Development of cross-priming amplification (CPA) combined with colorimetric and lateral flow dipstick visualization for scale drop disease virus (SDDV) detection.

Scale drop disease virus (SDDV) is one of the most important pathogens that causes scale drop disease (SDD) in Asian sea bass (Lates calcarifer). The outbreaks of this disease are one of the factors causing substantial losses in Asian sea bass aquaculture. In this study, the uracil-DNA glycosylase (UDG)-supplemented cross-priming amplification (UCPA) combined with a colorimetric detection method using the hydroxynaphthol blue (HNB) and lateral flow dipstick (LFD) for detection of SDDV was developed. The UDG was utilized to prevent carryover contamination, and the CPA reactions can be readily observed by HNB and LFD. The CPA primers and probe were designed to target the major capsid protein (MCP) gene of the SDDV. The optimized UCPA conditions were performed at the temperature of 61°C for 60 min. The UCPA assays demonstrated specificity to SDDV without cross-reaction to other tested viruses including red-spotted grouper nervous necrosis virus (RGNNV), infectious spleen and kidney necrosis virus (ISKNV) and Lates calcarifer herpes virus (LCHV), and other bacterial species commonly found in aquatic animals. The sensitivity of the UCPA-HNB and UCPA-LFD was 100 viral copies/µl and 10 pg of extracted total DNA, which was 10-fold more sensitive than that of conventional PCR. The UCPA-HNB and UCPA-LFD assays could be used to detect the SDDV infection in all 25 confirmed SDDV-infected fish samples. Therefore, the UCPA coupled with HNB and LFD was rapid, simple and effective and might be applied for diagnosis of SDDV infection.