Title Studies on prevention measure of white spot disease of kuruma shrimp Marsupenaeus japonicus
Authers Jun SATOH
Keywords WSD, PAV, kuruma shrimp, Marsupenaeus japonicus, vaccine
Citation Bull. Fish. Res. Agen. No.36, 57-106, 2012
Abstract
WSD (white spot disease), the equivalent of penaeid acute viremia (PAV), has become one of the most serious problems not only in the shrimp farming industry but also in hatchery production in Southeast Asian countries and the Americas. The major infection route of the causative virus (WSSV: white spot syndrome virus= PRDV)is considered to be vertical transmission from spawners to larvae/juveniles via eggs in the seed production process of kuruma shrimp(Marsupenaeus japonicus). Therefore, in order to inhibit vertical trans mission, eggs are selected based on PCR (polymerase chain reaction)detection of WSSV from the receptaculum seminis after spawning. In addition, the fertilized eggs are disinfected with povidone iodine (5 mg/l for 5 min). In order to prevent horizontal transmission, larval and juvenile rearing seawater is treated with UV irradiation. Stable production of specific-pathogen-free shrimp was accomplished by these countermeasures for the prevention of WSSV transmission. However, in kuruma shrimp farms, horizontal transmission by cannibalism and waterborne routes is also very important among reared shrimp and cohabiting crustaceans in those environments. Thus, it is still difficult to prevent horizontal infection by WSSV at shrimp farms. Recently, a “quasi-immune response”was founded in kuruma shrimp, wherein naturally survived from WSD were protected against a re-challenge with WSSV. Moreover, we developed oral vaccine with WSSV recombinant proteins, rVP26 and rVP28, meaning that shrimp protection against WSSV-infection was inducible by the oral vaccine with rVPs.
 Chapter 1. - In 1996, a hatchery of Japan Sea-Farming Association obtained kuruma shrimp(Marsupenaeus japonocus)eggs for seedling culture from wild broodstocks, among which some were found to be white spot syndrome virus(WSSV)infected by PCR test. Eggs were washed once with filtrated seawater and reared in the hatchery. During the culture, those post-larval groups in which WSSV were detected were excluded. Although no WSSV was found in the seedlings prior to the transportation to the nursery by PCR test, WSD occurred among them during the culture in the nursery facilities. In 1997, the hatchery again obtained prawn eggs from wild broodstocks. However, in this year, PCR-check was applied to select non-WSSV infected spawners. Furthermore, eggs were disinfected with iodine before being served for rearing. No WSD infection occurred throughout the culture in the hatchery and the nursery facility in this year. These results strongly suggest that the infec tion source of WSD occurred in 1996 originated from spawners.
 Chapter 2. - Conditions suitable for disinfection of fertilized eggs of Marsupenaeus japonicus using povidone-iodine were investigated. Eggs 10 h after fertilization were exposed to 0, 2.5, 5.0 and 10.0 mg/L of active iodine for 5, 10, 15 and 20 min. Hatching rate showed no significant difference between control and test groups in concentration of 2.5 mg/L of active iodine for 5 to 20 min and 5.0 mg/L for 5 to 15 min. Viable bacterial counts in these conditions decreased by more than 90% when compared with those of control groups. There was no significant difference in the hatching rates of eggs with eight different developmental stages between control and tested groups exposed to 5.0 mg/L of active iodine for 5 min.
 Chapter 3. - We compared WSSV infection induction in kuruma shrimp by oral, immersion, and intramuscular injection(IM)exposure methods and evaluated the oral vaccine prepared from the recombinant WSSV proteins rVP26 and rVP28. The 50% lethal doses (LD 50) of WSSV by oral, immersion, and IM challenges were 10-0.4, 10-4.4, and 10-7.7g shrimp-1, respectively, indicating that WSSV infection efficiency by oral challenge was significantly less than the other 2 challenge routes. However, in shrimp farms it is believed that WSSV infection is easily and commonly established by the oral route as a result of cannibalization of WSSV-infected shrimp. Kuruma shrimp vaccinated orally with WSSV rVP26 or rVP28 were challenged with WSSV by oral, immersion, and IM routes to compare protection efficacy. The relative percent survival values were 100% for oral challenge, 70 to 71% for immersion, and 34 to 61% for IM. Thus, the protection against WSSV-infection that was induced in kuruma shrimp by oral vaccination with rVP26 or rVP28 seemed equivalent to that obtained through IM vaccination.
 Chapter 4. - The phylaxis against WSSV was also inducible by oral vaccination with recombinant WSSV proteins, rVP26 and rVP28. In the present study, kuruma shrimp orally vaccinated with rVPs were sequentially challenged with WSSV to evaluate onset and duration of phylactic response and booster effect. The phylactic response of shrimp against WSSV-challenge peaked at day 45 after the vaccination with rVP26 (RPS: 100%)and at day 55 with rVP28 (RPS: 93%), and decreased within 10-20 days. The phylaxis against WSSVchallenge was boosted by the secondary vaccination with homologous rVPs, but not by those with heterologous rVPs. Phylactic responses by the secondary vaccination appeared more rapid than those by the primary vaccination. These results demonstrated that the duration of phylaxis induced by oral vaccination with rVPs was relatively short, but could be extended by booster vaccination with homologous rVPs.
URI http://www.fra.affrc.go.jp/bulletin/bull/bull36/36-2.pdf