Development and evaluation of reverse transcription loop-mediated isothermal amplification assay for the detection of the fathead minnow nidovirus

Fathead minnow nidovirus (FHMNV) is a serious baitfish-pathogenic virus in North America. Studies to trace the spread of the virus and determine its host range are hampered by the absence of reliable diagnostic assays. In this study, a one-step, reverse transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed that targets a region in the FHMNV spike protein gene. The assay was optimized, and the best results were obtained at 8 mM of Mg2+ with an incubation time of 40 min at 63 °C in the presence of calcein. The analytical sensitivity of the RT-LAMP method was estimated to be as low as 5 viral copies and was 1000-fold more sensitive than the conventional reverse transcription polymerase chain reaction (RT-PCR) method. The diagnostic sensitivity and specificity of the developed RT-LAMP assay versus the RT-PCR assay was 100% and 95.7%, respectively. A quantitative RT-LAMP of FHMNV with a high correlation coefficient (r2 = 0.9926) was also developed and the result of quantitation of viral copies in tissue samples of infected fish showed that the viral loads of the infected fish tissue samples reached up to 4.7 × 1010 copies per mg. It is anticipated that the developed RT-LAMP and quantitative RT-LAMP methods will be instrumental for diagnosis and surveillance of FHMNV.