Clinical signs of naturally white spot syndrome virus (WSSV)-infected kuruma shrimp Marsupenaeus japonicas, based on their physiological and behavioural states

White spot syndrome virus (WSSV) proliferates rapidly and causes a high fatality rate in shrimp. Additionally, there is a lack of effective prevention and treatment methods for this disease. Although WSSV pathology is well-studied by molecular biology techniques, little research has have focused on the clinical signs of this disease based on individual shrimp performances. In this study, we recorded the clinical signs of diseased shrimp by observing their physiological and behavioural states. The physiological state of shrimp, including their heart rate (HR), heart rate variability (HRV), and ventilatory rate (VR), was recorded with a high-speed camera, which was used for crustacean disease diagnosis for the first time. The behavioural state of the shrimp consisted of their behaviour in the pond before sampling and the action demonstrated by the shrimp while falling into the water. Quantitative of WSSV in the shrimp was performed after measuring their physiological and behavioural characteristics. The virus was detected in both shrimp with and without white spots. The amount of virus in shrimp with or without white spots in the sand was relatively low and that of shrimp with white spots swimming in the water was high. The HR and VR both first increased and then decreased as the amount of virus increased gradually. There was a significant negative correlation between HRV and virus quantity in diseased shrimp. The swimming ability of shrimp with high virus quantities was weaker than that of shrimp with low virus quantities. Based on the data, we infer that the burrowing behaviour of shrimp is a disease resistance strategy; the slow swimming or knocking at the wall are symptoms of shrimp suffering from severe infection stage; WSSV destroys the heart, gill and muscle tissue of shrimp, causing damage to their basic motor function. Using the HRV index, we uncovered a fast and effective method with which to determine the virus quantity in diseased shrimp. On the basis of the above symptoms, WSSV is demonstrated as a shrimp disease not only related to the immune system, but also to the motor nervous system and respiratory circulatory systems.