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. |
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. |