PREVENTION OF VIRAL INFECTIONS
There are no treatments for viral infections in fish or shrimps. Hence, preventive measures must be adapted to keep the viral pathogens away. The basic consideration in preventing the occurrence of viral diseases is avoidance, and the use of virus-free fry for stocking in ponds is highly recommended. It should also be borne in mind that semi-intensive and intensive culture systems pro-mote conditions conducive for disease development. As such, reduction of stress and the application of good husbandry or efficient technology may deter the occurrence of disease. Details on the general methods of disease prevention are discussed in Chapters 9 and 10.
In addition, specific precautions of egg washing with ozone-disinfected seawa-ter; using fine screens for inlet water and adherence to strict hygiene; stress test of shrimp postlarvae (PLs) with 100 ppm formalin for 30 min with aeration and stocking only tolerant PLs; the use of only dry commercial feeds and pasteur-ized fresh feed at 60°C for 15 min before use; and feeding with feeds contain-ing 100 ppm phosphated ascorbic acid (MAP) for 92 days were reported effec-tive for viral infections. For WSSV management, the effective use of immunostimulants mixed with feed such as peptidoglycan (PG) at 0.2 mg/kg body weight/day for 2-3 months or Fucoidan, an extract from Cladosiphonokamurans at 50-100 mg/kg shrimp/day for 15 days, were reported to increasesurvival of WSSV-exposed shrimps.
It is also important that surveillance for early signs of disease and stressful factors become essential components of farm/hatchery management in order to monitor the health status of fish stocks, to assess adequacy of rearing proce-dures and to prevent introduction of pathogens. Early detection of a disease outbreak will reduce mortalities and prevent a catastrophic spread of the virus. Finally, since virus can remain viable outside the living host for at least 72 hr, water change should be contemplated only at least 5 days after effluents from infected ponds in the area have been discharged.
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