure: The diagnosis procedure of SEMBV infection
is based on the appearance of the intranuclear hypertrophy in stained histological
sections and the presence of virus particles in the nucleus of the infected cells
observed under the electron microscope. PCR technique is recently used to detect
SEMBV in shrimp larval and other stages, including broodstock and subclinical virus
carriers.
Treatment: No treatment is available for SEMBV infection.
Prevention and Control: Prevention practices through avoidance
are strongly recommended for the farmers, involving the combinations of efficient
pond management, use of proper feed, selection of good quality of PL, reduction
of possible carriers, avoidance of introduction of contaminated water into the pond,
and disinfection of all equipment and utensils.
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White spot disease in shrimp
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White spots on the cephalothorax
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(Source: FAO technical paper)
(v) Infectious Hepatopancreatic and Lymphoid Organ
Necrosis (IHLN)
Etiological Agent: The primary cause of the disease is attributed
to viral etiology.
Clinical Signs: Light pinkish to yellowish discolouration of the
cephalothorax region. Often fouling by ciliate protozoan Zoothamnium seen.
Blackened and necrotic hepatopancreas. Secondary bacterial infection from bacteria
such as Vibrio alginolyticus seen.
Treatment: No treatment is available for IHLN infection.
Prevention and Control: Keep the physico-chemical condition of
pond environment within acceptable levels. To avoid bacterial and viral pathogen
entering from outside, closed culture could be useful in prevention of IHLN disease.
IHLN infected shrimps
Infectious Hypodermal and Haematopoietic Necrosis
(IHHN) disease
(Source: FAO technical paper)
B. Bacterial
Infection
(i) Luminous Vibriosis
Etiological Agent: Vibrio harveyi, Vibrio vulnificus
Clinical Signs: High mortality rate in young juvenile shrimp (one
month syndrome). Moribund shrimp hypoxic often come to the pond surface and edges
of pond. Vertical swimming behavior immediately before onset of acute mortality.
Presence of luminescent shrimp in ponds.
Treatment: Disinfection of intake water with Formalin (100-200
ppm). Administration of Oxolinic acid (0.6 ppm) and Sarafloxacin (5mg/kg) through
feed for 5 days.
Prevention and Control: Proper pond and water management. Utilization
of reservoir for intake water.
(ii) Vibriosis
Etiological Agent: Vibrio vulnificus, V. parahemolyticus, V. alginolyticus,
V. anguillarum, V. damsella, V. fluvialis and V. mimicus.
Clinical Signs: High mortality rates, particularly in young juvenile
shrimp. Moribund shrimp with corkscrew swimming behavior appear at edge of pond.
Reddish discoloration of juvenile shrimp.
External Fouling: Black spots, chronic soft shelling
Treatment: Disinfection of intake water i.e. formalin 100-200 ppm.
Anti-microbial preparation application through feeds (Oxolinic acid 0.6 ppm and
Sarafloxacin 5 mg/kg).
Prevention and Control: Proper pond and water management and utilization
of reservoir for intake water.
Shrimp affected with Systemic Vibriosis
disease
C. Fungal Infestation
(i) Larval Mycosis
Etiological Agent: Filamentous fungi of genus Lagenidium
spp. and other filamentous fungi, such as Sirolpidium spp. and Haliphthoros
spp.
Clinical Signs: Eggs and larve are weak and appear whitish. Moratlities
may reach 100% within two days. Fungal mycelium replaces the larval tissues and
ramifies into all parts of the body and protrudes out of the body and develops into
sporangia.
Prevention and Control: General hatchery management practices such
as use of UV sterilised and filtered seawater, adequate water exchange etc., must
be strictly followed. Rearing water, equipment used in the hatchery and all hatchery
facilities must be thoroughly disinfected before retarting the hatchery operations.
D. Protozoan and Parasitic
Infestation
(i) Black Gill Disease
Etiological Agent: Fusarium spp
Clinical Signs: Brownish to blackish discoloration on the gills
of juvenile shrimp.
Treatment: No treatment is available for fungal infestation without
harming the shrimp.
Prevention and Control: No information on prevention and control.
However, good management of the pond bottom and prevention of the entry of wild
crustaceans into the pond, which may carry pathogen, can be effective control practices.
Black gill disease
(ii) Surface Fouling Diseases
Etiological Agent: Many species of bacteria, algae and protozoa
such as filamentous bacteria, Leucouthrix sp., Flavobacterium
sp. and Zoothamnium sp.
Clinical Signs: Infected shrimps show black/ brown gills or appendage
discoloration or fuzzy/cottony appearance due to a heavy colony of the organisms.
In some cases, the severely affected shrimp die during the molting period.
Treatment: Chlorine and formalin are often used to treat those
commensal organisms if shrimp display heavy infection. Changing water is the most
preferable management, which stimulates molting of the shrimp in order to reduce
the infestation.
Prevention and Control: Prevention and control of the occurrence
of surface fouling are usually done through maintenance of good sanitary conditions
at the pond bottom and the overall pond area. Organic matters and suspended solids
in the pond should be reduced to prevent the attachment of those fouling organisms.
This is achieved by changing the water or applying lime.
(iii) Microsporidosis (Cotton shrimp disease or
Milk shrimp disease)
Etiological Agent: Microsporidia such as Thelohania spp.,
Nosema spp., and Pleistophora spp.
Clinical Signs: Infected shrimps appear opaque and cooked. Gradual
and low levels of mortalities are observed. Microsporidia invade and replace gill,
muscle, heart, gonads and hepatopancreas, and cause necrosis in these regions.
Prevention and Control: Maintain of good sanitary conditions at
the pond bottom and the overall pond area.
Cotton shrimp disease
(Source: CIBA Extension Series)
Non-Infectious Diseases
(i) Soft-shell syndrome
Etiological Agent: The exact cause of soft-shell syndrome is not
known. However, low saline condition in the culture pond and deterioration of pond
bottom condition are some physico-chemical factors that causes this disease. Shrimps
fed with low protein diet, contamination through agricultural run-off, high soil
pH, low water phosphate and low organic matter in soil all have an impact on soft-shell
disease.
Clinical Signs: Shrimps are weak, usually off-feed, have a loose
thin exoskeleton. Rostrum is stiff as healthy shrimps. Wavy undulating intestine
is clearly visible.
Prevention and Control: Low stocking density, feeding with high
quality feed and frequent water exchange are likely to reduce the recurrence of
the disease.
Shrimps with persistent soft shell disease
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