3.1. The epidemic time and clinical symptoms of diseasedH. nipponia
Cultivation of H. nipponia is shown in Figure 1. The epidemic time of H. nipponia was from May to October each year, when temperature is between 25-35 ℃. Diseased H. nipponia obviously showed reduced food intake, swam slowly and floated on the water. The incidence and mortality rates of H. nipponia were very high, causing serious economic damage to farmers. Only a small part of the body surface of diseased H. nipponia was congested and most of the body surface was normal without debonding (Figure 2). Internal organs were found to be edematous and erosive.