A Novel, Reverse Transcription, Droplet Digital PCR Assay for the Combined, Sensitive Detection of Severe Acute Respiratory Syndrome Coronavirus 2 with Swine Acute Diarrhea Syndrome Coronavirus

Background Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has rapidly spread over the world since its emergence. Although the dominant route of SARS-CoV-2 infection is respiratory, a number of studies revealed infection risk from contaminated surfaces and products, including porcine-derived food and other products. The SARS-CoV-2 outbreak has been severely threatening public health, and disrupting porcine products trade and the pig industry. Swine acute diarrhea syndrome coronavirus (SADS-CoV), which was responsible for large-scale, fatal disease in piglets, emerged in 2017 and has caused enormous economic losses in the pig industry. Currently, reverse transcription real-time PCR (RT-rPCR) is the gold standard method for SARS-CoV-2 diagnosis and is most commonly used for SADS-CoV detection. However, inaccurate detection of the SARS-CoV-2 infection obtained by RT-rPCR is increasingly reported, especially in specimens with low viral load. Objective This study aimed to develop an accurate reverse transcription droplet digital PCR (RT-ddPCR) assay for the detection of SARS-CoV-2 and SADS-CoV simultaneously. Methods Two pairs of primers and one double-quenched probe targeting the RNA-dependent RNA polymerase (RDRP) region of the open reading frame 1ab (ORF1ab) gene of SARS-CoV-2 and the corresponding ORF1ab region of SADS-CoV were designed to develop the RT-ddPCR assay. The sensitivity, specificity, repeatability, and reproducibility were tested using complementary RNAs (cRNAs) and clinical specimens. Results The detection limits of RT-ddPCR were 1.48 +/- 0.18 and 1.38 +/- 0.17 copies in a 20 mu L reaction for SARS-CoV-2 and SADS-CoV cRNAs, respectively (n = 8), showing approximately 4- and 10-fold greater sensitivity than the RT-rPCR assay. This assay also exhibited good specificity, repeatability, and reproducibility. Conclusion The established RT-ddPCR assay was shown to be a highly effective, accurate, and reliable method for the sensitive detection of SARS-CoV-2 and SADS-CoV.