Differences in virulence between white spot syndrome virus (WSSV) isolates and testing of some control strategies in WSSV infected shrimp

White spot syndrome virus (WSSV) is a fatal pathogen of shrimp. In this thesis, differences in virulence of WSSV isolates and control strategies against WSSV infection in Penaeus vannamei juvenile, including treatment with cidofovir an antiviral, feeding a diet supplemented with Spirulina platensis and exposure to high water temperature (33°C), were evaluated. In all experiments shrimp were inoculated intramuscularly or orally with a standardized protocol using a fixed WSSV infectious titre. Virulence was compared between three WSSV isolates based on time of onset of mortality and time to reach a cumulative mortality of 50 and 100%. Quantification of WSSV infected cells (VP28 positive) in tissues with an important role in shrimp physiology including gills, stomach cuticular epithelium, cuticular epithelium of the body wall, hematopoietic tissues and antennal glands was done to find the basis of differences in virulence. Mortality started at 36 h post inoculation (hpi) in shrimp inoculated with two Thai isolates and 36-60 hpi with the Viet isolate. Cumulative mortalities reached 100% at 72-84, 108 and 204-348 hpi with WSSV Thai-1, WSSV Thai-2 and WSSV Viet, respectively. The mean numbers of infected cells were significantly higher in each examined tissue of naturally deceased shrimp inoculated with a highly virulent WSSV Thai-1 than with the low virulent WSSV Viet isolate except in hematopoietic tissues. The difference was prominent in gills (6x). Cidofovir was not effective in blocking WSSV infection in shrimp, which was in contrast with the results obtained with DNA viruses in man. This could be due to (i) limited cellular uptake, (ii) disability of shrimp cells to metabolize cidofovir in its active form, and/or (iii) differences between WSSV and other DNA viruses. A diet supplemented with S. platensis did not reduce mortality. In this thesis, three water temperatures (27°C, 30°C and 33°C) were tested on the clinical and virological outcome of WSSV infection in shrimp. Two temperatures, 27°C and 30°C were permissive for virus replication, which resulted in disease and mortality of shrimp. At 33°C continuously before and after inoculation or after inoculation only, shrimp did not show clinical signs, mortality was almost absent and virus replication was inhibited. The effect of 33°C was independent of inoculation route and infectious titre used. To explore the potential of 33°C to control white spot syndrome, the impact of 33°C for 6 h, 12 h, 18 h and 24 h per day in shrimp infected with a high dose of WSSV Thai-1 or WSSV Viet isolate was investigated. A duration of 33°C for 6 h per day had no beneficial effect. Longer durations (12-24h) were effective to delay and reduce mortality in shrimp inoculated with both isolates. The results with 18 h per day at 33°C broaden the application potential of 33°C to prevent disease and reduce mortality by minimizing the negative effects on shrimp physiology. The effect of increasing temperature from 27°C to 33°C at different stages of WSSV infection (0, 12, 24, 48 and 96 hpi) in shrimp inoculated with a low or a high dose of WSSV Thai-1 or WSSV Viet was examined. Increasing temperature to 33°C was effective in inducing protection only at an early stage of infection (≤ 24 hpi) in shrimp inoculated with each isolate. At a later stage of infection, it had no or even an adverse effect, a sharp rise of mortality. The cause of this sharp rise of mortality could be a combination of progression of infection in tissues and negative effects of 33°C on shrimp physiology such as increased requirement of oxygen and reduced oxygen exchange in gills