A time-course study on the resistance of Penaeus japonicus induced by artificial infection with white spot syndrome virus.

The onset and duration of resistance in experimental survivors of Penaeus japonicus produced by an intramuscular injection with white spot syndrome virus (WSSV) were surveyed by re-challenge tests with the virus conducted at weeks 1-4 and months 1-3 post initial exposure (PIE) to the virus. Virus neutralising activity in the survivors' plasma was also examined. Plasma-treated WSSV was separated from the plasma by centrifugation and then injected into naïve shrimp, in parallel with each re-challenge test. Re-challenge tests of the survivors conducted at weeks 1-4 PIE revealed that the resistance commenced at week 3 (relative percent survival, RPS: 39%) and almost fully developed at week 4 (RPS: 58%), because statistically significant differences in survival rates were observed between the test (previously virus exposed) and control groups at weeks 3 and 4. Re-challenge at months 1-3 PIE resulted in RPS values of 67, 54 and 6%, respectively, indicating the resistance persisted until month 2. RPS values in neutralisation tests performed at weeks 1-4 and months 1-3 PIE were -5, 14, 36, 50, 100, 38 and 6%, respectively, which coincided with the RPS values in each re-challenge test conducted in parallel. The present results demonstrated that resistance of P. japonicus against the viral pathogen developed 3 or 4 weeks after an exposure to the virus, and it persisted for another month at 24 degrees C. The resistance was paralleled by a humoral neutralising factor(s) in the plasma of shrimp.