Development of three-valent vaccine against streptococcal infections in olive flounder, Paralichthys olivaceus

Abstract

Streptococcus iniae and S. parauberis types I and II are major bacterial pathogens affecting the olive flounder, Paralichthys olivaceus. With the introduction of formalin-killed vaccine for S. iniae, recent epidemiological studies have indicated that S. parauberis infections are becoming increasingly severe and frequent in the olive flounder farming industry. Here, the formalin-killed bacterins were used to develop a three-valent vaccine against three streptococcal infections, and its efficacy was assessed in laboratory- and field-challenge trials. In the laboratory-challenge test, it obtained 75%, 75% and 90% relative percent survival (RPS) values in vaccinated olive flounders challenged with S. iniae, S. parauberis type I, and S. parauberis type II, respectively. For the field-challenge trial, the three-valent vaccine was administered to over 100,000 olive flounders, which were compared to non-injected control fish. An RPS of 74% was observed when S. parauberis infection naturally occurred at 11 fish farms. Serum antibody measurements using olive flounder immunoglobulin-specific antibodies indicated that immunized fish had significantly higher serum antibody levels than control fish up to 6months post-vaccination. These results demonstrate that the three-valent vaccine using formalin-killed bacterins could effectively protect olive flounders against three major streptococcal infections. Statement of relevanceThe olive flounder, P. olivaceus, is an economically important fish species in Northeast Asia and the most valuable fish species in South Korea, corresponding to 56.5% of the total mariculture production in 2010 (Baeck et al., 2006). However, streptococcal infections often break out among farmed olive flounders subjected to certain conditions (e.g., high stocking density, poor water quality and high water temperature), leading to mass mortality and severe economic losses (Baeck et al., 2006; Cho et al., 2008) Antibiotic treatments are usually recommended to address such outbreaks, but the prevalence of multi-drug-resistant bacteria means that antibiotic compounds are not always able to eradicate the infections (Creeper and Buller, 2006).The Gram-positive bacteria, Streptococcus iniae and S. parauberis, cause streptococcosis and have severe effects on aquaculture of the olive flounder, barramundi, channel catfish, European sea bass, rainbow trout, striped bass, tilapia, turbot and yellow tail (Gudmundsdóttir and Björnsdóttir, 2007; Han et al., 2011). In the olive flounder, fish infected with S. iniae usually show severe pathological changes, such as darkening, exophthalmia, rectal hernia, abdominal distension, ascites and congestion of intestinal organs, whereas fish infected with S. parauberis rarely exhibited any remarkable pathological finding except for darkening of the skin (Kim et al., 2006).Some vaccines have been reported to prevent S. iniae infection, showing high relative percent survival (RPS) among olive flounders in laboratory trials; such vaccines are now commercially available (Cheng et al., 2010a). However, recent epidemiological studies have suggested that S. parauberis infection has increased and become more severe and frequent as much as infections of S. iniae, which was previously considered the major bacterial pathogen among olive flounder (Park et al., 2012; Park, 2009; Perera et al., 1998). S. parauberis can be divided into two serotypes (types I and II) based on serological testing of a distinctive capsular polysaccharide layer (Bromage and Owens, 2002; Plant and Lapatra, 2011). Since the two serotypes possess specific antigenic characteristics, a vaccine should be able to address both types in order to effectively prevent S. parauberis infection.The present study was undertaken to develop a three-valent vaccine against both serotypes of S. parauberis plus S. iniae using formalin-killed vaccine preparation method. The efficacy of this vaccine against the major streptococcal infections of olive flounder was calculated as RPS and conducted in both laboratory trials and large-scale field trial. This vaccine will be highly useful and more advantageous since the use of this does not require several antibiotics which means safer food production and more sustainable environment.