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Scale drop disease virus (SDDV) is an emerging viral pathogen that has caused outbreaks in farmed barramundi (Lates calcarifer Bloch, 1,790) in several SouthEast Asian countries including Singapore,

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Infectious spleen and kidney necrosis virus (ISKNV) is the sh pathogenic virus belonging to the genus Megalocytivirus of the family Iridoviridae. In 2018, disease occurrences (40-50% cumulative mortality) associated with ISKNV infection have been reported in grown-out Asian sea bass (Lates calcarifer) cultured in the inland freshwater system in Thailand. Clinical samples were collected from seven distinct farms located in the eastern and central regions of Thailand. The moribund sh showed various abnormal signs including lethargy, pale gill, darkened body, and skin hemorrhage, while the basophilic hypertrophied cell in gill, liver, and kidney tissue was observed microscopically. ISKNV infection was con rmed in 6 out of 7 farms using virus-speci c semi-nested PCR. MCP and ATPase genes showed 100% identity among virus isolates which also classi ed the virus into ISKNV genotype I clade. Koch's postulates were later con rmed by challenge assay and the mortality of the experimentally infected sh at 21 days post-challenge was 50-90% depending on the challenge dose. The complete genome of two ISKNV isolates, namely KU1 and KU2, was recovered directly from the infected specimens using the shotgun metagenomics approach. The genome length of ISKNV KU1 and KU2 were 111,487 and 111,610 bp, respectively. In comparison to the closely related ISKNV strains, the ISKNV KU1 and KU2 harbored nine unique genes, including caspase recruitment domaincontaining protein potentially involved in apoptosis inhibition. Collectively, this study indicated the homologous ISKNV strains affecting the inland cultured Asian sea bass which emphasized that the ISKNV genotype I should be prioritized for future vaccine research.

The present study describes a simultaneous infection of a novel Chlamydia-like organism (CLO) with a Myxozoa parasite, Henneguya sp. in snakeskin gourami Trichopodus pectoralis in Thailand. A new CLO is proposed “Candidatus Piscichlamydia trichopodus” (CPT) based on 16S rRNA phylogenetic analysis. Systemic intracellular CPT infection was confirmed by histological examination, in situ hybridization, PCR assay, and sequencing of 16S rRNA. This novel pathogen belongs to the order Chlamydiales but differs in certain aspects from other species. The histopathological changes associated with CPT infection were different from the typical pathological lesions of epitheliocystis caused by previously known CLO. Unlike other CLO, CPT localized in the connective tissue rather than in the epithelial cells and formed smaller clumps of intracellular bacteria that stained dark blue with hematoxylin. On the other hand, typical myxospores of the genus Henneguya with tails were observed in the gill sections. Infection with Henneguya sp. resulted in extensive destruction of the gill filaments, most likely leading to respiratory distress. Due to the frequency of co-infections and the unavailability of culture methods for CLO and Henneguya sp., it was difficult to determine which pathogens were directly responsible for the associated mortality. However, co-infections may increase the negative impact on the host and the severity of the disease. Given the commercial importance of the snakeskin gourami and its significant aquaculture potential, the findings of this study are important for further studies on disease prevention.

Tilapia lake virus (TiLV), a major pathogen of farmed tilapia, is known to be vertically transmitted. Here, we hypothesize that Nile tilapia (Oreochromis niloticus) broodstock immunized with a TiLV inactivated vaccine can mount a protective antibody response and passively transfer maternal antibodies to their fertilized eggs and larvae. To test this hypothesis, three groups of tilapia broodstock, each containing four males and eight females, were immunized with either a heat-killed TiLV vaccine (HKV), a formalin-killed TiLV vaccine (FKV) (both administered at 3.6 ×106 TCID50 per fish), or with L15 medium. Booster vaccination with the same vaccines was given 3 weeks later, and mating took place 1 week thereafter. Broodstock blood sera, fertilized eggs and larvae were collected from 6–14 weeks post-primary vaccination for measurement of TiLV-specific antibody (anti-TiLV IgM) levels. In parallel, passive immunization using sera from the immunized female broodstock was administered to naïve tilapia juveniles to assess if antibodies induced in immunized broodstock were protective. The results showed that anti-TiLV IgM was produced in the majority of both male and female broodstock vaccinated with either the HKV or FKV and that these antibodies could be detected in the fertilized eggs and larvae from vaccinated broodstock. Higher levels of maternal antibody were observed in fertilized eggs from broodstock vaccinated with HKV than those vaccinated with FKV. Low levels of TiLV-IgM were detected in some of the 1–3 day old larvae but were undetectable in 7–14 day old larvae from the vaccinated broodstock, indicating a short persistence of TiLV-IgM in larvae. Moreover, passive immunization proved that antibodies elicited by TiLV vaccination were able to confer 85% to 90% protection against TiLV challenge in naïve juvenile tilapia. In conclusion, immunization of tilapia broodstock with TiLV vaccines could be a potential strategy for the prevention of TiLV in tilapia fertilized eggs and larvae, with HKV appearing to be more promising than FKV for maternal vaccination.

Snakeskin gourami is among a few native species of Southeast Asia that has a long cultural history with Thailand is the biggest producer. Diseases are among the major setbacks in the farming of this species. However, studies of disease occurrence in snakeskin gourami are still very scarce. In the present study, we describe firstly, the systemic pathology caused by a novel Chlamydia-like organism (CLO) and comment on the gill parasite Henneguya sp. (Myxosporea, Myxobolidae) infecting the same fish, based on molecular analysis and histopathology observations.

Tilapia lake virus (TiLV), a major pathogen of farmed tilapia, is known to be vertically transmitted. Here, we hypothesize that Nile tilapia (Oreochromis niloticus) broodstock immunized with a TiLV inactivated vaccine can mount a protective antibody response and passively transfer maternal antibodies to their fertilized eggs and larvae. To test this hypothesis, three groups of tilapia broodstock, each containing 4 males and 8 females, were immunized with either a heat-killed TiLV vaccine (HKV), a formalin-killed TiLV vaccine (FKV) (both administered at 3.6 ×106 TCID50 per fish), or with L15 medium. Booster vaccination with the same vaccines was given 3-weeks later, and mating took place 1 week thereafter. Broodstock blood sera, fertilized eggs and larvae were collected from 6-14 weeks post-primary vaccination for measurement of TiLV-specific antibody (anti-TiLV IgM) levels. In parallel, passive immunization using sera from the immunized female broodstock was administered to naïve tilapia juveniles to assess if antibodies induced in immunized broodstock were protective. The results showed that anti-TiLV IgM was produced in the majority of both male and female broodstock vaccinated with either the HKV or FKV and that and that these antibodies could be detected in the fertilized eggs and larvae from vaccinated broodstock. Higher levels of maternal antibody were observed in fertilized eggs from broodstock vaccinated with HKV than those vaccinated with FKV. Low levels of TiLV-IgM were detected in some of the 1-3-day old larvae but were undetectable in 7-14-day old larvae from the vaccinated broodstock, indicating a short persistence of TiLV-IgM in larvae. Moreover, passive immunization proved that antibodies elicited by TiLV vaccination were able to confer 85% to 90% protection against TiLV challenge in naïve juvenile tilapia. In conclusion, immunization of tilapia broodstock with TiLV vaccines could be a potential strategy for the prevention of TiLV in tilapia fertilized eggs and larvae, with HKV appearing to be more promising than FKV for maternal vaccination.