CRISPR-based point-of-care diagnostics for African swine fever virus: From Cas12a detection to integrated microfluidic devices

Introduction

African swine fever virus (ASFV) is a large, enveloped DNA virus belonging to the family Asfarviridae and the sole member of the genus Asfivirus [1]. ASFV causes a highly contagious and often fatal hemorrhagic disease in domestic swine and wild boar, with mortality rates approaching 100 percent in naive populations [1]. The virus is endemic in sub-Saharan Africa, Sardinia, and parts of Eastern Europe and Asia, and its continued spread poses a severe threat to global swine production and food security [1]. Rapid and accurate detection of ASFV at the point of care is critical for implementing effective biosecurity measures, controlling outbreaks, and preventing further geographic dissemination [1].

Conventional molecular diagnostics for ASFV rely on real-time polymerase chain reaction (qPCR) performed in centralized laboratories [1]. While qPCR offers high sensitivity and specificity, it requires expensive thermal cycling equipment, trained personnel, and a stable electricity supply, all of which are often unavailable in resource-limited field settings [1]. Isothermal amplification methods such as loop-mediated isothermal amplification (LAMP) and recombinase polymerase amplification (RPA) have been developed to circumvent the need for thermal cyclers, but they still require specialized detection instruments or post-amplification processing steps [1]. The emergence of CRISPR-based diagnostic platforms has introduced a new paradigm for point-of-care molecular detection, combining the specificity of CRISPR-associated nucleases with the convenience of isothermal amplification and simple signal readout modalities [1].

This review provides a detailed technical examination of CRISPR-Cas12a and Cas13a based diagnostics for ASFV, from fundamental assay design principles to integration with microfluidic devices for field-deployable detection. The discussion encompasses crRNA targeting strategies, pre-amplification methods, signal transduction mechanisms, analytical performance metrics, and the engineering challenges associated with miniaturization and multiplexing.

CRISPR-Cas system overview for nucleic acid detection

CRISPR (clustered regularly interspaced short palindromic repeats) and CRISPR-associated (Cas) proteins constitute an adaptive immune system in bacteria and archaea [1]. For diagnostic applications, two class 2 type V and type VI effector proteins, Cas12a and Cas13a respectively, have been extensively characterized [1]. Cas12a is a DNA-guided DNA endonuclease that, upon recognition of a specific double-stranded DNA target complementary to its CRISPR RNA (crRNA), introduces a double-strand break [1]. Critically, Cas12a exhibits collateral (trans) cleavage activity: after specific target recognition, it non-specifically cleaves any single-stranded DNA (ssDNA) molecules in the reaction [1]. This property forms the basis of the DETECTR (DNA Endonuclease-Targeted CRISPR Trans Reporter) platform [1].

Cas13a, in contrast, is a RNA-guided RNA endonuclease that, upon binding to a complementary single-stranded RNA target, activates its collateral RNase activity, cleaving nearby non-target RNA molecules [1]. This mechanism underpins the SHERLOCK (Specific High-sensitivity Enzymatic Reporter Unlocking) platform [1]. Both systems enable the conversion of a nucleic acid binding event into a detectable signal, typically fluorescence or colorimetric change, through the use of quenched fluorophore-reporter molecules [1].

Assay design for ASFV detection

Target gene selection and crRNA design

The most commonly targeted genomic region for ASFV molecular detection is the p72 (B646L) gene, which encodes the major capsid protein and is highly conserved across all 24 known ASFV genotypes [1]. The p72 gene is present in all ASFV isolates and exhibits sufficient sequence divergence from other swine viruses to ensure diagnostic specificity [1]. crRNAs for Cas12a are designed to recognize a protospacer adjacent motif (PAM) sequence, typically 5'-TTTV-3' for Cas12a from Lachnospiraceae bacterium (LbCas12a) or Acidaminococcus sp. (AsCas12a), located immediately upstream of the target region [1]. For Cas13a, crRNA design requires a protospacer flanking site (PFS) motif, which varies by ortholog; for Leptotrichia wadei Cas13a (LwaCas13a), the preferred PFS is a single non-G base [1].

The crRNA spacer length is typically 20 to 24 nucleotides for Cas12a and 28 to 30 nucleotides for Cas13a [1]. Computational tools such as CRISPR design algorithms are used to select crRNA sequences with minimal off-target complementarity to the swine genome and other porcine viral genomes [1]. Multiple crRNAs targeting different regions of the p72 gene are often evaluated empirically to identify the candidate with the highest on-target cleavage efficiency and collateral activity [1].

Pre-amplification strategies

Direct detection of ASFV genomic DNA by CRISPR-Cas12a or Cas13a without target amplification is generally insufficient for clinical sensitivity, as the limit of detection (LOD) of unamplified CRISPR assays is typically in the picomolar to nanomolar range [1]. To achieve the attomolar sensitivity required for detecting viral nucleic acids in clinical specimens, a pre-amplification step is essential [1].

Recombinase polymerase amplification (RPA) is the most widely used pre-amplification method for CRISPR-based diagnostics [1]. RPA operates at a constant temperature of 37 to 42 degrees Celsius and uses a recombinase, single-stranded DNA binding proteins, and a strand-displacing DNA polymerase to exponentially amplify target DNA [1]. RPA primers are designed to flank the crRNA target region, and the resulting amplicon contains the PAM or PFS sequence required for Cas protein recognition [1]. RPA is compatible with both DNA (for Cas12a) and RNA (for Cas13a, after a reverse transcription step) targets [1].

Loop-mediated isothermal amplification (LAMP) is an alternative pre-amplification method that uses four to six primers recognizing six to eight distinct regions of the target sequence [1]. LAMP is performed at 60 to 65 degrees Celsius and produces high yields of amplified DNA with a characteristic stem-loop structure [1]. LAMP amplicons can be detected by Cas12a, provided the crRNA target site is present within the amplified region [1]. However, the complex secondary structure of LAMP products can sometimes interfere with Cas12a binding and collateral cleavage [1].

For RNA virus detection or for detecting ASFV mRNA transcripts, a reverse transcription step is incorporated prior to or concurrent with RPA or LAMP [1]. The combined RT-RPA or RT-LAMP reaction is performed in a single tube, and the resulting cDNA is then subjected to CRISPR detection [1].

Signal readout modalities

Fluorescence-based detection

The most straightforward readout for CRISPR-based diagnostics uses a quenched fluorophore reporter [1]. For Cas12a, the reporter is a short ssDNA oligonucleotide labeled with a fluorophore at one end and a quencher at the other [1]. Upon collateral cleavage by activated Cas12a, the reporter is cleaved, separating the fluorophore from the quencher and generating a fluorescence signal [1]. For Cas13a, the reporter is a ssRNA oligonucleotide with a fluorophore-quencher pair [1]. Fluorescence can be measured in real-time using a plate reader or a portable fluorometer, or endpoint fluorescence can be visualized under a blue light transilluminator [1].

Lateral flow readout

Lateral flow assays (LFAs) provide a visual, instrument-free readout suitable for field deployment [1]. In a CRISPR-based LFA, the collateral cleavage activity of Cas12a or Cas13a is used to cleave a dual-labeled reporter that is subsequently captured on a nitrocellulose strip [1]. A common design uses a FAM-biotin labeled ssDNA or ssRNA reporter [1]. Intact reporters are captured at the control line by anti-biotin antibodies, while cleaved reporters (with only the FAM label) flow to the test line and are captured by anti-FAM antibodies [1]. The presence of a visible test line indicates a positive result [1]. The LOD of CRISPR-LFA assays for ASFV is typically in the range of 1 to 10 copies per microliter of input nucleic acid [1].

Electrochemical and colorimetric sensors

Beyond fluorescence and lateral flow, electrochemical biosensors have been developed for CRISPR-based ASFV detection [1]. In these systems, collateral cleavage of a reporter molecule immobilized on an electrode surface alters the electrochemical signal, which can be measured using a potentiostat [1]. Colorimetric readouts, such as gold nanoparticle aggregation or enzymatic color development, have also been explored [1]. These modalities offer the potential for quantitative detection and integration with portable electronic readers [1].

Microfluidic integration for field deployment

Rationale for microfluidic devices

Microfluidic lab-on-a-chip platforms offer several advantages for point-of-care molecular diagnostics: reduced reagent consumption, shorter reaction times, precise fluid handling, and the ability to integrate multiple assay steps (sample preparation, amplification, and detection) into a single closed system [1]. For ASFV surveillance in the field, microfluidic devices can minimize the risk of amplicon contamination and simplify the workflow for non-specialist operators [1].

Chip design and fluidic control

Microfluidic chips for CRISPR-based ASFV detection are typically fabricated from polydimethylsiloxane (PDMS), cyclic olefin copolymer (COC), or thermoplastics using soft lithography or injection molding [1]. The chip architecture includes separate chambers for sample lysis, RPA or LAMP amplification, CRISPR detection, and signal readout [1]. Fluid movement between chambers is controlled by pneumatic valves, capillary action, or centrifugal forces [1]. For example, a centrifugal microfluidic disc can sequentially release pre-loaded reagents through rotation at controlled speeds [1].

Integration of pre-amplification and CRISPR detection

One of the key challenges in microfluidic integration is the incompatibility of RPA and LAMP reagents with the Cas12a or Cas13a reaction [1]. RPA and LAMP require specific buffer compositions (e.g., high magnesium ion concentrations, crowding agents) that can inhibit Cas nuclease activity [1]. To address this, two-step microfluidic designs physically separate the amplification and detection steps [1]. The amplified product is transferred from the amplification chamber to the detection chamber via a microchannel or a valve, where it mixes with pre-loaded CRISPR reagents [1]. Alternatively, one-pot reactions using engineered Cas proteins or optimized buffer conditions have been reported, but their robustness in microfluidic formats requires further validation [1].

Multiplexed detection

Microfluidic devices can be designed to perform multiplexed detection of ASFV alongside other swine pathogens, such as classical swine fever virus, porcine reproductive and respiratory syndrome virus, and swine influenza A virus [1]. This is achieved by incorporating multiple detection chambers, each containing a different crRNA targeting a specific pathogen [1]. Spatial separation of crRNAs on a microarray or in individual wells of a microfluidic array allows simultaneous detection of multiple targets from a single sample [1]. Multiplexing is particularly valuable for differential diagnosis of swine febrile diseases in the field [1].

Analytical performance and validation

Limit of detection and sensitivity

The analytical sensitivity of CRISPR-based ASFV assays is determined by the combined efficiency of the pre-amplification step and the CRISPR detection step [1]. For RPA-Cas12a assays targeting the p72 gene, LOD values as low as 1 to 10 copies of synthetic ASFV DNA per reaction have been reported [1]. For RT-RPA-Cas13a assays, LOD values are typically in the range of 10 to 100 copies of RNA per reaction [1]. These sensitivities are comparable to those of qPCR, the current gold standard [1].

Specificity and cross-reactivity

Specificity is assessed by testing the assay against a panel of other swine viruses, including classical swine fever virus, porcine circovirus type 2, porcine reproductive and respiratory syndrome virus, and swine influenza A virus [1]. Well-designed crRNAs targeting conserved regions of the ASFV p72 gene show no cross-reactivity with these pathogens [1]. Additionally, the requirement for PAM or PFS recognition by Cas12a or Cas13a provides an additional layer of specificity, as non-target sequences lacking the appropriate motif are not cleaved [1].

Clinical sample testing

Validation of CRISPR-based ASFV diagnostics using clinical specimens, including whole blood, serum, and oral fluids, is essential for demonstrating field utility [1]. Oral fluids are particularly attractive for point-of-care testing because they can be collected non-invasively using rope or sponge samplers [1]. However, oral fluids contain inhibitors of isothermal amplification, such as mucins and polysaccharides, which can reduce assay sensitivity [1]. Sample preparation methods, including simple dilution, heat treatment, or the use of commercial nucleic acid extraction kits, are employed to mitigate inhibition [1]. The diagnostic sensitivity and specificity of CRISPR-based assays on clinical samples are generally reported to be above 90 percent compared to qPCR [1].

Challenges and future directions

Despite the rapid progress in CRISPR-based diagnostics for ASFV, several challenges remain [1]. The requirement for a pre-amplification step adds complexity and increases the risk of carryover contamination [1]. The development of amplification-free CRISPR detection methods with sufficient sensitivity for clinical specimens is an active area of research [1]. Additionally, the stability of CRISPR reagents (Cas proteins, crRNAs, and reporters) under field conditions (elevated temperatures, humidity) needs to be improved through lyophilization or other formulation strategies [1].

Integration with microfluidic devices continues to advance, but issues related to chip fabrication cost, scalability, and user interface design must be addressed for widespread adoption [1]. The incorporation of smartphone-based fluorescence or colorimetric readers can further enhance the portability and data connectivity of these devices [1].

Future directions include the development of multiplexed panels for simultaneous detection of ASFV and other high-consequence swine pathogens, the integration of internal amplification controls for quantitative analysis, and the coupling of CRISPR diagnostics with genomic surveillance platforms for real-time monitoring of viral evolution [1].

Conclusion

CRISPR-based point-of-care diagnostics, leveraging Cas12a and Cas13a collateral cleavage activity combined with isothermal pre-amplification and simple signal readout, represent a transformative approach for ASFV detection in field settings. The integration of these assays into microfluidic devices promises to deliver rapid, sensitive, and specific molecular diagnostics that can be deployed at the point of care, supporting outbreak control and biosecurity efforts. Continued optimization of assay chemistry, device engineering, and validation in diverse field conditions will be essential for translating these technologies into routine veterinary practice.

References

[1] Zhang X, Zhao X, Song Y et al. Advances in CRISPR-Cas12a/13a-Based Nucleic Acid Detection for Porcine Viral Diseases: A Comprehensive Review. Vet Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41745936/ *** 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.