Cyprinid herpesvirus 2 (CyHV‐2) is a viral pathogen worldwide and causing high mortality on goldfish and silver crucian carp (Carassius auratus gibelio). In order to establish a stable and sensitive immunological diagnostic approach, the recombinant ORF121 protein encoded by the CyHV‐2 ORF121 gene, was selected as a capture antigen to identify cells and tissues infected with CyHV‐2 by immunological methods in this study. Firstly, the open reading frame of CyHV‐2 ORF121 was cloned into the PGEX‐4T‐3 vector and expressed in Escherichia coli. Purified recombinant ORF121 protein was then used as an antigen to prepare monoclonal antibodies, and an efficient hybridoma cell line was selected by dot‐blot assay. The resulting mAb‐3D9 was applied to detect CyHV‐2 in infected caudal fin of Carassius auratus gibelio (GiCF) cells and fish tissues by western blotting, immunofluorescence assays and immunohistological asays. The monoclonal antibody could specifically identify CyHV‐2 in infected GiCF cells and the gills, the kidney and the spleen tissues, and it could attenuate CPE by CyHV‐2 in vitro, suggesting it can be applied for CyHV‐2 detection in the crucian carp and ORF121 may be a candidate vaccine against CyHV‐2.

Infections of Cyprinid herpesvirus 2 in goldfish and farmed crucian carp (Carassius auratus gibelio) are still an urgent problem worldwide. Detection and prevention are necessary for the control of haematopoietic necrosis disease caused by CyHV‐2. Although many sensitive molecular diagnostic methods have been developed, effective immunodiagnosis and neutralization approaches based on monoclonal antibodies (MAbs) against CyHV‐2 are still important to CyHV‐2 study. In this experiment, purified CyHV‐2 was used as antigens to produce monoclonal antibodies (Mabs). Six Mabs bound to different proteins were selected by Dot‐blot screening and Western‐blot analysis, and no one had cross‐reactivity with closely related koi herpesvirus. Among them, Mabs 2E1‐B10, 1F5‐A3 and 4C4‐A7 belonged to IgG1 isotype, while other three Mabs 3G9‐B11, 3B4‐G5 and 4F4‐B7 belonged to IgM isotype. These six Mabs all could specifically detect CyHV‐2 in CyHV‐2 infected caudal fin of Carassius auratus gibelio (GiCF) cells by immunofluorescence assays. Then, the neutralization ability was tested in vitro, and the result showed that all six Mabs can attenuate CPE by CyHV‐2 in vitro among which 2E1‐B10 had the best neutralization ability. The virus proteins recognized by these six Mabs were identified by mass spectrometry identification, and the result showed they probably were ORF88, ORF55R, ORF115 and ORF151R. This study is the first to prepare Mabs by purifying CyHV‐2, which will provide a practical basis for the in‐depth study of CyHV‐2 virus.

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Cyprinid herpesvirus 3 (CyHV‐3) is the main pathogen of koi herpesvirus disease (KHVD), which has caused serious damage to the ornamental and food‐producing carp industry. Effective and rapid on‐site detection methods are needed for early diagnosis of CyHV‐3. A lateral flow immuno‐chromatographic assay (LFIA) using two specific anti‐CyHV‐3 monoclonal antibodies has been developed and validated for on‐site detection of CyHV‐3. MAb 3C9 was used to bio‐conjugate CyHV‐3 antigen with colloidal gold, and MAb 2A8 was used to capture antigen bound colloidal gold on the test line. The control line was lined with goat anti‐mouse IgG to capture unbound colloidal gold to validate performance. The test results can be viewed within 10 min after putting the strip into CyHV‐3 virus infection fluid. The lowest limit of detection for the LFIA test was found to be 1.5 × 104 copies/μL and it showed no cross‐reactivity with other fish viral pathogens. The specificity of the strip was 100% when spleen and kidney tissues of CyHV‐3‐infected and healthy koi were validated at the field level. The LFIA strip will be an effective device for the early detection of CyHV‐3 in the future.