CRISPR-Cas13-Based Direct Detection of Porcine Reproductive and Respiratory Syndrome Virus in Oral Fluids: A Field-Deployable Molecular Platform

Abstract

Porcine reproductive and respiratory syndrome virus (PRRSV) remains a major economic burden to global swine production. Oral fluid sampling offers a non-invasive, population-level surveillance method [1, 2]. However, current molecular diagnostics rely on reverse transcription quantitative polymerase chain reaction (RT-qPCR), which requires thermal cycling infrastructure and skilled personnel [3, 4]. CRISPR-Cas13 systems provide an isothermal alternative that leverages the collateral ribonuclease activity of Cas13 to cleave fluorescent or lateral-flow reporter probes upon target RNA recognition [reviewed in general literature]. This article reviews the design, optimization, and validation of a CRISPR-Cas13-based direct detection platform for PRRSV RNA in oral fluids. Key considerations include guide RNA (gRNA) design targeting conserved genomic regions, reporter system selection (fluorescence vs. lateral flow), sample preparation methods to mitigate inhibitors and RNA degradation, and lyophilization strategies for field stability. Analytical sensitivity is compared to RT-qPCR using data from controlled and field studies [3, 4, 5]. The platform offers a rapid, instrument-free alternative for point-of-care PRRSV surveillance in swine herds.

Introduction

PRRSV is an enveloped, positive-sense single-stranded RNA virus belonging to the family Arteriviridae. It causes reproductive failure in sows and respiratory disease in growing pigs [6, 7, 8]. Oral fluids have emerged as a practical specimen for PRRSV surveillance because they aggregate viral shedding from multiple animals and are simple to collect [1, 2, 9, 10]. Numerous studies have validated oral fluid diagnostics using RT-qPCR, demonstrating high sensitivity and specificity [3, 4, 11]. However, RT-qPCR requires expensive reagents and instruments, limiting its use in low-resource or field settings.

CRISPR-Cas systems have been repurposed for nucleic acid detection. Cas13 (formerly C2c2) is an RNA-guided RNase that, upon binding a specific target RNA, activates a non-specific collateral cleavage activity. This collateral activity can be harnessed to cleave fluorescently labeled reporter RNA molecules, generating a detectable signal. Isothermal amplification steps (e.g., recombinase polymerase amplification, RPA) can precede Cas13 detection to enhance sensitivity, but direct detection (without pre-amplification) is desirable for simplicity. The present review focuses on the direct detection approach, where PRRSV genomic RNA is directly targeted by Cas13.

Oral fluid matrices contain inhibitors such as mucins, polysaccharides, and nucleases that can compromise RNA integrity [5, 11]. Sample processing methods including centrifugation, filtration, and chemical stabilization are critical. Moreover, field deployment requires lyophilized, ambient-temperature-stable reagents. We address each component of the assay workflow.

CRISPR-Cas13 Mechanism and Rationale for PRRSV Detection

The Cas13 effector protein (e.g., Cas13a from Leptotrichia wadei or Cas13b from Prevotella sp.) is guided by a CRISPR RNA (crRNA) that contains a spacer complementary to the target RNA sequence. Upon crRNA-target RNA hybridization, the Cas13 catalytic site undergoes a conformational change that activates its HEPN (higher eukaryotes and prokaryotes nucleotide-binding) domains, enabling non-specific RNase activity. This collateral cleavage degrades nearby single-stranded RNA molecules, including reporter probes. For detection, a synthetic reporter RNA labeled with a fluorophore and quencher is included; cleavage separates the fluorophore from the quencher, producing a fluorescence signal. Alternatively, reporters conjugated to gold nanoparticles or biotin can be used for lateral flow readout.

The direct detection of PRRSV RNA without pre-amplification is appealing for field use. Sensitivity of direct Cas13 detection is typically in the femtomolar to picomolar range (10e10 to 10e12 copies/mL) for viral RNA. For PRRSV, which may be present at moderate to high titers in acutely infected pigs (e.g., 10e6 to 10e9 copies/mL oral fluid), direct detection may be feasible when sample concentration is optimized.

Guide RNA Design and Target Selection

Design of crRNAs is critical for specificity and sensitivity. The conserved regions of the PRRSV genome, such as the 5' untranslated region (UTR), ORF1a, ORF7 (nucleocapsid), and the non-structural protein coding regions, should be targeted to cover both PRRSV-1 (European) and PRRSV-2 (North American) genotypes [12, 13, 14]. A multi-target approach using a pool of crRNAs can reduce the risk of false negatives due to genomic variation. In silico predictions using tools like Cas13 design algorithms can minimize off-target binding to host or commensal RNA.

Experimental validation involves synthesizing candidate crRNAs and testing against PRRSV RNA extracted from cell culture or known positive oral fluids. Comparative studies show that crRNAs targeting the ORF7 region often yield the highest sensitivity due to high copy number abundance [13]. Mismatches in the seed region (6–8 nucleotides adjacent to the protospacer flanking site) dramatically reduce cleavage efficiency, so crRNAs should be designed against sequences conserved across circulating lineages.

Reporter Systems: Fluorescence and Lateral Flow

Two major readout modalities exist for Cas13 diagnostics:

• Fluorescence-based real-time detection: A quenched fluorophore reporter (e.g., FAM–RNA–BHQ1) is added to the reaction. As Cas13 cleaves the reporter, fluorescence increases and can be measured with a simple handheld fluorometer or plate reader. This approach provides quantitative kinetic data but requires a light source and detector.
• Lateral flow strip detection: A dual-labeled reporter (e.g., FAM and biotin separated by a short RNA) is cleaved by Cas13. The resulting fragments are applied to a lateral flow strip containing anti-FAM antibodies and streptavidin. Intact reporters are captured at the control line, while cleaved reporters migrate to the test line. This yields a visible color change, eliminating the need for instruments.

Table 1 compares the two detection methods.

The lateral flow format is more field-deployable and has been used for CRISPR-based detection of other swine viruses, such as African swine fever virus, as reviewed in the article entitled "CRISPR Cas12a Based Lateral Flow Assay for Rapid Point of Care Detection of African Swine Fever Virus in Porcine Blood and Oral Fluids" (available on this portal). For PRRSV, lateral flow readout may be preferred for barn-side testing.

Sample Preparation Challenges and Solutions

Oral fluids present multiple challenges for molecular detection. The presence of mucins, proteases, RNases, and bacterial nucleic acids can inhibit Cas13 activity and degrade target RNA [5, 11]. Moreover, PRRSV RNA is labile, especially at ambient temperatures. Studies have shown that RT-qPCR detection in oral fluids is significantly affected by freeze-thaw cycles, storage temperature, and the addition of stabilizing buffers [5, 11]. For CRISPR-Cas13, similar issues apply.

Recommended sample processing steps include:

1. Collection: Use sterile cotton ropes or swabs as per standardized protocols [9, 15, 10, 16].
2. Clarification: Centrifuge oral fluid at 2000 x g for 10 min to remove debris and large inhibitory substances.
3. RNA extraction (optional): For direct detection, extraction may be omitted if the sample is diluted to reduce inhibitors. However, dilution reduces sensitivity. A rapid, heat-based lysis step (e.g., 95°C for 5 min in a buffer containing Tris-EDTA and 0.1% Tween-20) can liberate RNA without full extraction.
4. Inhibitor removal: The addition of polyvinylpyrrolidone (PVP) or bovine serum albumin (BSA) can mitigate some inhibition [5]. Filtration through 0.22 µm filters can also remove bacteria and large mucins.
5. RNA stabilization: Addition of RNase inhibitors and chelating agents (e.g., EDTA) prevents degradation.

A critical advance is the use of an internal sample control (e.g., a porcine endogenous reference gene) to monitor RNA integrity and inhibition, as described for RT-qPCR [4]. For Cas13, a synthetic RNA control can be spiked into the reaction.

Lyophilization for Field Stability

Lyophilization (freeze-drying) of Cas13 reagents enables room-temperature transport and storage, a prerequisite for field deployment. The complete reaction mixture including Cas13 enzyme, crRNA, reporter probe, buffer, and stabilizers (trehalose, sucrose, or mannitol) can be lyophilized in single-use tubes. Upon rehydration with a sample lysate, the reaction proceeds. Studies on related CRISPR platforms (e.g., Cas12a) have demonstrated that lyophilized reagents retain activity for months at ambient temperature, as reviewed in the article on this portal titled "CRISPR-Cas12a-Based Diagnostics for Rapid Detection of African Swine Fever Virus in Field Samples". For Cas13, similar formulations are effective, though careful optimization of excipient concentrations is required to prevent enzyme denaturation.

Stability testing involves accelerated aging at 37°C for several weeks and periodic testing against PRRSV RNA standards. The lyophilized formulation should have a final moisture content below 3% and be sealed under inert gas (argon) to prevent oxidation.

Analytical Sensitivity and Comparison with RT-qPCR

The limit of detection (LOD) of the direct CRISPR-Cas13 assay for PRRSV RNA in oral fluids is a key performance metric. In side-by-side comparisons with RT-qPCR (using validated assays [3, 4]), the Cas13 direct detection platform typically shows 10- to 100-fold lower sensitivity (higher LOD). For example, RT-qPCR can detect as few as 1–10 copies per reaction, whereas direct Cas13 detection may require 10e4–10e6 copies per reaction. However, because infected pigs shed high viral loads (10e7–10e9 copies/mL oral fluid), the assay can still identify positive samples during acute infection. For subclinically infected or low-shedding animals (e.g., during the late convalescent phase), sensitivity may be insufficient.

To improve sensitivity, a short isothermal amplification step (e.g., RPA, NASBA) can be added. This hybrid approach (RPA-Cas13) achieves sensitivity comparable to RT-qPCR, as demonstrated for other RNA viruses. However, this review focuses on direct detection without amplification. Strategies to boost direct detection include sample concentration (e.g., ultrafiltration, precipitation) and increasing reaction volume to accommodate larger sample input.

Table 2 summarizes analytical performance parameters.

Field Validation and Deployment

Field validation of the Cas13 assay should follow established guidelines for diagnostic test evaluation. Accuracy is assessed using oral fluids from herds with known PRRSV status as determined by RT-qPCR and sequencing [2, 17, 18, 19, 20, 21, 22]. The assay should be evaluated for cross-reactivity with other swine respiratory viruses such as swine influenza A virus, porcine circovirus type 2, and porcine respiratory coronavirus, none of which should produce a positive signal [23, 24]. Multiplexing capabilities, though not detailed here, could be added using multiple crRNA pools and orthogonal reporters.

Field deployment requires a simple, stepwise workflow:

flowchart TD
    A[Collect oral fluid from rope], > B[Centrifuge at 2000xg, 10 min]
    B, > C[Mix clarified fluid 1:1 with TE-Tween buffer]
    C, > D[Heat at 95°C for 5 min]
    D, > E[Add lysate to lyophilized Cas13 reaction]
    E, > F[Incubate at 37°C for 30 min]
    F, > G[Read fluorescence or dip lateral flow strip]
    G, > H[Interpret result: positive if signal > threshold]

This workflow can be executed by farm personnel with minimal training. A portable heat block or a warm water bath provides the incubation temperature. The lateral flow readout eliminates the need for instruments.

Conclusion

CRISPR-Cas13-based direct detection of PRRSV RNA in oral fluids represents a promising field-deployable diagnostic method. While sensitivity is lower than RT-qPCR, the assay can detect the virus during peak shedding and provides results rapidly without complex equipment. Continued improvements in Cas13 enzyme engineering, crRNA multiplexing, and sample processing will likely close the sensitivity gap. The platform is particularly valuable for outbreak screening and for farms lacking access to centralized diagnostic laboratories. Integration with swine health management strategies, as discussed in related articles on this portal (e.g., "High-Throughput Multiplex Real-Time RT-PCR for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV), Porcine Circovirus Type 2 (PCV2), and Swine Influenza A Virus (SIV) in Oral Fluids: Analytical Sensitivity and Field Validation"), can enhance herd-level surveillance.

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