For the past two years, farmers have been recommended to carefully check the health status of post-larvae before purchasing them. Despite such checks, viral diseases still occur in shrimp certified as virus-free. There are two possible reasons for this. First, randomly examined post- larvae samples may not represent the entire batch. Second, virus-free shrimp can become infected if put in poorly managed culture systems.
Most shrimp farmers choose to rely on their own experience to diagnose white spot disease.
The most common gross clinical signs are white spots of various sizes embedded in the carapace. Sometimes the spots disappear after moulting and the shrimp returns to normal. However, in most cases, shrimp which develop white spots do not survive. This situation has caused confusion amongst Thai farmers. Even technicians find it difficult to predict the outcome of shrimp which develop white spots.