Establishment of a recombinase polymerase amplification (RPA) fluorescence assay for the detection of swine acute diarrhea syndrome coronavirus (SADS-CoV)

Establishment of a novel double-antibody sandwich fluorescence microsphere immunochromatographic test strip for rapid detection of swine acute diarrhea syndrome coronavirus (SADS-CoV) infection

Introduction

Swine acute diarrhea syndrome coronavirus (SADS-CoV) is an enveloped, positive-sense, single-stranded RNA virus that causes clinical symptoms such as vomiting and diarrhea in 10-day-old piglets. SADS-CoV has caused significant economic losses in the swine industry in southern China. Currently, no effective treatments or vaccines are available for this disease, making it crucial to establish a point-of-care testing (POCT) technology for early diagnosis and prevention.

Methods

In this study, we first validated the specificity and immunogenicity of four monoclonal antibodies (mAbs) targeting the nucleocapsid (N) protein of swine acute diarrhea syndrome coronavirus (SADS-CoV). The optimal antibody pair for constructing the fluorescent microsphere-based immunochromatographic assay (FM-ICA) was determined through systematic pairwise screening. Critical parameters of the FM-ICA test strip, including antibody labeling concentration, coating concentration, incubation time, and sample dilution ratio, were subsequently optimized. Analytical performance characteristics of the developed FM-ICA were then rigorously evaluated. Finally, clinical validation was conducted by parallel testing of 72 field samples using both FM-ICA and quantitative PCR (qPCR), followed by concordance rate analysis.

Results

First, we demonstrated that all four monoclonal antibodies exhibited favorable immunogenicity and specificity. Subsequently, mAb 12E1 was identified as the coating antibody, and mAb 5G12 was selected as the labeled antibody, forming the optimal combination for FM-ICA preparation. After optimization, the ideal parameters were determined: a labeling concentration of 200 μg/mg for antibodies, a coating concentration of 1 mg/mL, an incubation time of 10 min, and a dilution factor of 10. The FM-ICA exhibited outstanding specificity, sensitivity, reproducibility, and stability, achieving a maximum detectable dilution factor of 1280 and a limit of detection (LOD) of 78 PFU mL⁻¹. Finally, the concordance rate between FM-ICA and qPCR for clinical samples reached 97.22%.

Discussion

These results indicate that FM-ICA is an excellent POCT technology that can be used for the early diagnosis of SADS-CoV, providing support for disease prevention and treatment.

A filter pad design-based multiplexed lateral flow immunoassay for rapid simultaneous detection of PDCoV, TGEV, and PEDV in swine feces

Swine Enteric Coronaviruses (SECoVs), with high lethality and infectiousness, are the main pathogens causing fatal and watery diarrhea in piglets and spreading globally. Moreover, these SECoVs can cause similar clinical manifestations and are often co-infected, requiring an accurate assay suitable for rapid, in situ, and differential detection. Here, we developed a multiplexed fluorescent-based lateral flow immunoassay (mFB-LFIA) for the detection of three SECoVs, including porcine delta coronaviruses (PDCoV), transmissible gastroenteritis virus (TGEV), and porcine epidemic diarrhea virus (PEDV), in swine fecal samples. Thanks to the filter pad design and reasonable optimization, the mFB-LFIA was achieved within 15 min for three SECoVs detection simultaneously and improved the tolerance of the strips for feces samples. The limit of detection (LoD) of detecting PDCoV, TGEV, and PEDV were 2.1 × 104 TCID50 mL-1, 3.4 × 102 TCID50 mL-1, and 3.6 × 102 TCID50 mL-1, respectively. Additionally, the proposed assay was successfully applied to the detection of PDCoV, TGEV, and PEDV in swine feces with high accuracy. Compared with the gold standard nucleic acid testing, the total coincidence rate of the proposed assay was more than 90 %. Moreover, the mFB-LFIA performed excellent stability and repeatability. The proposed mFB-LFIA allows for rapid, in situ, more cost-effective and simultaneous detection of PDCoV, TGEV, and PEDV compared with nucleic acid testing. To the best of our knowledge, this is the first report to describe a multiplexed point-of-care assay capable of detecting PDCoV, TGEV, and PEDV in swine fecal samples. We believe our approach has a great potential for application to pig farm.

Rapid detection of blood using a novel application of RT-RPA integrated with CRISPR-Cas: ALAS2 detection as a model

A rapid, sensitive and specific test for blood is reported based on a novel application of recombinase polymerase amplification integrated with CRISPR-Cas and lateral flow assay (LFA). The blood specific marker ALAS2 was used as the target to record the presence of blood. The assay used either RNA extracted from a body fluid as a template, or omitting this extraction step and using a direct approach where the questioned body fluid was added directly to the assay. The assay only detected blood (all peripheral blood and some menstrual blood samples) and no other body fluid (semen, saliva, or vaginal fluid). The limit of detection varied from an initial template of 0.195 ng extracted RNA (27 dilution) or 0.0218 μL (26 dilution) liquid peripheral blood. The assay gave the expected result when peripheral blood was mixed with saliva: ratios of peripheral blood/saliva at 19:1, 3:1, 1:1, 1:3 and 1:19 all gave a positive result using extracted RNA. By contrast, only three ratios of peripheral blood and saliva gave a positive result for blood (19:1, 3:1 and 1:1) when adding these two body fluids directly. When peripheral blood was mixed with semen there was a strong inhibition of the assay and ALAS2 could only be detected at ratio of 19:1 using RNA. Using reconstituted peripheral bloodstains gave comparable results to liquid peripheral blood. This is the first application of RT-RPA integrated CRISPR and combined with a LFA assay to detect body fluid-specific RNA. The proposed method opens up the potential to perform this method remote from laboratories such as at crime scenes.

A multiplex real-time RT-qPCR assay for simultaneous detection of porcine epidemic diarrhea virus, porcine deltacoronavirus, and swine acute diarrhea syndrome coronavirus

Porcine epidemic diarrhea virus (PEDV), porcine deltacoronavirus (PDCoV), and swine acute diarrhea syndrome coronavirus (SADS-CoV) cause intestinal diseases with similar manifestations in suckling piglets. In this study, we developed a multiplex real-time PCR for differential diagnosis of PEDV, PDCoV, and SADS-CoV. The assay demonstrated high specificity with a detection limit of 5 copies/µl for each virus. The assay specifically detected PEDV, PDCoV, and SADS-CoV and excluded all other swine pathogens circulating in pigs. Furthermore, the assay exhibited satisfactory performance in analyzing clinical samples. The data indicate that the newly developed multiplex real-time PCR method can be applied for differential diagnosis of porcine enteric coronaviruses.

Emergent Molecular Techniques Applied to the Detection of Porcine Viruses

Molecular diagnostic tests have evolved very rapidly in the field of human health, especially with the arrival of the recent pandemic caused by the SARS-CoV-2 virus. However, the animal sector is constantly neglected, even though accurate detection by molecular tools could represent economic advantages by preventing the spread of viruses. In this regard, the swine industry is of great interest. The main viruses that affect the swine industry are described in this review, including African swine fever virus (ASFV), porcine reproductive and respiratory syndrome virus (PRRSV), porcine epidemic diarrhea virus (PEDV), and porcine circovirus (PCV), which have been effectively detected by different molecular tools in recent times. Here, we describe the rationale of molecular techniques such as multiplex PCR, isothermal methods (LAMP, NASBA, RPA, and PSR) and novel methods such as CRISPR-Cas and microfluidics platforms. Successful molecular diagnostic developments are presented by highlighting their most important findings. Finally, we describe the barriers that hinder the large-scale development of affordable, accessible, rapid, and easy-to-use molecular diagnostic tests. The evolution of diagnostic techniques is critical to prevent the spread of viruses and the development of viral reservoirs in the swine industry that impact the possible development of future pandemics and the world economy.