CRISPR Cas12a Based Lateral Flow Assay for Rapid Point of Care Detection of African Swine Fever Virus in Porcine Blood and Oral Fluids

Introduction

African swine fever virus (ASFV) is the etiologic agent of a highly contagious and often fatal hemorrhagic disease affecting domestic swine and wild boar. The virus, a large double stranded DNA virus of the family Asfarviridae, causes substantial economic losses and trade restrictions worldwide [1]. Rapid and accurate detection of ASFV is critical for outbreak containment and surveillance. Conventional diagnostic methods include quantitative real time PCR (qPCR), virus isolation, and serological assays such as ELISA. However, qPCR requires expensive thermocyclers and trained personnel, making it challenging for field deployment [2, 3]. In contrast, CRISPR Cas12a based lateral flow assays offer a rapid, isothermal, and equipment light alternative for point of care detection of viral nucleic acids [4, 5]. This article provides a comprehensive review of the CRISPR Cas12a lateral flow platform for ASFV detection in porcine blood, serum, and oral fluids, with an emphasis on assay design, analytical performance, and practical advantages over conventional methods.

Assay Architecture and Mechanism

The CRISPR Cas12a (also known as DETECTR) system couples isothermal nucleic acid amplification with CRISPR mediated collateral cleavage for target specific signal generation. The assay workflow involves three principal steps: sample preparation, isothermal amplification (typically recombinase polymerase amplification, RPA, or loop mediated isothermal amplification, LAMP), and Cas12a detection with lateral flow readout [4, 6].

Target Selection: Conserved p72 (B646L) Gene

The assay targets the B646L gene encoding the p72 major capsid protein, a highly conserved genomic region across all ASFV genotypes [1, 4]. A CRISPR RNA (crRNA) is designed to complement a specific 20 24 nucleotide sequence within the p72 amplicon. The crRNA guides the Cas12a nuclease to bind the target DNA, activating its non specific single stranded DNase (ssDNase) activity [5]. This collateral cleavage activity is harnessed to cleave a reporter molecule in the lateral flow detection step.

Isothermal Pre Amplification

For field deployable detection, isothermal amplification is required to increase target DNA concentration to detectable levels within 20 40 minutes. Both RPA and LAMP have been validated for ASFV detection with Cas12a [4, 6]. RPA operates at a constant temperature (37 42 degrees Celsius) using a recombinase, single stranded binding proteins, and a strand displacing polymerase. LAMP requires a higher temperature (60 65 degrees Celsius) and a set of four to six primers. Both methods achieve amplification yields comparable to PCR without the need for thermal cycling equipment [4, 6].

Lateral Flow Readout

The lateral flow strip incorporates two capture lines: a test line and a control line. The reporter molecule is a dual labeled ssDNA probe (e.g., a FAM biotin conjugate) that is cleaved by activated Cas12a [6]. In a typical format, gold nanoparticles conjugated to anti FAM antibodies are dried on the conjugate pad. When the reporter is intact, it bridges the gold nanoparticles to the test line via biotin streptavidin interaction, producing a visible signal. Cleavage of the reporter prevents this bridging, resulting in a reduction or absence of test line signal. A positive sample (ASFV DNA present) therefore yields a negative test line and a positive control line; a negative sample yields both lines [6]. Some assay designs invert this logic, but the underlying principle remains collateral cleavage triggered signal modulation.

Diagnostic Workflow

The following Mermaid diagram illustrates the step by step process from sample collection to result interpretation.

graph TD
    A[Porcine blood or oral fluid sample], > B[DNA extraction (rapid column or boil method)]
    B, > C[Isothermal amplification (RPA or LAMP) 20-40 min]
    C, > D[Cas12a detection reaction with crRNA and reporter]
    D, > E[Lateral flow strip incubation 5-10 min]
    E, > F[Visual or automated readout]
    F, > G[Positive: test line absent / weak; control line present]
    F, > H[Negative: both test and control lines present]

Analytical Sensitivity and Specificity

Reported limits of detection for CRISPR Cas12a based lateral flow assays for ASFV range from 1 to 10 copies per microliter of input DNA [4, 6]. This sensitivity is comparable to that of qPCR, which typically detects 10 100 copies per reaction [2, 3]. The isothermal amplification step is critical for achieving these detection thresholds; without pre amplification, the Cas12a collateral cleavage alone cannot generate sufficient signal from low copy number targets [5].

Cross reactivity testing against other porcine viruses is essential to confirm assay specificity. The crRNA targeting the p72 gene shows no cross reactivity with porcine reproductive and respiratory syndrome virus (PRRSV), porcine circovirus type 2 (PCV2), or swine influenza A virus (SIV) [4, 6]. These results are consistent with the high sequence conservation of the p72 target region among ASFV genotypes and its absence in other swine pathogens.

Validation on Porine Blood, Serum, and Oral Fluids

Validation studies have been conducted using spiked and field derived samples. Whole blood, serum, and oral fluids are the primary specimen types [1, 4, 6]. In spiking experiments, EDTA whole blood and serum were inoculated with known concentrations of inactivated ASFV. Oral fluids were collected using cotton ropes and processed by centrifugation to remove debris. All sample types underwent a rapid DNA extraction step (e.g., boil and spin or commercial silica column kits) before isothermal amplification [4]. The lateral flow assay correctly identified positive samples at concentrations as low as 10 copies per reaction in all three matrices [4, 6].

A study by Wang et al. demonstrated 100% concordance with qPCR for 50 field samples (25 blood, 25 oral fluid) from ASFV infected herds [6]. Mao et al. reported similar performance using multiple cross displacement amplification combined with a nanoparticle based lateral flow biosensor, achieving a limit of detection of 2 copies per microliter and no cross reactivity with PRRSV, PCV2, or SIV [4].

Comparison with Conventional Diagnostic Methods

Table 1 provides a comparison of the CRISPR Cas12a lateral flow assay with qPCR and ELISA for ASFV detection.

Table 1: Comparative Performance of Diagnostic Methods for African Swine Fever Virus

Advantages and Limitations

The primary advantage of the CRISPR Cas12a lateral flow platform is its field readiness. The elimination of thermal cycling and fluorescence detection reduces instrument cost and power requirements, enabling testing in low resource settings and on farm environments [4, 5, 6]. The total turnaround time of under one hour, from sample to result, supports rapid outbreak response and culling decisions [1, 4]. Additionally, the visual readout eliminates the need for specialized software or trained technicians [6].

Limitations include the need for a rapid DNA extraction step, which adds 10 15 minutes and consumable costs [4]. The lateral flow strips are single use and subject to lot to lot variability. The assay also requires cold chain storage for reagents, though lyophilized formats are in development [5]. False positives can occur due to contamination with amplicons, requiring strict adherence to spatial separation of pre and post amplification steps.

Linkages to Related Diagnostics and Surveillance

This technology aligns with broader advances in point of care molecular diagnostics for swine and other veterinary species. Readers are directed to the article on CRISPR Cas12a and Cas13a Platforms for Rapid Veterinary Viral Diagnostics for a general overview. For applications in other pathogens, see Point of Care Molecular Diagnostics for Feline Upper Respiratory Pathogens: FHV 1, FCV, and Bordetella. Multiplex detection strategies using digital droplet PCR for PRRSV and SIV are described in Multiplex Digital Droplet PCR (ddPCR) for Simultaneous Detection of Porcine Reproductive and Respiratory Syndrome Virus (PRRSV) and Swine Influenza A Virus (SIV) in Oral Fluid Samples. For comprehensive disease information, the African Swine Fever Virus reference page provides detailed virology, epizootiology, and biosecurity measures. Computational modeling of ASFV spread based on diagnostic data is covered in African Swine Fever: Computational Models for Early Detection and Spread Prediction in Wild Boar Populations.

Future Directions

Future improvements include the development of multiplex CRISPR Cas12a assays for simultaneous detection of ASFV and other swine pathogens, such as classical swine fever virus and PRRSV [3, 4]. Integration with microfluidic lab on a chip platforms (see Microfluidic Lab on a Chip for Point of Care Veterinary Diagnostics) could further reduce sample volume and automate extraction and amplification. The use of lyophilized reagents and preloaded lateral flow strips would enhance shelf life and field stability [5]. Automated readout using smartphone based image analysis is also under investigation and could improve consistency and data recording for surveillance programs [1].

Conclusion

The CRISPR Cas12a based lateral flow assay represents a significant advancement in rapid, point of care detection of African swine fever virus. By combining isothermal amplification with Cas12a collateral cleavage and lateral flow readout, the platform achieves analytical sensitivity comparable to qPCR while eliminating the need for thermal cycling and fluorescence detection. Validation studies on porcine blood, serum, and oral fluids demonstrate robust performance with no cross reactivity to common porcine viruses. Field deployable, cost effective, and rapid, this diagnostic approach is well suited for on farm surveillance, outbreak response, and resource limited settings. Continued optimization and integration with other emerging technologies will further enhance its utility in global ASFV control efforts.

References

[1] Olesen AS, Valadkhani B, Johnston CM, et al. A rapid detection method for African swine fever virus antigens in serum and whole blood samples using an automated machine reading tool for lateral flow assays. J Virol Methods. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42229692/

[2] Chen J, Shi Z, Ru Y, et al. Rapid and visual dual-color lateral flow immunoassay for african swine fever virus antibody detection. Appl Microbiol Biotechnol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41673330/

[3] Chen J, Shi Z, Ru Y, et al. Establishment and Evaluation of a Multicolor Latex Microsphere-Based Lateral Flow Immunoassay for the Simultaneous Detection of Antibodies Against African and Classical Swine Fever Viruses. Transbound Emerg Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41550267/

[4] Mao S, Zhang R, Yang X, et al. Ultra-rapid and sensitive detection of African swine fever virus using multiple cross displacement amplification combined with nanoparticle-based lateral flow biosensor. Front Microbiol. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39651348/

[5] Aira C, Monedero A, Hernández-Antón S, et al. Improving African Swine Fever Surveillance Using Fluorescent Rapid Tests. Pathogens. 2023. URL: https://pubmed.ncbi.nlm.nih.gov/37375501/

[6] Wang Z, Yu W, Xie R, et al. A strip of lateral flow gene assay using gold nanoparticles for point-of-care diagnosis of African swine fever virus in limited environment. Anal Bioanal Chem. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/34018036/ *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.