Per os infectivity of white spot syndrome virus (WSSV) in white-legged shrimp (Litopenaeus vannamei) and role of peritrophic membrane.

As earlier observations on peroral infectivity of WSSV in white-legged shrimp are conflicting, here, a standardized peroral intubation technique was used to examine (i) the role of the physical composition of the viral inoculum and (ii) the barrier function of the PM. In a first experiment, the infectivity of a WSSV stock was compared by determining the SID50 by intramuscular injection, peroral inoculation or via feeding. The following titers were obtained: 10(8.77) SID50/g by intramuscular injection, 10(1.23) SID50/g by peroral inoculation and 10(0.73) SID50/g by feeding. These results demonstrated that 10(7.54)-10(8.03) infectious virus is needed to infect shrimp by peroral inoculation and via feeding. Next, it was examined if damage of the PM may increase the susceptibility for WSSV by peroral route. The infectivity of a virus stock was tested upon peroral inoculation of shrimp with and without removal of the PM and compared with the infectivity upon intramuscular inoculation. The virus titers obtained upon intramuscular injection and peroral inoculation of shrimp with and without PM were 10(8.63), 10(1.13) and 10(1.53) SID50/mL, respectively. This experiment confirmed the need of 10(7.1)-10(7.5) infectious virus to infect shrimp via peroral route and showed that the removal of the PM slightly but not significantly (p > 0.05) facilitated the infection of shrimp. This study indicated that WSSV contaminated feed is poorly infectious via peroral route, whereas it is highly infectious when injected into shrimp. The PM plays a minor role as internal barrier of shrimp against WSSV infection.

Susceptibility of juvenile Macrobrachium rosenbergii to different doses of high and low virulence strains of white spot syndrome virus (WSSV).

As some literature on the susceptibility of different life stages of Macrobrachium rosenbergii to white spot syndrome virus (WSSV) is conflicting, the pathogenesis, infectivity and pathogenicity of 2 WSSV strains (Thai-1 and Viet) were investigated here in juveniles using conditions standardized for Penaeus vannamei. As with P. vannamei, juvenile M. rosenbergii (2 to 5 g) injected with a low dose of WSSV-Thai-1 or a high dose of WSSV-Viet developed comparable clinical pathology and numbers of infected cells within 1 to 2 d post-infection. In contrast, a low dose of WSSV-Viet capable of causing mortality in P. vannamei resulted in no detectable infection in M. rosenbergii. Mean prawn infectious dose 50% endpoints (PID50 ml⁻¹) determined in M. rosenbergii were in the order of 100-fold higher for WSSV-Thai-1 (105.3 ± 0.4 PID50 ml⁻¹) than for WSSV-Viet (103.2 ± 0.2 PID50 ml⁻¹), with each of these being about 20-fold and 400-fold lower, respectively, than found previously in P. vannamei. The median lethal dose (LD50 ml⁻¹) determined in M. rosenbergii was also far higher (~1000-fold) for WSSV-Thai-1 (105.4 ± 0.4 LD50 ml⁻¹) than for WSSV-Viet (102.3 ± 0.3 LD50 ml⁻¹). Based on these data, it is clear that juvenile M. rosenbergii are susceptible to WSSV infection, disease and mortality. In comparison to P. vannamei, however, juvenile M. rosenbergii appear more capable of resisting infection and disease, particularly in the case of a WSSV strain with lower apparent virulence.