STUDIES ON WHITE SPOT SYNDROME VIRUS (WSSV) CHALLENGE IN Penaeus vannamei JUVENILES

White Spot Syndrome Virus (WSSV) is an enveloped, double-stranded DNA virus causing severe mortalities (up to 100% within 3 to 10 days) of cultured penaeid shrimp globally including India. Development of effective therapeutic strategies requires lab scale challenge studies with the virus. The WSSV challenge studies have been carried out mostly in adult specimens through oral, immersion and intramuscular injection routes. However, the immersion challenge of early life stages has been ineffective as reported by several authors. The present study was carried out to standardize WSSV challenge protocol in P. vannamei juveniles.

The WSSV infected P. vannamei were collected from a shrimp farm in Navsari, Gujarat. The infection was confirmed by the presence of prominent white spots on the exoskeleton and PCR. Virus inoculum was prepared and the viral copy number was estimated by absolute quantification using qunatitative PCR assay. Infectivity of the inoculum was confirmed on the adult shrimps (30 to 40g). The WSSV challenge study was performed on three body weight (bw) groups (10-20mg, 30-40mg and 50-60mg) following identical experimental design consisting of two treatments with three replicates each. Animals were immersion challenged with 100µl of two different viral titers (10⁵ and 10⁴ virions per µl) in 1L of sea water for 2h, transferred to clean sea water and monitored for 20 days post-immersion (dpi).

No WSSV positive mortality was recorded in the experimental group of 10 to 20mg body weight. The PCR and nested PCR assays revealed the absence of virus. Similarly, in the second experiment (30 to 40mg), no WSSV positive mortality was recorded. Nested PCR detected the virus, but the animals did not die. In the third experiment (50 to 60mg), 100% mortality was recorded at 10 and 13 dpi in animals challenged with 100µl of 10⁵ and 10⁴ copies/µl inoculum, respectively. First mortality was recorded on 4^th dpi and 5^th dpi, respectively. The qPCR assay indicated an increasing trend in viral copy number till the end of the experiment. The experimental details are given in Table 1. The present study suggests 50-60 mg body weight to be the minimum size for WSSV challenge studies in P. vannamei juveniles.