We examined the effectiveness of a formalin-inactivated vaccine against viral hemorrhagic septicemia (VHS) in Japanese flounder Paralichthys olivaceus by the pathogen challenge. Japanese flounder (average weight, 10 g) kept at 12°C, 20°C and 28°C were vaccinated by intraperitoneal injection of a formalin-inactivated VHS virus (VHSV) and maintained at each temperature for 21, 19 and 17 days post vaccination (dpv), respectively. The fish vaccinated at 20°C and 28°C were acclimated to 12°C, and then all vaccinated fish were challenged with VHSV at 12°C at 21 dpv. Fish that received the vaccine at 20°C showed relative percent survival (RPS) of 48%, compared with 0% for those given the vaccine at 12°C and 28°C. In other experiment, efficient protection against VHSV lasted at least 7 mo in Japanese flounder (average weight, 100 g) vaccinated at 20°C and maintained at ambient temperature, where high RPS values (71-100%) were obtained after viral challenges at 18, 77, 98 and 213 dpv. We obtained promising results that a single immunization with the formalin-inactivated vaccine at the physiologically optimal temperature (20°C) provided significant protective and long-lasting immunity to Japanese flounder against VHS.
Ostreid herpesvirus 1 (OsHV-1) μVar is a variant of OsHV-1 and suspected of being the causative agent of acute mass mortality events of Pacific oysters during summers in Europe since 2008. In this study, the distribution of OsHV-1 was surveyed in the six main oyster-producing areas of Japan, using PCR targeting a C2/C6 fragment including ORF4. PCR products were amplified from 123 out of 1,714 oysters of three species of Crassostrea (C. gigas, C. sikamea and C. ariakensis), and 23 different nucleotide sequences, showing 96% to 99% similarity to the reference OsHV-1, were obtained. Although 18 sequences among the 23 obtained possessed a microsatellite deletion unique to OsHV-1 μVar, all PCR products contained two conserved nucleotides that were shared with the reference OsHV-1 and not with OsHV-1 μVar. Here, we found variable types of OsHV-1 in oysters in Japan, but their nucleotide sequences were not identical to those of OsHV-1 μVar.
Red seabream iridovirus (RSIV) causes significant mortality in many marine fishes. Polyinosinic-polycytidylic acid [Poly(I:C)] immunization with a live virus confers protection of fishes from viral infection. Thus, we applied this immunization with live RSIV to rock bream Oplegnathus fasciatus, red seabream Pagrus major and yellowtail Seriola quinqueradiata. No significant difference was observed in mortalities due to RSIV infection between the fishes that did or did not receive Poly(I:C), indicating that fishes administered Poly(I:C) were not protected from RSIV infection. It was confirmed that the Mx gene, an indicator of induced interferon, was well expressed in rock bream that received Poly(I:C). The results suggest that RSIV is probably insensitive to the transient innate immune response induced by Poly(I:C).
In July 2003, we observed abnormal opaqueness of both the yolk in the embryos and recently hatched larvae of Pacific bluefin tuna Thunnus orientalis spawned at the Ohshima Experimental Station in Wakayama, Japan. The yolks were infected with a number of a unicellular parasite. All the infected yolk sacs burst one day post-hatch. Consequently, all the host larvae died and the parasites were released into seawater. The infection rate of this disease was less than 16% in 2003 but spiked up to 98% in 2004. However, the disease has not been observed since 2006. The pathogen has been identified as Ichthyodinium sp. belonging to Alveolata.