IMIS

For the intramuscular route, Penaeus (Litopenaeus) vannamei shrimp (n = 450) were injected with 10^-2.3 up to 10^2.7 shrimp infectious dose 50% end point (SID₅₀) of WSSV in early and late post-molt, inter-molt, early and late pre-molt; resp. A-, B-, C-, D1- and D2-stage. The resulting infection titers demonstrated that no difference (p > 0.05) in susceptibility existed between different molt stages when virus was injected. For the waterborne route, shrimp in different molt stages were immersed in seawater containing 10⁴ SID₅₀ ml^-1 of WSSV. In a first study, P. vannamei (n = 125) incubated in cell culture flasks, became infected with WSSV mostly in post-molt stages. In a second study, 2 groups of P. vannamei (n = 100) and P. monodon (n = 100) were transferred into plastic bags to prevent damage to the cuticle; and in 1 group a pleopod was cut off prior to incubation. Induction of damage increased infection significantly (p < 0.05) in A-stage from 0–40% to 60–100%, in B-stage from 0–20% to 40–60%, in C-stage from 0–20 to 20–60%, while infection was 0% in D-stages with both immersion methods. This study proved that shrimp are more susceptible to WSSV infection via immersion after molting than in the period before molting and wounding facilitates infection.