Report of significant WSSV-resistance in the Pacific white shrimp, Litopenaeus vannamei, from a Panamanian breeding program

Three Litopenaeus vannamei families, from a breeding program in Panama and with possible WSSV resistance, were challenged per os with a reference isolate of White spot syndrome virus originally obtained from China in 1995 (WSSV-CN95). These F8, F9 and F12 generation families were developed from founder stocks a decade ago and were survivors of white spot disease. Juvenile shrimp used for WSSV challenge averaged 1.5. g, and they were stocked at 50 to 96 animals per tank into nine 1000. L fiberglass tanks containing artificial seawater at 30. ppt salinity and 26. °C. Three of the 1000. L tanks were used as negative control tanks, with one tank for each family. Six 1000. L tanks were used for challenging the three families with WSSV, with two replicate tanks for each family. A positive control consisting of 20 "Kona" SPF reference line L. vannamei (average weight 1.5. g) was included and challenged with WSSV in a 90. L glass aquarium. The Kona stock was fed the same batch of WSSV infected tissue as the three Panamanian families to confirm infectivity and to provide a basis with which to compare final survival. WSSV infected minced frozen shrimp tissue was fed at a rate of 5% of average body weight one time on day 0. All tanks were equipped with air diffusers to provide sufficient aeration and an acclimated crushed oyster shell internal recirculating biological filter. Each tank was covered with a plastic sheet to contain aerosols and minimize water temperature fluctuations. The experimental tanks were checked daily and moribund animals were collected when observed and preserved in Davidson's AFA fixative. Mortalities in the three Panamian families ceased at 17. days post challenge. Two survivors from each tank were preserved for histology and five shrimp per tank were individually tested by qPCR to determine their WSSV status and viral load. Survival at termination in the negative control families was 95%, 98% and 100%. Survival in the Kona line WSSV positive control was 0% with all the Kona line shrimp dead by day 6 post infection. At termination on day 17, survival of Panamanian selected families in the WSSV challenged groups was 23%, 57% and 26% for families LP-1, LP-2 and LP-3, respectively. This is the first time in the scientific literature that significant resistance of L. vannamei against WSSV under controlled conditions is reported.