|
Disease Management
Health, Predation and Disease
Continuous exchange of a small proportion of the water is the normal way of maintaining
good water quality. However, some farmers change water more suddenly every two weeks,
and in much larger proportions, because this tends to make the prawns moult. The
more that moult (and are therefore soft-shelled) at the same time, the less potential
losses there may be due to cannibalism. Low dissolved oxygen should be suspected
if prawns begin to crawl out of the ponds or congregate at the edges of the pond
in daylight. If this problem occurs, flush the pond. Very high pH levels in freshwater
prawn ponds can cause prawn mortalities, both because of the direct effect of the
pH itself and because of the greater solubility of waste ammonia at high pH. High
pH is often caused by dense phytoplankton blooms.
Major problems that may arise during culture are mortality of the stock due to low
dissolved oxygen in the pond water. Heavy plankton bloom, very low water level and
lack of water exchange leads to low dissolved oxygen levels. Continuous rainy/cloudy
days precipitate this problem. Immediate water exchange or aeration of ponds during
night hours prevents this problem. Development of bottom algae due to high transparency
of water is another problem during monoculture of prawns. To avoid this problem
always maintain transparency in 30-40 cm range by frequent fertilization. Predation
is one of the greatest problems for any aquaculture enterprise, including freshwater
prawn farming. Predation is caused mainly by other aquatic species, birds, snakes
and humans. Normally, insects (mainly dragonfly nymphs), carnivorous fish and birds
are the most serious predators in freshwater prawn farming.
Major diseases known to
affect freshwater prawns, and their symptoms
|
Virus Diseases
|
Bacterial And Rickettsial Diseases
|
Fungal Diseases
|
|
Macrobrachium hepatopancreatic parvo-like virus (MHPV):
None , Not associated with significant morbidity or mortality.
|
Black spot (sometimes called brown spot or shell disease): One
or many melanized lesions on the cuticle; often caused by opportunistic bacteria
which enter following physical damage; problem may disappear at the following moult
but sometimes develops into deep spreading lesions; reduces marketable value of
harvested prawns.
|
Lagenidium infection: Affects larvae: an extensive mycelial network
can be seen through the exoskeleton; can decimate hatchery populations within 24
hours.
|
|
Macrobrachium muscle virus (MMV): Muscle tissues
become opaque, followed by necrosis; occurs within 10 days of stocking PL and may
cause upto 50% mortality.
|
Appendage necrosis: Larval appendages become necrotic and melanized;
affected larvae do not eat and may become bluish in colour; may be associated with
a heavy surface burden of the filamentous bacterium Leucothrix.
|
Infections by Fusarium and Saprolegnia: Cause necrosis and melanization;
follow physical damage.
|
|
White spot syndrome baculovirus (WSBV): Targets
the cuticular epidermis, stomach, gills and hepatopancreas; important disease in
marine shrimp; Macrobrachium is known to be a carrier but it is not yet certain
whether WSBV causes mortalities in it.
|
Internal infections: Caused by a variety of Gram negative bacteria
such as Vibrio spp. and Aeromonas spp.; feeding discontinues; discolouration of
the body (usually pale and white) occurs; animals listless; infections by luminous
vibrios are usually serious.
|
Yeast infections: Muscles appear yellowish, bluish or grey; causes
heavy mortalities in grow-out ponds; particularly prevalent when temperatures are
lower than optimal and organic matter is allowed to accumulate and eutrophication
occurs.
|
|
Nodavirus (M R NV): Opaque whitish
appearance of the abdomen, followed by severe mortalities.
|
Bacterial infection caused by Enterococcus: Necrosis in muscles
and hepatopancreas; begins in the head portion and proceeds to the tail; animal
appears opaque; exacerbated in high temperature (33-34°C) and high pH (8.8-9.5)
conditions. Rickettsial disease: larvae become white throughout
their bodies and generally inactive before death; infected populations experience
significant mortalities.
|
|
(Source: Food and Agriculture Organisation, 2002)
Prevention and Treatment of freshwater prawn
diseases
|
Disease
|
Prevention and Treatment
|
|
Macrobrachium hepatopancreatic parvo-like virus (MHPV)
|
Obtain and maintain disease-free stock; good management. No treatment reported.
|
|
Macrobrachium muscle virus (MMV)
|
Obtain and maintain disease-free stock; good management. No treatment reported.
|
|
White spot syndrome baculovirus (WSBV)
|
Obtain and maintain disease-free stock; good management. No treatment reported.
|
|
Nodavirus (M R NV)
|
Obtain and maintain disease-free stock; good management. No treatment reported.
|
|
Black spot (sometimes called brown spot or shell disease)
|
Good management, especially maintaining good water quality and avoiding physical
damage by handling (by transfer, sampling) or by other prawns (may be caused by
overstocking, poor feeding, etc.). Treatment by immersion in 10 ppm oxolinic acid
for 1 hour, or 2 ppm nifurpirinol for 96 hours reported.
|
|
Appendage necrosis
|
Good management, especially maintaining good water quality and avoiding physical
damage by handling (by transfer, sampling) or by other prawns (may be caused by
overstocking, poor feeding, etc.). Treatment by 0.65-1.0 ppm erythromycin or 2 ppm
of a penicillin-streptomycin mixture, or 1.5 ppm chloramphenicol reported.
|
|
Internal infections
|
Good management, especially good filtration and/or treatment of incoming hatchery
water. Treatment by 2 ppm chloramphenicol combined with 2 ppm furazolidone for 5-7
days reported.
|
|
Bacterial infection caused by Enterococcus
|
Good management, especially by avoiding constructing farms in areas where (or operating
farms at times when) temperature and pH are too high. No treatment reported.
|
|
Rickettsial disease
|
Obtain and maintain disease-free stock; good management; treatment of tanks and
equipment with lime (CaO) before stocking. Treatment by application of 10 ppm oxytetracycline
combined with 10 ppm furazolidone reported.
|
|
Lagenidium infection
|
Good management. Treatment by maintaining 10-100 ppb trifluralin in hatchery tanks,
or treatment with 20 ppm of Merthiolate ® has been reported.
|
|
Infections by Fusarium and Saprolegnia
|
Good management, especially maintaining good water quality and avoiding physical
damage by handling (by transfer, sampling) or by other prawns (may be caused by
overstocking, poor feeding, etc.). No treatment reported.
|
|
Yeast infections
|
Good management, especially the avoidance of lower than optimal water temperatures,
the accumulation of organic matter and eutrophication; use better water exchange,
aeration and circulation and lower feeding rates. No treatment reported.
|
(Source: Food and Agriculture Organisation, 2002)
Top
|