PCR Detection and Phylogenetic Analysis of Megalocytivirus Isolates in Farmed Giant Sea Perch Lates calcarifer in Southern Taiwan

Protective efficiency and immune responses to single and booster doses of formalin-inactivated scale drop disease virus (SDDV) vaccine in Asian seabass (Lates calcarifer)

BACKGROUND

Scale drop disease virus (SDDV) threatens Asian seabass (Lates calcarifer) aquaculture production by causing scale drop disease (SDD) in Asian seabass. Research on the development of SDDV vaccines is missing an in-depth examination of long-term immunity and the immune reactions it provokes. This study investigated the long-term immune protection and responses elicited by an SDDV vaccine. The research evaluated the effectiveness of a formalin-inactivated SDDV vaccine (SDDV-FIV) using both prime and prime-booster vaccination strategies in Asian seabass. Three groups were used: control (unvaccinated), single-vaccination (prime only), and booster (prime and booster). SDDV-FIV was administered via intraperitoneal route, with a booster dose given 28 days post-initial vaccination.

RESULTS

The immune responses in vaccinated fish (single and booster groups) showed that SDDV-FIV triggered both SDDV-specific IgM and total IgM production. SDDV-specific IgM levels were evident until 28 days post-vaccination (dpv) in the single vaccination group, while an elevated antibody response was maintained in the booster group until 70 dpv. The expression of immune-related genes (dcst, mhc2a1, cd4, ighm, cd8, il8, ifng, and mx) in the head kidney and peripheral blood lymphocytes (PBLs) of vaccinated and challenged fish were significantly upregulated within 1-3 dpv and post-SDDV challenge. Fish were challenged with SDDV at 42 dpv (challenge 1) and 70 dpv (challenge 2). In the first challenge, the group that received booster vaccinations demonstrated notably higher survival rates than the control group (60% versus 20%, P < 0.05). However, in the second challenge, while there was an observable trend towards improved survival rates for the booster group compared to controls (42% versus 25%), these differences did not reach statistical significance (P > 0.05). These findings suggest that the SDDV-FIV vaccine effectively stimulates both humoral and cellular immune responses against SDDV. Booster vaccination enhances this response and improves survival rates up to 42 dpv.

CONCLUSIONS

This research provides valuable insights into the development of efficient SDDV vaccines and aids in advancing strategies for immune modulation to enhance disease management in the aquaculture of Asian seabass.

Molecular epidemiology of Megalocytivirus in freshwater angelfish (Pterophyllum scalare) from Johor, Malaysia

Background and Aim

Malaysia has more than 630 culturists who are involved in the ornamental fish industry and culture 250 species, including local and exotic species. Among these viruses, megalocytiviruses have been associated with severe systemic diseases and economic losses in ornamental fish. The intensity of Megalocytivirus infection in Pterophyllum scalare in Malaysia remains unknown. Thus, this study aimed to investigate the occurrence of Megalocytivirus while discovering its associated risk factors and the genotypes of its causative agents in an ornamental fish farm in Malaysia.

Materials and Methods

Seven broodstock pairs of P. scalare were used in this study to follow the life stages of fish, from egg to market size. Water samples and other samples, such as mucus swabs, gill swabs, P. scalare eggs, fries, juveniles, snails, snail eggs, live feed (Tubifex worms and Moina spp.), sediment samples, and wild fish, were collected periodically for initial environmental sampling from day 0 to day 60. Nested polymerase chain reaction amplifications were performed for megalocytivirus-related sequences. The phylogenetic tree, including the sampled causative agents of megalocytiviruses, was inferred from the major capsid protein genes of all known Iridoviridae species. Pearson's correlation coefficients were calculated to determine the strength of the correlation between the presence of megalocytiviruses in P. scalare samples and the associated risk factors.

Results

A total of 312 out of 935 pooled and individual samples tested positive for the presence of Megalocytivirus-related sequences, except snail eggs and wild fish (Poecilia reticulata). No clinical symptoms were observed in any fish samples. Megalocytivirus-associated viruses detected in water samples indicate horizontal transmission of the virus. All the nucleotide sequences found in this study had high nucleotide identities of 95%-99 % and were closely related to Megalocytivirus genotype I infectious spleen and kidney necrosis virus. Risk factors associated with Megalocytivirus include water temperature, dissolved oxygen (DO), pH, ammonia, nitrate, nitrite, and the life stages of P. scalare. High Megalocytivirus infection was detected when the water temperature, DO, and pH were high in P. scalare, high water temperature and nitrate in the water samples, and the same rate of Megalocytivirus infection in P. scalare fry and juveniles.

Conclusion

This is the first study to confirm the existence of different possible routes of megalocytivirus distribution in ornamental fish farms in Malaysia. Nevertheless, the connection between the mode of transmission and the risk factors for this virus needs to be explored further to recognize the evolution and potential new host species.

The Inactivated ISKNV-I Vaccine Confers Highly Effective Cross-Protection against Epidemic RSIV-I and RSIV-II from Cultured Spotted Sea Bass Lateolabrax maculatus

The genus Megalocytivirus of the family Iridoviridae is composed of two distinct species, namely, infectious spleen and kidney necrosis virus (ISKNV) and scale drop disease virus (SDDV), and both are important causative agents in a variety of bony fish worldwide. Of them, the ISKNV species is subdivided into three genotypes, namely, red seabream iridovirus (RSIV), ISKNV, and turbot reddish body iridovirus (TRBIV), and a further six subgenotypes, RSIV-I, RSIV-II, ISKNV-I, ISKNV-II, TRBIV-I, and TRBIV-II. Commercial vaccines derived from RSIV-I , RSIV-II and ISKNV-I have been available to several fish species. However, studies regarding the cross-protection effect among different genotype or subgenotype isolates have not been fully elucidated. In this study, RSIV-I and RSIV-II were demonstrated as the causative agents in cultured spotted seabass, Lateolabrax maculatus, through serial robust evidence, including cell culture-based viral isolation, whole-genome determination and phylogeny analysis, artificial challenge, histopathology, immunohistochemistry, and immunofluorescence as well as transmission electron microscope observation. Thereafter, a formalin-killed cell (FKC) vaccine generated from an ISKNV-I isolate was prepared to evaluate the protective effects against two spotted seabass original RSIV-I and RSIV-II. The result showed that the ISKNV-I-based FKC vaccine conferred almost complete cross-protection against RSIV-I and RSIV-II as well as ISKNV-I itself. No serotype difference was observed among RSIV-I, RSIV-II, and ISKNV-I. Additionally, the mandarin fish Siniperca chuatsi is proposed as an ideal infection and vaccination fish species for the study of various megalocytiviral isolates. IMPORTANCE Red seabream iridovirus (RSIV) infects a wide mariculture bony fish and has resulted in significant annual economic loss worldwide. Previous studies showed that the phenotypic diversity of infectious RSIV isolates would lead to different virulence characteristics, viral antigenicity, and vaccine efficacy as well as host range. Importantly, it is still doubted whether a universal vaccine could confer the same highly protective effect against various genotypic isolates. Our study here presented enough experimental evidence that a water in oil (w/o) formation of inactivated ISKNV-I vaccine could confer almost complete protection against RSIV-I and RSIV-II as well as ISKNV-I itself. Our study provides valuable data for better understanding the differential infection and immunity among different genotypes of ISKNV and RSIV isolates in the genus Megalocytivirus.

Genotypic Characterization of Infectious Spleen and Kidney Necrosis Virus (ISKNV) in Southeast Asian Aquaculture

Infectious spleen and kidney necrosis virus (ISKNV) is a species within the genus Megalocytivirus (family Iridoviridae), which causes high mortality disease in many freshwater and marine fish species. ISKNV was first reported in Asia and is an emerging threat to aquaculture with increasing global distribution, in part due to its presence in ornamental fish with clinical and subclinical infections. The species ISKNV includes three genotypes: red seabream iridovirus (RSIV), turbot reddish body iridovirus (TRBIV), and ISKNV. There is an increasing overlap in the recognized range of susceptible fish hosts and the geographic distribution of these distinct genotypes. To better understand the disease caused by ISKNV, a nucleic acid hybridization capture enrichment was used prior to sequencing to characterize whole genomes from archived clinical specimens of aquaculture and ornamental fish from Southeast Asia (n = 16). The method was suitable for tissue samples containing 2.50 × 104–4.58 × 109 ISKNV genome copies mg−1. Genome sequences determined using the hybridization capture method were identical to those obtained directly from tissues when there was sufficient viral DNA to sequence without enrichment (n = 2). ISKNV genomes from diverse locations, environments, and hosts had very high similarity and matched established genotype classifications (14 ISKNV genotype Clade 1 genomes with >98.81% nucleotide similarity). Conversely, two different genotypes were obtained at the same time and location (RSIV and ISKNV from grouper, Indonesia with 92.44% nucleotide similarity). Gene-by-gene analysis with representative ISKNV genomes identified 59 core genes within the species (>95% amino acid identity). The 14 Clade 1 ISKNV genomes in this study had 100% aa identity for 92–105 of 122 predicted genes. Despite high overall sequence similarity, phylogenetic analyses using single nucleotide polymorphisms differentiated isolates from different host species, country of origin, and time of collection. Whole genome studies of ISKNV and other megalocytiviruses enable genomic epidemiology and will provide information to enhance disease control in aquaculture.

Partial validation of a TaqMan quantitative polymerase chain reaction for the detection of the three genotypes of Infectious spleen and kidney necrosis virus

Megalocytiviruses (MCVs) are double-stranded DNA viruses known to infect important freshwater and marine fish species in the aquaculture, food, and ornamental fish industries worldwide. Infectious spleen and kidney necrosis virus (ISKNV) is the type species within the genus Megalocytivirus that causes red seabream iridoviral disease (RSIVD) which is a reportable disease to the World Animal Health Organization (WOAH). To better control the transboundary spread of this virus and support WOAH reporting requirements, we developed and partially validated a TaqMan real-time qPCR assay (ISKNV104R) to detect all three genotypes of ISKNV, including the two genotypes that cause RSIVD. Parameters averaged across 48 experiments used a 10-fold dilution series of linearized plasmid DNA (107-101 copies), carrying a fragment of the three-spot gourami iridovirus (TSGIV) hypothetical protein revealed that the assay was linear over 7 orders of magnitude (107-101), a mean efficiency of 99.97 ± 2.92%, a mean correlation coefficient of 1.000 ± 0.001, and a limit of detection (analytical sensitivity) of ≤10 copies of TSGIV DNA. The diagnostic sensitivity and specificity for the ISKNV104R qPCR assay was evaluated and compared to other published assays using a panel of 397 samples from 21 source populations with different prevalence of ISKNV infection (0-100%). The diagnostic sensitivity and specificity for the ISKNV104R qPCR assay was 91.99% (87.28-95.6; 95% CI) and 89.8% (83.53-94.84). The latent class analysis showed that the ISKNV104R qPCR assay had similar diagnostic sensitivities and specificities with overlapping confidence limits compared to a second TaqMan qPCR assay and a SYBR green assay. This newly developed TaqMan assay represents a partially validated qPCR assay for the detection of the three genotypes of the species ISKNV. The ISKNV104R qPCR assay once fully validated, will serve as an improved diagnostic tool that can be used for ISKNV surveillance efforts and diagnosis in subclinical fish to prevent further spread of MCVs throughout the aquaculture and ornamental fish industries.

Scale Drop Disease Virus Associated Yellowfin Seabream (Acanthopagrus latus) Ascites Diseases, Zhuhai, Guangdong, Southern China: The First Description

Scale drop disease virus (SDDV), an emerging piscine iridovirus prevalent in farmed Asian seabass Lates calcarifer in Southeast Asia, was firstly scientifically descripted in Singapore in 2015. Here, an SDDV isolate ZH-06/20 was isolated by inoculating filtered ascites from diseased juvenile yellowfin seabream into MFF-1 cell. Advanced cytopathic effects were observed 6 days post-inoculation. A transmission electron microscopy examination confirmed that numerous virion particles, about 140 nm in diameter, were observed in infected MFF-1 cell. ZH-06/20 was further purified and both whole genome and virion proteome were determined. The results showed that ZH-06/20 was composed of 131,122 bp with 135 putative viral proteins and 113 of them were further detected by virion proteome. Western blot analysis showed that no (or weak) cross-reaction was observed among several major viral proteins between ZH-06/20 and ISKNV-like megalocytivirus. An artificial challenge showed that ZH-06/20 could cause 100% death to juvenile yellowfin seabream. A typical sign was characterized by severe ascites, but not scale drop, which was considerably different from SDD syndrome in Asian seabass. Collectively, SDDV was confirmed, for the first time, as the causative agent of ascites diseases in farmed yellowfin seabream. Our study offers useful information to better understanding SDDV-associated diseases in farmed fish.