Detection of white spot syndrome virus in filtered shrimp-farm water fractions and experimental evaluation of its infectivity in Penaeus (Litopenaeus) vannamei

Abstract

White spot syndrome virus (WSSV) may spread through water to neighbor ponds or farms. Routine water exchange and wastewater released during white spot disease (WSD)-emergency harvests may preserve WSSV in shrimp farming areas. To test this hypothesis, on-site experiments were performed in a WSSV-affected farm in Guasave, Sinaloa, Mexico. Plankton and shrimp hemolymph were collected from 12 ponds during a WSD outbreak. PCR analyses showed that 72% of the hemolymph pools (26 out of 36) were WSSV-positive. In contrast, only 14% (4 of 28) plankton samples (filtered through 10 and 0.45 µm) from three ponds (2, 7 and 10) were WSSV-positive. Plankton from pond 9 was WSSV-negative, but 14 days later, shrimp began to die. At this point, a differential filtration experiment was performed in pond 9. WSSV-positive samples were only found in three fractions [particulate fraction (PF) 1 µm and liquid fractions (LF) < 100 and < 40 µm]. Both LFs and PFs were used for insitu infectivity assays by water-borne routes in WSSV-negative whiteleg shrimp Penaeus (Litopenaeus) vannamei. Some shrimp exposed to different PFs and LFs (100 µm to > 0.65 µm) became WSSV-positive. Results indicate that water fractions between 100 and 0.65 µm induced WSSV infection to shrimp. Results showed that pond water and/or particulate fractions are vehicles for WSSV dispersion via virus suspended in water, attached to microalgae, or carried by zooplankton.