African swine fever virus serodiagnosis: a general review with a focus on the analyses of African serum samples

Identification of a linear B-cell epitope in B117L-213 protein of ASFV using monoclonal antibodies

African Swine Fever (ASF) is a highly infectious, acute, hemorrhagic Swine disease caused by the African Swine Fever Virus (ASFV). It is lethal to both domestic pigs and wild boars, and has caused significant economic losses to the global pig industry. Vaccines represent the most significant means of preventing and treating viral diseases. However, due to the complex genome and strong immune evasion mechanism of ASFV, there has been no successful development of a vaccine that has been made commercially available on a global scale. Thus, the focus of present research efforts is the study of its viral protein. B117L is a non-structural protein encoded by the ASFV, the function of which has not yet been fully defined. It belongs to a variable region in the viral genome. Direct studies on the B117L protein are relatively scarce, but analogous proteins (MGF360/505 family) have been found to have immunoregulatory functions. B117L may be a potential immunoregulatory protein of ASFV involved in viral immune evasion, replication regulation, or pathogenesis, but further investigation is required to determine its specific function. The amino acid sequence of African swine fever B117L protein is as follows: MGYTIQLDGDYCWDEDPTHHDPYMQANATSHVATSYATTSHAATPHAAAHHTFHEPFIKLNLTDKNIFNGLGFILIVIFIYLLLITLQQMLTRHIYNTVQHCVKAHLDSKNLQ. In order to facilitate a more effective expression of the B117L protein, the hydrophobic side B117L - 213 in amino acid protein is utilized, and its amino acid sequence is MGYTIQLDKDGDYCWDEDPTHHDPYMQANATSHVATSYATTSHAATPHAAAHHTFHEPFIKLNLTDKNIFN. The preparation of B117L-213 monoclonal antibody has the potential to facilitate further understanding of the B117L protein, the development of an ASFV vaccine, and the control of ASFV epidemics. The study encompassed of three strains of ASFV B117L-213 protein-specific monoclonal antibodies: 7F8D2A6, 8B11C8F9 and 8H1D5H8 (IgG2b/Kappa subtypes). The findings of this study are of considerable significance for the design and development of ASFV vaccines, and also provide relevant information for the study of the function of the B117L protein.

Establishment of an immunoperoxidase monolayer assay for the detection of African swine fever virus antibodies

African swine fever (ASF) is a lethal infectious disease affecting domestic and wild pigs, caused by the African swine fever virus (ASFV), with a mortality rate of up to 100 %. The evolving prevalence and variation of ASFV has led to the emergence of low-virulence strains, which induced chronic infections and posed challenges in nucleic acid-based diagnostics due to potential false negatives. This underscores the urgent need for reliable antibody monitoring to facilitate early diagnosis. In this study, based on a highly attenuated BK2258 cell-adapted strain HLJ18/BK33, we established an immunoperoxidase monolayer assay (IPMA) for ASFV antibodies detection. After optimization using a total of 608 pig sera, the performance of the assay was better than that of the commercial iELISA with higher sensitivity and specificity. The newly established IPMA method demonstrated high specificity with no cross-reactivity with positive sera for six other important porcine pathogens. The IPMA method developed in the present study could serve as the potential gold standard for serological diagnosis and evaluation of other detection methods for ASFV antibodies, owing to its high sensitivity and specificity. Furthermore, the IPMA method will provide a new and effective strategy for ASF monitoring, prevention and control in China.

The Novel Antigenic Epitopes of African Swine Fever Virus Inner Membrane p54 Protein Revealed by Monoclonal Antibodies

African swine fever (ASF) is caused by the African swine fever virus (ASFV); infection in domestic pigs and wild boars leads to a highly contagious, hemorrhagic disease. The p54 protein is encoded by the ASFV E183L gene and is an important structural protein located on the inner envelope of the virus. It is involved in processes of virus assembly, apoptosis induction, and neutralizing antibody production. In this study, three specific monoclonal antibodies (mAbs) against ASFV p54 protein were generated, namely 6B11, 3E3, and 3C10, from mice who were immunized with recombinant prokaryotic p54-truncated protein. Three novel linear B cell epitopes, recognized by the mAbs, were revealed: 60AAIEEEDIQFINP72, 128MATGGPAAAPAAASAPAHPAE148, and 163MSAIENLRQRNTY175. The epitopes 60AAIEEEDIQFINP72 and 163MSAIENLRQRNTY175 were highly conserved in genotype I and II ASFV strains. In addition, the epitope peptide ELISA can be used for the detection of ASFV antibodies. Our work provides new insights for p54 antigenicity and an alternative tool for serological diagnosis of ASF.

The capsid protein p72 specific mAb and the corresponding novel epitope based ELISAs for detection of ASFV infection

The African Swine Fever Virus (ASFV) poses a significant threat to the global pig farming industry. The major icosahedral capsid protein p72 plays a key role in the assembly of virus particles and infection process. As one major antigen of ASFV, p72 has been widely utilized as a marker for diagnosing infection. In this study, five p72 specific monoclonal antibodies (mAbs) were generated through the immunization of mice followed by cell fusion. Among the five hybridomas, clones 1B7, 2F3, 5D3, and 5D4 recognized a novel epitope of 101FHDMVGHHILGACH114, while clone 1D7 recognized a new epitope, 239GPLLCNIHDL248. Both linear epitopes were found to be highly conserved across all genotypes I and II ASFV isolates, with the first located at the pseudo hexagonal base of the p72 trimer and the latter situated at the FGN insertion loop. The antigenic epitopes were capable of competing with the binding of corresponding mAbs in p72 protein based ELISA, and peptide based ELISA effectively detected ASFV antibodies in clinical samples. Furthermore, we developed and optimized a sandwich ELISA using the mAb 2F3 for efficient detection of ASFV p72 antigen. Our study not only provides valuable tools for assessing p72 function assay, but also lays the foundation for serological diagnosis, prevention and control of ASF.

Rapid detection of African swine fever virus by a blue latex microsphere immunochromatographic strip

African swine fever (ASF) is a contagious viral disease in pigs, caused by the African swine fever virus (ASFV). Currently, there are no effective vaccines and drugs for ASFV, making diagnostic methods crucial for the prevention and control of ASF. In this study, the ASFV p30 protein was successfully expressed using a prokaryotic expression system and used as an immunogen to prepare monoclonal antibody (mAb) 2A5. The antigenic epitope recognized by the mAb 2A5 was identified as 58VKYDIVKSARIYAGQGY74. Then, a blue latex microsphere immunochromatographic strip method for detecting ASFV antigens was established. Wherein a rabbit polyclonal antibody (pAb) against p30 stored in our laboratory was coupled with blue latex microspheres to prepare an immune probe, with mAb 2A5 as the test line and goat anti-rabbit IgG as the control line. The immunochromatographic strip exhibited excellent sensitivity, with a minimum detection limit of 10 ng/mL for the p30 protein. It demonstrated good specificity, showing no cross-reactivity with PRRSV, PCV2, PPV, and PEDV. Moreover, it maintained good stability and could be stored for at least 100 days at 37 °C. The concordance rate between the immunochromatographic strip and qPCR test kit was 92.2%. The immunochromatographic strip method can meet the requirements of clinical testing and provide technical support for ASFV diagnosis.

Development and Validation of an Indirect and Blocking ELISA for the Serological Diagnosis of African Swine Fever

African swine fever (ASF) is an economically devastating viral disease of pigs caused by the ASF virus (ASFV). The rapid global spread of ASF has increased the demand for ASF diagnostics to be readily available and accessible. No commercial ASF enzyme-linked immunosorbent assay (ELISA) kits are manufactured and licensed in North America. Here, we report the development of two serological diagnostic assays, a blocking ELISA (bELISA) based on ASFV glycoprotein p54 and an indirect ELISA (iELISA) based on ASFV glycoproteins p54 and p72. The assays showed high sensitivity and specificity and detected anti-ASFV antibodies in serum samples from experimentally infected animals as early as 8 days post-infection. The two assays were produced commercially (AsurDx™ bELISA and iELISA) and subjected to extensive validation. Based on data from a set of characterized reference sera, the prototype commercial assays, while maintaining 100.00% specificity, showed 97.67% (AsurDx™ bELISA) and 83.72% (AsurDx™ iELISA) sensitivity. Both prototype assays detected anti-ASFV antibodies in serum samples collected from pigs experimentally infected with multiple ASFV strains and field samples collected from sick, recovering, and vaccinated animals. The two commercially available assays can be used in routine ASF diagnostics, serological surveys, and for evaluating serological responses to ASF vaccine candidates.

Development of quantum dot-based immunochromatographic strip for detection of antibodies against ASFV pp62

ASFV is the only known double-stranded insect-borne DNA virus, which can rapidly infect domestic pigs and wild boars with ticks as transmission medium. Since it was first discovered in 1921, it quickly spread to all parts of the world and brought huge economic losses to the pig industry all over the world. At present, there is still no safe and effective vaccine for ASFV. Here, we developed a quantum-dot labeled antibody test strip for the detection of antibodies against ASFV pp62. The pp62 protein was labeled with quantum dots, and the antibody test strip was developed uses it in a detection mode of labeled antigen-SPA interceptor-monoclonal antibody quality control. The test strip showed high sensitivity, the positive detection limit of the strip was 1: 106 by continuous multiple dilution using the positive standard serum of ASFV antibody as reference. The test strip showed good specificity, and there was no cross reaction with other swine diseases virus (PCV2, PRRSV, CSFV, PPV). Using the detection results of commercialized kit for African swine fever virus as reference, 80 ASFV antibody negative serum and 4 different ASFV antibody positive serum were detected using the ASFV pp62 quantum-dot labeled antibody test strip. The results were consistent with the commercial kit. This study provides a new detection method for the prevention and control of African swine fever.

Development of plate-type and tubular chemiluminescence immunoassay against African swine fever virus p72

African swine fever (ASF) is a highly contagious and fatal viral disease that has caused huge economic losses to the pig and related industries worldwide. At present, rapid, accurate, and sensitive laboratory detection technologies are important means of preventing and controlling ASF. However, because attenuated strains of African swine fever virus (ASFV) are constantly emerging, an ASFV antibody could be used more effectively to investigate the virus and control the disease on pig farms. The isolation of ASFV-specific antibodies is also essential for the diagnosis of ASF. Therefore, in this study, we developed two chemiluminescence immunoassays (CLIAs) to detect antibodies directed against ASFV p72: a traditional plate-type blocking CLIA (p72-CLIA) and an automatic tubular competitive CLIA based on magnetic particles (p72-MPCLIA). We compared the diagnostic performance of these two methods to provide a feasible new method for the effective prevention and control of ASF and the purification of ASFV. The cut-off value, diagnostic sensitivity (Dsn), and diagnostic specificity (Dsp) of p72-CLIA were 40%, 100%, and 99.6%, respectively, in known background serum, whereas those of p72-MPCLIA were 36%, 100%, and 99.6%, respectively. Thus, both methods show good Dsn, Dsp, and repeatability. However, when analytical sensitivity was evaluated, p72-MPCLIA was more sensitive than p72-CLIA or a commercial enzyme-linked immunosorbent assay. More importantly, p72-MPCLIA reduced the detection time to 15 min and allowed fully automated detection. In summary, p72-MPCLIA showed superior diagnostic performance and offered a new tool for detecting ASFV infections in the future. KEY POINTS: • Two chemiluminescence immunoassay (plate-type CLIA and tubular CLIA) methods based on p72 monoclonal antibody (mAb) were developed to detect ASFV antibody. • Both methods show good diagnostic performance (Dsn (100%), Dsp (99.6%), and good repeatability), and p72-MPCLIA detects antibodies against ASFV p72 with high efficiency in just 15 min.

Identification of two novel B cell epitopes on E184L protein of African swine fever virus using monoclonal antibodies

African swine fever virus (ASFV) is a large double-stranded DNA virus with a complex structural architecture and encodes more than 150 proteins, where many are with unknown functions. E184L has been reported as one of the immunogenic ASFV proteins that may contribute to ASFV pathogenesis and immune evasion. However, the antigenic epitopes of E184L are not yet characterized. In this study, recombinant E184L protein was expressed in prokaryotic expression system and four monoclonal antibodies (mAbs), designated as 1A10, 2D2, 3H6, and 4C10 were generated. All four mAbs reacted specifically with ASFV infected cells. To identify the epitopes of the mAbs, a series of overlapped peptides of E184L were designed and expressed as maltose binding fusion proteins. Accordingly, the expressed fusion proteins were probed with each E184L mAb separately by using Western blot. Following a fine mapping, the minimal linear epitope recognized by mAb 1A10 was identified as 119IQRQGFL125, and mAbs 2D2, 3H6, and 4C10 recognized a region located between 153DPTEFF158. Alignment of amino acids of E184L revealed that the two linear epitopes are highly conserved among different ASFV isolates. Furthermore, the potential application of the two epitopes in ASFV diagnosis was assessed through epitope-based ELISA using 24 ASFV positive and 18 negative pig serum and the method were able to distinguish positive and negative samples, indicating the two epitopes are dominant antigenic sites. To our knowledge, this is the first study to characterize the B cell epitopes of the antigenic E184L protein of ASFV, offering valuable tools for future research, as well as laying a foundation for serological diagnosis and epitope-based marker vaccine development.

Evaluation of early African swine fever virus detection using CP204L gene encoding the p30 protein using quantitative polymerase chain reaction

Background and Aim

The African swine fever virus (ASFV), spanning 170-193 kb, contains over 200 proteins, including p72 and p30, which play crucial roles in the virus's entry and expression. This study investigated the capability of detecting ASFV early through the analysis of genes B646L and CP204L, encoding p72 and p30 antigen proteins, by employing ASFV, diagnosis, immunohistochemistry (IHC), quantitative polymerase chain reaction (qPCR), and IHC techniques.

Materials and Methods

Samples were taken from both experimentally and field-infected pigs to evaluate the effectiveness of qPCR and IHC in detecting ASFV. Twenty-two infected pigs were necropsied at 3-, 5-, 7-, and 9-day post-infection to obtain the first set of samples, collecting anticoagulated blood and tissues each time. The thymus, spleen, and lymph nodes were processed by fixing in 10% formalin, paraffin-blocking, and undergoing IHC staining. Forty anticoagulated blood samples were collected from clinically infected sows at a pig farm for the second batch of samples. Based on the lowest Ct values, three blood samples were diluted fivefold for qPCR DNA testing, and their tissues were used for both qPCR and IHC analyses.

Results

At 1-day post-infection, p30-qPCR identified more ASFV-positive pigs and measured lower Ct values compared to p72-qPCR. At later time points, both methods showed similar levels of detection. ASFV was detected earlier and with lower Ct values in lymphoid tissues using p30-qPCR compared to p72-qPCR, particularly in the spleen and lymph nodes. In a field outbreak study, p30-qPCR demonstrated superior sensitivity and lower Ct values when detecting ASFV in blood samples compared to p72-qPCR.

Conclusion

The early detection of the CP204L gene encoding p30 and its corresponding antigenic protein in ASFV diagnosis compared to the gene encoding p72 suggests that CP204L and p30 are promising candidates for the development of more effective antigen and antibody testing methods.

Epitope mapping and establishment of a blocking ELISA for mAb targeting the p72 protein of African swine fever virus

The African swine fever virus (ASFV) has the ability to infect pigs and cause a highly contagious acute fever that can result in a mortality rate as high as 100%. Due to the viral epidemic, the pig industry worldwide has suffered significant financial setbacks. The absence of a proven vaccine for ASFV necessitates the development of a sensitive and reliable serological diagnostic method, enabling laboratories to effectively and expeditiously detect ASFV infection. In this study, four strains of monoclonal antibodies (mAbs) against p72, namely, 5A1, 4C4, 8A9, and 5E10, were generated through recombinant expression of p72, the main capsid protein of ASFV, and immunized mice with it. Epitope localization was performed by truncated overlapping polypeptides. The results indicate that 5A1 and 4C4 recognized the amino acid 20-39 aa, 8A9 and 5E10 are recognized at 263-282 aa, which is consistent with the reported 265-280 aa epitopes. Conserved analysis revealed 20-39 aa is a high conservation of the epitopes in the ASFV genotypes. Moreover, a blocking ELISA assay for detection ASFV antibody based on 4C4 monoclonal antibody was developed and assessed. The receiver-operating characteristic (ROC) was performed to identify the best threshold value using 87 negative and 67 positive samples. The established test exhibited an area under the curve (AUC) of 0.9997, with a 95% confidence interval ranging from 99.87 to 100%. Furthermore, the test achieved a diagnostic sensitivity of 100% (with a 95% confidence interval of 95.72 to 100%) and a specificity of 98.51% (with a 95% confidence interval of 92.02 to 99.92%) when the threshold was set at 41.97%. The inter- and intra-batch coefficient of variation were below 10%, demonstrating the exceptional repeatability of the method. This method can detect the positive standard serum at a dilution as high as 1:512. Subsequently, an exceptional blocking ELISA assay was established with high diagnostic sensitivity and specificity, providing a novel tool for detecting ASFV antibodies. KEY POINTS: • Four strains of ASFV monoclonal antibodies against p72 were prepared and their epitopes were identified. • Blocking ELISA method was established based on monoclonal antibody 4C4 with an identified conservative epitope. • The established blocking ELISA method has a good effect on the detection of ASFV antibody.

Small ubiquitin-like modifier-tag and modified protein purification significantly increase the quality and quantity of recombinant African swine fever virus p30 protein

Background and Aim

African swine fever (ASF) is a highly virulent and contagious viral disease caused by the ASF virus (ASFV). It has a significant impact on swine production throughout the world, while existing vaccines and specific treatments remain ineffective. ASFV p30 is a potent antigenic protein that induces protective antibodies immediately after infection; however, most recombinant p30 is insoluble. This study aimed to improve the solubility, yield, and purity of recombinant p30 by tagging it with a small ubiquitin-like modifier (SUMO) and modifying the protein purification process.

Materials and Methods

SUMO fused with ASFV p30 (SUMO-p30) and p30 alone were cloned and expressed in Escherichia coli. SUMO-p30 and p30 solubility and expression levels were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). Protein purification was modified by combining ammonium sulfate precipitation method with affinity chromatography. In addition, large-scale production of all versions of p30 were compared using SDS-PAGE and western blotting, and the purified p30 was used to develop the indirect enzyme-linked immunosorbent assay (ELISA).

Results

The solubility and expression levels of SUMO-p30 were dramatically enhanced compared with that of p30. Modification of the purification process significantly increased purified and soluble SUMO-p30 and p30 yields by 6.59 and 1.02 μg/mL, respectively. Large-scale production confirmed that this procedure increased the quantity of recombinant p30 while maintaining protein purity and immunogenicity. The p30-based indirect ELISA was able to discriminate between positive and negative serum samples with statistically significant differences in mean optical density 450 values (p < 0.001).

Conclusion

This study demonstrates the enhancement of solubility, purity, and yield of ASFV p30 expressed in E.coli by SUMO fusion tagging and combining ammonium sulfate precipitation with affinity chromatography for protein purification. These positive effects were sustained in large-scale production. Cleavage and removal of hexahistidine-SUMO tag from the fusion protein by protease may not be suitable when handling a large amount of the protein. However, the SUMO-fused p30 retained strong immunoreactivity to convalescent swine serum, indicating its application in immunization and diagnostic purposes. The expression and purification procedures in this study could be applied to increase solubility, quality, and quantity of other recombinant proteins as well.

A highly efficient blocking ELISA based on p72 monoclonal antibody for the detection of African swine fever virus antibodies and identification of its linear B cell epitope

Due to the absence of effective vaccine and treatment, African swine fever virus (ASFV) control is entirely dependent on accurate and early diagnosis, along with culling of infected pigs. The B646L/p72 is the major capsid protein of ASFV and is an important target for developing a diagnostic assays and vaccines. Herein, we generated a monoclonal antibody (mAb) (designated as 2F11) against the trimeric p72 protein, and a blocking ELISA (bELISA) was established for the detection of both genotype I and II ASFV antibodies. To evaluate the performance of the diagnostic test, a total of 506 porcine serum samples were tested. The average value of percent of inhibition (PI) of 133 negative pig serum was 8.4 % with standard deviation (SD) 6.5 %. Accordingly, the cut-off value of the newly established method was set at 28 % (mean + 3SD). Similarly, a receiver operating characteristic (ROC) was applied to determine the cut off value and the p72-bELISA exhibited a sensitivity of 100 % and a specificity of 99.33 % when the detection threshold was set at 28 %. The bELISA was also able to specifically recognize anti-ASFV sera without cross-reacting with other positive serums for other major swine pathogens. Moreover, by designing a series of overlapped p72 truncated proteins, the linear B cell epitope recognized by 2F11 mAb was defined to be 283NSHNIQ288. Amino acid sequence comparison revealed that the amino acid sequence 283NSHNIQ288 is highly conserved between different ASFV isolates. Our findings indicate that the newly established mAb based blocking ELISA may have a great potential in improving the detection of ASFV antibodies and provides solid foundation for further studies.

In vitro SELEX and application of an African swine fever virus (ASFV) p30 protein specific aptamer

The African swine fever virus (ASFV) has caused severe economic losses in the pig industry. To monitor ASFV spread, the p30 protein has been identified as an ideal infection marker due to its early and long-term expression during the ASFV infection period. Timely monitoring of ASFV p30 enables the detection of ASFV infection and assessment of disease progression. Aptamers are an outstanding substitute for antibodies to develop an efficient tool for ASFV p30 protein detection. In this study, a series of aptamer candidates were screened by in vitro magnetic bead-based systematic evolution of ligands by exponential enrichment (MB-SELEX). An aptamer (Atc-20) finally showed high specificity and affinity (Kd = 140 ± 10 pM) against ASFV p30 protein after truncation and affinity assessment. Furthermore, an aptamer/antibody heterogeneous sandwich detection assay was designed based on Atc20, achieving a linear detection of ASFV p30 ranging from 8 to 125 ng/ml and a detection limit (LOD) of 0.61 ng/ml. This assay showed good analytical performances and effectively detected p30 protein in diluted serum samples, presenting promising potential for the development of ASFV biosensors.

Establishment and characterization of a novel indirect ELISA method based on ASFV antigenic epitope-associated recombinant protein

African Swine Fever (ASF) is an acute and highly lethal disease in pigs caused by African Swine Fever Virus (ASFV). Viral proteins have been commonly used as antigenic targets for the development of ASF diagnostic methods. However, the prokaryotic expression of viral proteins has deficiencies such as instability, insolubility, and high cost in eukaryotic situations. This study screened and verified ASFV-encoded p72, p54, and p30 protein antigenic epitopes. Subsequently, a novel antigenic epitope-associated recombinant protein was designed based on an ideal structural protein and expressed in Escherichia coli (E. coli). Western blot analysis indicated that the recombinant protein could specifically react with the monoclonal antibody (mAb) of p72 and polyclonal antibodies of p54 and p30, respectively. Next, an ASF indirect ELISA (iELISA) method was established based on the recombinant protein, which has no specific reaction with sera of other important pig viral diseases. Meanwhile, it shows a sensitivity to detecting dilutions of ASF-positive reference serum up to 1:6400. The clinical sample detection results showed a high coincidence rate of 98 % with a commercial competition ELISA kit. In conclusion, we established a novel specific, and sensitive ASF serologic detection method that opens new avenues for ASF serodiagnostic method development.

A Robust Quadruple Protein-Based Indirect ELISA for Detection of Antibodies to African Swine Fever Virus in Pigs

African swine fever (ASF) emerged in domestic pigs and wild boars in China in 2018 and rapidly spread to neighboring Asian countries. Currently, no effective vaccine or diagnostic tests are available to prevent its spread. We developed a robust quadruple recombinant-protein-based indirect enzyme-linked immunosorbent assay (QrP-iELISA) using four antigenic proteins (CD2v, CAP80, p54, and p22) to detect ASF virus (ASFV) antibodies and compared it with a commercial kit (IDvet) using ASFV-positive and -negative serum samples. The maximum positive/negative value was 24.033 at a single antigen concentration of 0.25 μg/mL and quadruple ASFV antigen combination of 1 μg/mL at a 1:100 serum dilution. Among 70 ASFV-positive samples, 65, 67, 65, 70, 70, and 14 were positive above the cut-offs of 0.121, 0.121, 0.183, 0.065, 0.201, and 0.122, for CD2v, CAP80, p54, p22-iELISA, QrP-iELISA, and IDvet, respectively, with sensitivities of 92.9%, 95.7%, 92.9%, 100%, 100%, and 20%, respectively, all with 100% specificity. The antibody responses in QrP-iELISA and IDvet were similar in pigs infected with ASFV I. QrP-iELISA was more sensitive than IDvet for early antibody detection in pigs infected with ASFV II. These data provide a foundation for developing advanced ASF antibody detection kits critical for ASF surveillance and control.

Tick-Borne Diseases of Humans and Animals in West Africa

Ticks are a significant group of arthropod vectors that transmit a large variety of pathogens responsible for human and animal diseases worldwide. Ticks are the second biggest transmitters of vector-borne diseases, behind mosquitoes. However, in West Africa, there is often only limited knowledge of tick-borne diseases. With the scarcity of appropriate diagnostic services, the prevalence of tick-borne diseases is generally underestimated in humans. In this review, we provide an update on tick-borne pathogens reported in people, animals and ticks in West Africa by microscopic, immunological and molecular methods. A systematic search was conducted in PubMed and Google Scholar. The selection criteria included all studies conducted in West Africa reporting the presence of Rickettsia, Borrelia, Anaplasma, Ehrlichia, Bartonella, Coxiella burnetii, Theileria, Babesia, Hepatozoon and Crimean-Congo haemorrhagic fever viruses in humans, animals or ticks. Our intention is to raise awareness of tick-borne diseases amongst human and animal health workers in West Africa, and also physicians working with tourists who have travelled to the region.

Novel Epitope Mapping of African Swine Fever Virus pI215L Protein Using Monoclonal Antibodies

The African swine fever virus (ASFV) is one of the most important pathogens that causes huge damage to worldwide swine production. The pI215L protein is found within the virion and expressed at a high level in infected porcine alveolar macrophages (PAMs), indicating a possible role of pI215L protein in ASFV detection and surveillance. In the present study, female BALB/c mice (5-6-week-old) were immunized with rpI215L protein, and six hybridomas, 1C1, 2F6, 2F10, 3C8, 5E1 and 5B3, steadily secreted anti-pI215L monoclonal antibodies (mAbs). Among them, 1C4, 5E1, and 5B3 had the IgG1 isotype with a Lambda light chain, 2F10 and 3C8 had the IgG1 isotype with a Kappa light chain, and 2F6 had the IgG2a isotype with a Kappa light chain. Western blot showed a good reactivity of the six mAbs against ASFV. Eight truncated polypeptides were produced for epitope mapping. Two novel B cell epitopes, 67LTFTSEMWHPNIYS80 and 167IEYFKNAASN176, were identified by the mAbs. Further analysis revealed that 2F6 mAb could be widely used in ASFV surveillance and 5B3 mAb might serve as a tool in the distinguishment of different ASFV genotypes. This study provides tools of monoclonal antibodies for further study of I215L function and contributes to the development of serological diagnosis and vaccine research.

Establishment of a Dual-Antigen Indirect ELISA Based on p30 and pB602L to Detect Antibodies against African Swine Fever Virus

African swine fever (ASF) is an acute, virulent, and highly fatal infectious disease caused by the African swine fever virus (ASFV). There is no effective vaccine or diagnostic method to prevent and control this disease currently, which highlights the significance of ASF early detection. In this study, we chose an early antigen and a late-expressed antigen to co-detect the target antibody, which not only helps in early detection but also improves accuracy and sensitivity. CP204L and B602L were successfully expressed as soluble proteins in an Escherichia coli vector system. By optimizing various conditions, a dual-antigen indirect ELISA for ASFV antibodies was established. The assay was non-cross-reactive with antibodies against the porcine reproductive and respiratory syndrome virus, classical swine fever virus, porcine circovirus type 2, and pseudorabies virus. The maximum serum dilution for detection of ASFV-positive sera was 1:1600. The intra-batch reproducibility coefficient of variation was <5% and the inter-batch reproducibility coefficient of variation was <10%. Compared with commercial kits, the dual-antigen indirect ELISA had good detection performance. In conclusion, we established a detection method with low cost, streamlined production process, and fewer instruments. It provides a new method for the serological diagnosis of ASF.

A new B cell epitope of pC129R protein of African swine fever virus identified by monoclonal antibodies

African swine fever virus (ASFV) is a most important pathogen which causes huge damage in swine production in the world. pC129R protein is one of the most abundant ASFV proteins in infected Vero cells and WSL-HP cells, which consequently could be a target for ASF detection and surveillance. In this study, 5-6-week-old female BALB/c mice were immunized with rpC129R protein expressed by a prokaryotic system. And three hybridomas, 1B1, 1B4 and 4H4, steadily secreted anti-pC129R monoclonal antibodies were screened by an indirect enzyme linked immunosorbent assay (ELISA). Among them, 1B4 and 4H4 had IgG2a isotype with Kappa light chain, while 1B1 had IgG1 isotype with Kappa light chain. Western blot and indirect immunofluorescence assay showed that three monoclonal antibodies (mAbs) specifically reacted with ASFV. Epitope mapping was performed with truncated polypeptides. And a new B cell epitope, 18KHYVLIPK25 was identified by the mAbs, which was highly conserved in most genotypes of ASFV. These findings not only provide a monoclonal antibody tool for further study of the function of C129R, but also lay the foundation for serological diagnosis and vaccine development.

Development of an effective one-step double-antigen sandwich ELISA based on p72 to detect antibodies against African swine fever virus

African swine fever (ASF), caused by ASF virus (ASFV), is a highly contagious and lethal disease of domestic pigs leading to tremendous economic losses. As there are no vaccines and drugs available. An effective diagnosis to eliminate ASFV-infected pigs is a crucial strategy to prevent and control ASF. To this end, ASFV capsid protein p72 was expressed using Chinese hamster ovary (CHO) cells and subsequently conjugated with horseradish peroxidase (HRP) to develop a one-step double-antigen sandwich enzyme-linked immunosorbent assay (one-step DAgS-ELISA). The performance of this ELISA for detecting ASFV antibodies was evaluated. Overall, a diagnostic sensitivity of 97.96% and specificity of 98.96% was achieved when the cutoff value was set to 0.25. No cross-reaction with healthy pig serum and other swine viruses was observed. The coefficients of variation of the intra-assay and inter-assay were both <10%. Importantly, this ELISA could detect antibodies in standard serum with 12,800-fold dilution, and seroconversion started from the 7th day post-inoculation (dpi), showing excellent analytical sensitivity and great utility. Furthermore, compared to the commercial kit, this ELISA had a good agreement and significantly shorter operation time. Collectively, a novel one-step DAgS-ELISA for detecting antibodies against ASFV is developed, which will be reliable and convenient to monitor ASFV infection.

New perspective on African swine fever: a bibliometrics study and visualization analysis

Introduction

African swine fever (ASF) is a contagious viral disease that can have devastating effects on domestic pigs and wild boars. Over the past decade, there has been a new wave of this ancient disease spreading around the world, prompting many scholars to dedicate themselves to researching this disease. This research aims to use bibliometric methods to organize, analyze and summarize the scientific publications on ASF that have been amassed in the past two decades.

Methods

This paper used VOSviewer, CiteSpace, and a bibliometric online analysis platform to conduct performance analysis and visualization studies on 1,885 academic papers about ASF in the Web of Science from January 2003 to December 2022.

Results

The amount of literature published on ASF has increased exponentially in recent years, and the development trend of related research is good. A group of representative scholars have appeared in this research field, and some cooperative networks have been formed. Transboundary and Emerging Diseases is the journal with the most publications in this field, while Virus Research is the journal with the most citation per article. High-productivity countries are led by China in terms of the number of articles published followed by the United States and Spain. In regard to the average number of citations, the scholars in the UK are in the lead. The institution with the most articles was the Chinese Academy of Agricultural Sciences. The analysis of high-frequency keywords showed that the pathogens and epidemiology of ASF were the research hotspots in this field, and the research content was closely related to molecular biology and immunology. The burst keywords "transmission", "identification", "virulence", "replication", and "gene" reflects the research frontier. In addition, by collating and analyzing highly cited journals and highly co-cited references, we explored the knowledge structure and theoretical basis of this field.

Discussion

This is the first bibliometric analysis report on ASF research, which highlights the key characteristics of ASF research and presents the research status and evolution trend in this field from a new perspective. It provides a valuable reference for further research.

Screening and identification of the dominant antigens of the African swine fever virus

African swine fever is a highly lethal contagious disease of pigs for which there is no vaccine. Its causative agent African swine fever virus (ASFV) is a highly complex enveloped DNA virus encoding more than 150 open reading frames. The antigenicity of ASFV is still unclear at present. In this study, 35 proteins of ASFV were expressed by Escherichia coli, and ELISA was developed for the detection of antibodies against these proteins. p30, p54, and p22 were presented as the major antigens of ASFV, positively reacting with all five clinical ASFV-positive pig sera, and 10 pig sera experimentally infected by ASFV. Five proteins (pB475L, pC129R, pE199L, pE184L, and pK145R) reacted well with ASFV-positive sera. The p30 induced a rapid and strong antibody immune response during ASFV infection. These results will promote the development of subunit vaccines and serum diagnostic methods against ASFV.

A chemiluminescent magnetic microparticle immunoassay for the detection of antibody against African swine fever virus

The p30 protein is abundantly expressed in the early stage of African swine fever virus (ASFV) infection. Thus, it is an ideal antigen candidate for serodiagnosis with the use of an immunoassay. In this study, a chemiluminescent magnetic microparticle immunoassay (CMIA) was developed for the detection of antibodies (Abs) against ASFV p30 protein in porcine serum. Purified p30 protein was coupled to magnetic beads, and the experimental conditions including concentration, temperature, incubation time, dilution ratio, buffers, and other relevant variables were evaluated and optimized. To evaluate the performance of the assay, a total of 178 pig serum samples (117 negative and 61 positive samples) were tested. According to receiver operator characteristic curve analysis, the cut-off value of the CMIA was 104,315 (area under the curve, 0.998; Youden's index, 0.974; 95% confidence interval: 99.45 to 100%). Sensitivity results showed that the dilution ratio of p30 Abs in ASFV-positive sera detected by the CMIA is much higher when compared to commercial blocking ELISA kit. Specificity testing showed that no cross-reactivity was observed with sera positive for other porcine disease viruses. The intraassay coefficient of variation (CV) was < 5%, and the interassay CV was < 10%. The p30-magnetic beads could be stored at 4 °C for more than 15 months without loss of activity. The kappa coefficient between CMIA and INGENASA blocking ELISA kit was 0.946, showing strong agreement. In conclusion, our method showed superiority with high sensitivity, specificity, reproducibility, and stability and potentialized its application in the development of a diagnostic kit for the detection of ASF in clinical samples. KEY POINTS: • ASFV tag-free p30 was successfully purified. • High sensitivity, specificity, relatively simple, and time-saving to detect antibody against ASFV were developed. • The development of CMIA will help the clinical diagnosis of ASFV and will be useful for large-scale serological test.

Development and application of a TaqMan-based real-time PCR method for the detection of the ASFV MGF505-7R gene

African swine fever virus (ASFV), the etiological agent of African swine fever (ASF), causes deadly hemorrhagic fever in domestic pigs. ASF's high mortality and morbidity have had disastrous effects on the world's swine industry. In recent years, the number of African swine virus strains has increased and presented new challenges for detecting classical ASFV-p72-based viruses. In this study, we observed that the ASFV MGF505-7R gene, a member of the multigene family that can enhance ASFV virulence and pathogenesis, has the potential to be a candidate for vaccine formulations. We also developed a real-time PCR assay based on the ASFV MGF505-7R gene and validated it in multiple aspects. The results indicated that the approach could detect standard plasmids with a sensitivity and a specificity of up to 1 × 10¹ copies/μL. Moreover, the assay had no cross-reactions with other porcine viruses. In laboratory and clinical settings, the assay can detect ASFV-infected samples at an early stage (4 hpi) and show a consistency of 92.56% when compared with classical ASFV detection in clinically ASFV-infected materials. This study's results also indicated that the TaqMan-based quantitative real-time PCR assay we developed for detecting the ASFV MGF505-7R gene is both sensitive and specific. This assay can provide a quick and accurate method for detecting ASFV and has the potential to be used as an optional tool for screening and monitoring ASF outbreaks.

A Novel Linear B-Cell Epitope on the P54 Protein of African Swine Fever Virus Identified Using Monoclonal Antibodies

The African swine fever virus (ASFV) is a highly infectious viral pathogen that presents a major threat to the global pig industry. No effective vaccine is available for the virus. The p54 protein, a major structural component of ASFV, is involved in virus adsorption and entry to target cells and also plays a key role in ASFV vaccine development and disease prevention. Here, we generated species-specific monoclonal antibodies (mAbs), namely 7G10A7F7, 6E8G8E1, 6C3A6D12, and 8D10C12C8 (subtype IgG1/kappa type), against the ASFV p54 protein and characterized the specificity of these mAbs. Peptide scanning techniques were used to determine the epitopes that are recognized by the mAbs, which defined a new B-cell epitope, TMSAIENLR. Amino acid sequence comparison showed that this epitope is conserved among all reference ASFV strains from different regions of China, including the widely prevalent, highly pathogenic strain Georgia 2007/1 (NC_044959.2). This study reveals important signposts for the design and development of ASFV vaccines and also provides critical information for the functional studies of the p54 protein via deletion analysis.

Development of a Competitive Enzyme-Linked Immunosorbent Assay Targeting the-p30 Protein for Detection of Antibodies against African Swine Fever Virus

African swine fever (ASF) is a highly contagious hemorrhagic viral disease of domestic and wild pigs of all breeds and ages, caused by African swine fever virus (ASFV). Due to the absence of a safe and efficacious vaccine, accurate laboratory diagnosis is critical for the control of ASF prevention. The p30 protein is immunogenic and stimulates a high level of antibody response to ASFV infection. We developed a panel of 4 monoclonal antibodies (mAbs) against p30 protein, and mAb-2B4 showed the highest percent of inhibition (PI) of 70% in the solid phase blocking ELISA (bELISA). Epitope mapping revealed the mAb-2B4 recognized the epitope of aa 12-18 of p30, which is conserved among various ASFV genotypes. Subsequently, a competitive enzyme-linked immunosorbent assay (cELISA) was established using HRP-labeled mAb-2B4. The cutoff for discrimination between 98 negative sera and 40 positive sera against ASFV was determined by plotting a receiver operating characteristic (ROC) curve. It yielded the area under the curve (AUC) of 0.998, and a diagnostic specificity of 97.96% and a sensitivity of 97.5% were achieved when the cutoff value was determined at 37.1%. Furthermore, the results showed an excellent repeatability of the established cELISA and no cross-reaction to antisera against six other pig pathogens. Additionally, the cELISA detected a titer of 1:256 in the positive standard serum. Overall, mAb-2B4 showed a conserved epitope and high ability to be inhibited by positive sera in ASFV antibody detection. The cELISA based on HRP-labeled mAb-2B4 offers an alternative to other assays for a broader diagnostic coverage of ASFV infection.

Development of a chromatographic lateral flow immunoassay for detection of African swine fever virus antigen in blood

African swine fever (ASF) is a highly lethal disease of domestic and wild swine caused by African swine fever virus (ASFV). The disease currently circulates in Africa, Europe, Asia and on the island of Hispaniola. The ongoing epizootics in Europe and Asia have produced millions of animal deaths and severe economic losses. No effective vaccine is available for ASF, making rapid and accurate detection of ASFV essential for disease mitigation strategies. Currently available diagnostics for ASFV possess significant limitations related to assay performance, deployability, and/or turn-around time; therefore there is an unmet need for pen-side diagnostic tests with sufficient sensitivity and specificity. A chromatographic lateral flow immunoassay (LFIA) was developed for the detection of ASFV antigen in EDTA-treated whole blood using monoclonal antibodies targeting the viral p30 protein. The assay requires only water to perform and provides results in 25 min, making it well-suited for field use. The LFIA was capable of detecting genotype I and genotype II strains of ASFV in EDTA blood from experimentally infected pigs at varying time-points after infection, though it was unable to detect a genotype X ASFV strain. Diagnostic sensitivity correlated with clinical disease severity, body temperature, and viral DNA levels, and was over 90% in animals showing moderate to severe ASF-related symptoms after challenge with virulent genotype II virus. The LFIA also showed a robust diagnostic specificity of over 98%, which is essential to field testing for a high consequence to foregin animal disease. The LFIA targeting the viral p30 protein can reliably detect ASFV in whole blood from animals showing moderate to severe clinical signs of infection with virulent genotype I and II isolates, making it a promising candidate for use as a field-deployable antigen detection assay. Additional evaluation using field samples and different virus strains is required to further assess the utility of this rapid diagnostic test.

Indirect ELISA Using Multi-Antigenic Dominants of p30, p54 and p72 Recombinant Proteins to Detect Antibodies against African Swine Fever Virus in Pigs

African swine fever (ASF) caused by ASF virus (ASFV) is a fatal disease in pigs and results in great economic losses. Due to the lack of available vaccines and treatments, serological diagnosis of ASF plays a key role in the surveillance program, but due to the lack of knowledge and the complexity of the ASFV genome, the candidate target viral proteins are still being researched. False negativity is still a big obstacle during the diagnostic process. In this study, the high antigenic viral proteins p30, p54 and p72 were screened to find the antigenic dominant domains and the tandem His-p30-54-72 was derived. An indirect enzyme-linked immunosorbent assay (iELISA) coated with His-p30-54-72 was developed with a cut-off value of 0.371. A total of 192 clinical samples were detected by His-p30-54-72-coated indirect ELISA (iELISA) and commercial ASFV antibody kits. The results showed that the positive rate of His-p30-54-72-coated iELISA was increased by 4.7% and 14.6% compared with a single viral protein-based commercial ASFV antibody kits. These results provide a platform for future ASFV clinical diagnosis and vaccine immune effect evaluation.

Development and characterization of monoclonal antibodies against the extracellular domain of African swine fever virus structural protein, CD2v

African swine fever virus (ASFV), a DNA double-stranded virus with high infectivity and mortality, causing a devastating blow to the pig industry and the world economy. The CD2v protein is an essential immunoprotective protein of ASFV. In this study, we expressed the extracellular region of the CD2v protein in the 293F expression system to achieve proper glycosylation. Monoclonal antibodies (mAbs) were prepared by immunizing mice with the recombinant CD2v protein. Eventually, four mAbs that target the extracellular region of the ASFV CD2v protein were obtained. All four mAbs responded well to the ASFV HLJ/18 strain and recognized the same linear epitope, 154SILE157. The specific shortest amino acid sequence of this epitope has been accurately identified for the first time. Meaningfully, the 154SILE157 epitope was highly conformed in the ASFV Chinese epidemic strain and Georgia2008/1 strains according to the analysis of the conservation and have a fair protective effect. These findings contribute to further understanding of the protein function of CD2v and provide potential support for the development of diagnostic tools and vaccines for ASFV.

Development of an indirect ELISA against African swine fever virus using two recombinant antigens, partial p22 and p30

African swine fever (ASF) is a highly fatal viral disease affecting pigs. It is caused by the ASF virus (ASFV), and causes serious economic losses to the swine industry worldwide, including in Korea. Commercially available enzyme-linked immunosorbent assay (ELISA) kits for detecting anti-ASFV antibodies are used for the diagnosis and surveillance of ASF. In this study, an ELISA was developed to detect anti-ASFV antibodies using two recombinant proteins, p22 and p30, from genotype II ASFV. Recombinant transmembrane domain-deleted p22 (p22∆TM) and p30 were expressed in E.coli vector system pET32a and mixed for use as antigens in indirect ELISA. The p22∆TM/p30-based indirect ELISA was validated using 31 sera from genotype I ASFV-infected pigs and 1133 sera from uninfected pigs. Area under the curve of this test was 0.999 [95 % concentration interval 0.992-1.000], and sensitivity and specificity were 93.5 % and 99.8 %, respectively. The between run coefficient of variation for internal quality control serum was 6.61 %. In the seroconversion analysis, the p22∆TM/p30-based indirect ELISA showed equal or better ability to detect antibodies in pigs experimentally challenged with ASFV p72 genotypes I and II (p < 0.05). In conclusion, the p22∆TM/p30-based indirect ELISA is a reliable diagnostic method for detecting anti-ASFV antibodies.

Preparation of Monoclonal Antibodies against the Viral p54 Protein and a Blocking ELISA for Detection of the Antibody against African Swine Fever Virus

African swine fever virus (ASFV) causes a highly contagious viral disease in domestic and wild pigs, leading to serious economic losses. As there are no vaccines or drugs available, early accurate diagnosis and eradiation of infected animals are the most important measures for ASFV prevention and control. Therefore, improvement of available diagnostic assays and development of novel effective techniques are required. This study is devoted to generating a new detection platform of blocking monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) against ASFV p54 protein. Seven monoclonal antibodies against recombinant p54 protein were produced and four epitopes were identified. Three blocking ELISAs were developed with 6A5 and 6F9 mAbs labeled with HRP, respectively, of which the 6A5/6F9-based blocking ELISA displayed the best detection performance, with an AUC of 0.986, sensitivity of 98.36% and specificity of 92.36% in ROC analysis. Moreover, it has an excellent agreement at 96.59% (198/205) when compared to the commercial blocking ELISA (kappa value = 0.920). The method also has high repeatability, with CV <10%, and no cross reaction with the serum antibodies against PRV, PRRSV, CSFV, PCV2 or SVA. This indicates that the 6A5/6F9-based blocking ELISA has high accuracy with good sensitivity and specificity, suitable for viral detection, field surveillance and epidemiological studies.

Development of an Effective Double Antigen Sandwich ELISA Based on p30 Protein to Detect Antibodies against African Swine Fever Virus

African swine fever (ASF), the highly lethal swine infectious disease caused by the African swine fever virus (ASFV), is a great threat to the swine industry. There is no effective vaccine or diagnostic method to prevent and control this disease currently. The p30 protein of ASFV is an important target for serological diagnosis, expressed in the early stage of viral replication and has high immunogenicity and sequence conservatism. Here, the CP204L gene was cloned into the expression vector pET-30a (+), and the soluble p30 protein was successfully expressed in the E. coli prokaryotic expression system and then labeled with horseradish peroxidase (HRP) to be the enzyme-labeled antigen. Using the purified recombinant p30 protein, a double-antigen sandwich ELISA for ASFV antibody detection was developed. This method exhibits excellent specificity, sensitivity and reproducibility in clinical sample detection with lower cost and shorter production cycles. Taken together, this study provides technical support for antibody detection for ASFV.

An improved luciferase immunosorbent assay for ultrasensitive detection of antibodies against African swine fever virus

African swine fever (ASF), caused by African swine fever virus (ASFV), is a fatal infectious disease of pigs and causes great socioeconomic losses globally. The reliable diagnostic method is critical for prevention and control of the disease. In this study, an improved Luciferase immunosorbent assay (LISA) for detecting ASF was developed using the cell lysates containing ASFV p35 protein fused with a reporter Nano-luciferase (p35-Luc protein). The improved method avoids the complicate procedures of immobilizing the serum samples with protein G in the normal LISA method, and replaced by directly coating the serum samples with carbonate buffer, therefore reduces the productive cost and simplifies the operation procedures. The p35-Luc LISA exhibited high specificity for anti-ASFV sera while no cross-reactions with the sera against other swine viruses. The detection limit of the p35-Luc LISA was shown to be at least four times higher than that of the p35 based indirect ELISA established in our lab. The receiver operating characteristic (ROC) analysis showed the 96.36% relative specificity and 96.97% relative sensitivity of the p35-Luc LISA with the cutoff values of 3.55 as compared to the commercial Ingezim p72-ELISA kit. Furthermore, a total of 248 serum samples were tested by both the p35-Luc LISA and commercial Ingezim p72-ELISA kit, and there was a high degree of agreement (97.6%, kappa = 0.9753) in the performance of the two assays. Collectively, the improved LISA based on the p35-Luc protein could be used as a rapid, ultrasensitive, cost-effective and reliable diagnostic tool for serological survey of ASF in pig farms.

Development of a highly sensitive Gaussia luciferase immunoprecipitation assay for the detection of antibodies against African swine fever virus

In recent years, African swine fever (ASF) has caused a devastating blow to the swine industry globally. Since no effective vaccine is available, strict biosafety measures and rapid diagnosis are the most effective strategies for ASF control. ASFV p30 is one of the most antigenic viral proteins that have been widely used in the field for serological diagnosis of ASF infection. In this study, we developed a luciferase immunoprecipitation system (LIPS) assay for the detection of ASFV antibodies in pig serum using Gaussia luciferase (GLuc)-tagged p30 as a diagnostic antigen. The optimal GLuc-p30 input of 10⁷ luminance units (LU) and optimal serum dilution factor of 1/100 were set to achieve the highest P/N ratio. Based on 87 ASFV-positive and negative pig sera, the cutoff value of the S/N ratio could be set between 2.298 and 30.59 to achieve 100% sensitivity and 100% specificity. Moreover, the diagnostic sensitivity of this LIPS is comparable to that of a commercial enzyme-linked immunosorbent assay (ELISA) and the specificity of LIPS is even superior to the tested ELISA. In conclusion, we have established a LIPS assay for ASFV antibody detection, which could be a potential method for ASFV diagnosis in laboratories and farms.

A simple nanobody-based competitive ELISA to detect antibodies against African swine fever virus

African swine fever virus (ASFV) infection is a big threat to the global pig industry. Because there is no effective vaccine, rapid, low-cost, and simple diagnosis methods are necessary to detect the ASFV infection in pig herds. Nanobodies, with advantages of small molecular weight and easy genetic engineering, have been universally used as reagents for developing diagnostic kits. In this study, the recombinant ASFV-p30 was expressed and served as an antigen to immunize the Bactrian camel. Then, seven nanobodies against ASFV-p30 were screened using phage display technique. Subsequently, the seven nanobodies fused horseradish peroxidase (nanobody-HRP) were secretory expressed and one fusion protein ASFV-p30-Nb75-HRP was selected with the highest sensitivity in blocking ELISA. Using the ASFV-p30-Nb75-HRP fusion protein as a probe, a competitive ELISA (cELISA) was developed for detecting anti-ASFV antibodies in pig sera. The cut-off value of cELISA was determined to be 22.7% by testing 360 negative pig sera. The detection limit of the cELISA for positive pig sera was 1:320, and there was no cross-reaction with anti-other swine virus antibodies. The comparative assay showed that the agreement of the cELISA with a commercial ELISA kit was 100%. More importantly, the developed cELISA showed low cost and easy production as a commercial kit candidate. Collectively, a simple nanobody-based cELISA for detecting antibodies against ASFV is developed and it provides a new method for monitoring ASFV infection in the pig herds.

Carbon nanodots combined with loop-mediated isothermal amplification (LAMP) for detection of African swine fever virus (ASFV)

The spread of African swine fever virus (ASFV) caused huge economic costs, so early detection is particularly important. Here, we established a fluorescence biosensor based on carbon nanodots (CNDs) and loop-mediated isothermal amplification (LAMP) to ultra-sensitively detect ASFV. LAMP with high efficiency produced a large amount of pyro phosphoric acid and caused pH change in a short time. CNDs with strong light stability had a large fluorescence response at the emission wavelength of 585.5 nm to small pH change by the excitation wavelength of 550 nm. The biosensor realized “turn-off–on” mode for ASFV detection with the detection limit as low as 15.21 copies μL⁻¹. In addition, the biosensor had high accuracy in the actual sample assay. Therefore, the biosensor achieved rapid, sensitive, low-cost, and simple detection for ASFV. Moreover, the biosensor broadened the detection pathway of LAMP as a tool with great development prospect.

Development of an ELISA Method to Differentiate Animals Infected with Wild-Type African Swine Fever Viruses and Attenuated HLJ/18-7GD Vaccine Candidate

African swine fever (ASF) is a highly contagious hemorrhagic disease of pigs, posing a significant threat to the world pig industry. Several researchers are investigating the possibilities for developing a safe and efficient vaccine against ASF. In this regard, significant progress has been made and some gene-deleted ASFVs are reported as potential live attenuated vaccines. A seven-gene-deleted live attenuated vaccine candidate HLJ/18-7GD (among which CD2v is included) has been developed in our laboratory and reported to be safe and protective, and it is expected to be commercialized in the near future. There is an urgent need for developing a diagnostic method that can clearly discriminate between wild-type-ASFV-infected and vaccinated animals (DIVA). In the present study, a dual indirect ELISA based on p54 and CD2v proteins was successfully established to specifically distinguish serum antibodies from pigs infected with wild-type ASFV or possessing vaccine immunization. To evaluate the performance of the assay, a total of 433 serum samples from four groups of pigs experimentally infected with the wild-type HLJ/18 ASFV, immunized with the HLJ/18-7GD vaccine candidate, infected with the new lower virulent variant, and specific-pathogen-free pigs were used. Our results showed that the positive rate of immunized serum was 96.54% (p54) and 2.83% (CD2v), and the positive rate of the infection by wild-type virus was 100% (p54) and 97.8% (CD2v). Similarly, the positive rate to infection by the new low-virulent ASFV variant in China was 100% (p54) and 0% (CD2v), indicating the technique was also able to distinguish antibodies from wild-type and the new low-virulent ASFV variant in China. Moreover, no cross-reaction was observed in immune sera from other swine pathogens, such as CSFV, PEDV, PRRSV, HP-PRRSV, PCV2, and PrV. Overall, the developed dual indirect ELISA exhibited high diagnostic sensitivity, specificity, and repeatability and will provide a new approach to differentiate serum antibodies between wild virulent and CD2v-unexpressed ASFV infection, which will play a great role in serological diagnosis and epidemiological monitoring of ASF in the future.

Accurate, rapid and highly sensitive detection of African swine fever virus via graphene oxide-based accelerated strand exchange amplification

African swine fever is an acute, severe and highly contagious infectious disease caused by African swine fever virus (ASFV), posing a huge threat to the global swine industry. Rapid and accurate diagnostic methods are of great significance for the effective prevention and control of ASFV transmission. In this work, we established and evaluated a graphene oxide-based accelerated strand exchange amplification (GO-ASEA) method for rapid, highly sensitive, and quantitative detection of ASFV. The use of GO provided a novel solution reference for improving the specificity of strand exchange amplification and solving the potential false positive problem caused by primer dimers. The detection limit of the GO-ASEA assay was 5.8 × 10^-1 copies per μL of ASFV (equal to 2.9 copies per reaction) or 5.8 × 10⁰ copies per μL of ASFV in spiked swine nasal swabs. The selectivity of the GO-ASEA assay was supported by the ASFV DNA reference material and another seven porcine-derived viruses with similar clinical symptoms. The GO-ASEA assay took only about 29 minutes and was validated with 6 inactivated specimens and 52 swine nasal swabs, showing excellent clinical applicability. The novel assay is an accurate and practical method for rapid, highly sensitive detection of ASFV, and can potentially serve as a robust tool in epidemic prevention and point-of-care diagnosis.

HRP-conjugated-nanobody-based cELISA for rapid and sensitive clinical detection of ASFV antibodies

Abstract

African swine fever (ASF), which is caused by the ASF virus (ASFV), is a highly contagious hemorrhagic disease that causes high mortality to domestic porcine and wild boars and brings huge economic losses to world swine industry. Due to the lack of an effective vaccine, the control of ASF must depend on early, efficient, and cost-effective detection and strict control and elimination strategies. Traditional serological testing methods are generally associated with high testing costs, complex operations, and high technical requirements. As a promising alternative diagnostic tool to traditional antibodies, nanobodies (Nb) have the advantages of simpler and faster generation, good stability and solubility, and high affinity and specificity, although the system is dependent on the immunization of Bactrian camels to obtain the specific VHH library of the target protein. The application of Nbs in the detection of ASFV antibodies has not yet been reported yet. Using a phage display technology, one Nb against the ASFV p54 protein that exhibited high specificity and affinity, Nb8, was successfully screened. A HEK293T cell line stably expressing Nb8-horseradish peroxidase (HRP) fusion protein was established using the lentiviral expression system. Following the optimization of the reaction conditions, the Nb8-HRP fusion protein was successfully used to establish a competitive enzyme-linked immunosorbent assay (cELISA) to detect ASFV-specific antibodies in pig serum, for the first time. There was no cross-reaction with healthy pig serum, porcine pseudorabies virus (PRV), porcine reproductive and respiratory syndrome virus (PRRSV), classical swine fever virus (CSFV), porcine epidemic diarrhea virus (PEDV), and classical swine fever virus (CSFV) positive sera. The optimal cut-off value for the cELISA by ROC analysis was 52.5%. A total of 209 serum samples were tested using the developed cELISA and a commercial ELISA kit. The results showed that the relative specificity of the cELISA was 98.97%, and the relative sensitivity of the cELISA was 93.3%, with the percent agreement between the two ELISA methods being 98.56%. In conclusion, a specific, sensitive, and repeatable cELISA was successfully developed based on the Nb8 as a probe, providing a promising method for the detection of anti-ASFV antibodies in clinical pig serum.

Key points

• We successfully screened a specific, high affinity nanobody against ASFV p54 protein.

• We establish a method for continuous and stable expression of Nb-HRP fusion protein using a lentiviral packaging system.

• We establish a nanobody cELISA detection method that can monitor an ASF infection.

Supplementary Information

The online version contains supplementary material available at 10.1007/s00253-022-11981-4.

Development of a p72 trimer-based colloidal gold strip for detection of antibodies against African swine fever virus

Abstract

African swine fever virus (ASFV) causes a highly contagious and often lethal swine viral disease, and leads to tremendous economic losses to the swine industry. Unfortunately, there are no vaccines and effective antiviral agents available to prevent and control ASFV outbreaks. Therefore, it is necessary to develop simple and rapid strategies to monitor ASFV-infected pigs to restrain its spread. In the current study, ASFV capsid protein p72 was expressed along with its chaperone pB602L to form trimers in human embryonic kidney 293 (HEK293) cells. The p72 trimers were subsequently labeled with colloidal gold to develop a immunochromatographic strip. The strip showed high specificity to ASFV-positive serum and no cross-reactivity to other swine virus positive sera. Importantly, the strip showed a higher sensitivity of detecting ASFV antibodies in both positive standard serum and clinical serum samples than a commercial enzyme-linked immunosorbent assay (ELISA) kit. Taken together, these results demonstrate the strip as a reliable diagnostic tool against ASFV infection, which will be appropriate for application in prevention and control of ASFV.

Key points

• ASFV p72 trimers were successfully generated.

• A colloidal gold strip was developed based on ASFV p72 trimers.

• The strip is appropriate for detecting ASFV antibodies in the field.

Novel method for sub-grouping of genotype II African swine fever viruses based on the intergenic region between the A179L and A137R genes

BACKGROUND

African swine fever (ASF) is a highly contagious and deadly viral disease affecting domestic and wild pigs of all ages. African swine fever virus (ASFV) has spread rapidly through Eastern and Southeastern Asia first appearing in Vietnam in 2019.

OBJECTIVES

Molecular typing of African swine fever virus (ASFV) in Vietnam has identified two principal variants circulating based on the sequencing of the intergenic region (IRG) between the I73R and I329L genes. Identification of additional genetic markers would enable higher resolution tracing of outbreaks within the country.

METHODS

Sequence analysis suggested the IRG between the A179L and A137R genes may also exhibit variability, PCR primers were designed and samples from Vietnam were subject to Sanger sequencing.

RESULTS

We developed a novel method for sub-grouping of ASFV based on the IRG between the A179L and A137R genes of ASFV. Our results demonstrated that the finding of the insertion or deletion of an 11- nucleotide sequence (GATACAATTGT) between the A179L-A137R genes.

CONCLUSIONS

The sub-grouping method may provide useful insights into the evolution of genotype II ASFV as well as providing evidence of a relationship between geographically separated outbreaks.

Rapid Detection of Genotype II African Swine Fever Virus Using CRISPR Cas13a-Based Lateral Flow Strip

The African swine fever virus (ASFV) is a dsDNA virus that can cause serious, highly infectious, and fatal diseases in wild boars and domestic pigs. The ASFV has brought enormous economic loss to many countries, and no effective vaccine or treatment for the ASFV is currently available. Therefore, the on-site rapid and accurate detection of the ASFV is key to the timely implementation of control. The RNA-guided, RNA-targeting CRISPR effector CRISPR-associated 13 (Cas13a; previously known as C2c2) exhibits a “collateral effect” of promiscuous RNase activity upon the target recognition. The collateral cleavage activity of LwCas13a is activated to degrade the non-targeted RNA, when the crRNA of LwCas13a binds to the target RNA. In this study, we developed a rapid and sensitive ASFV detection method based on the collateral cleavage activity of LwCas13a, which combines recombinase-aided amplification (RAA) and a lateral flow strip (named CRISPR/Cas13a-LFD). The method was an isothermal detection at 37 °C, and the detection can be used for visual readout. The detection limit of the CRISPR/Cas13a-LFD was 101 copies/µL of p72 gene per reaction, and the detection process can be completed within an hour. The assay showed no cross-reactivity to eight other swine viruses, including classical swine fever virus (CSFV), and has a 100% coincidence rate with real-time PCR detection of the ASFV in 83 clinical samples. Overall, this method is sensitive, specific, and practicable onsite for the ASFV detection, showing a great application potential for monitoring the ASFV in the field.

Establishment of a Blocking ELISA Detection Method for Against African Swine Fever Virus p30 Antibody

African swine fever (ASF) is a highly lethal hemorrhagic viral disease of domestic pigs caused by African swine fever virus (ASFV). A sensitive and reliable serological diagnostic assay is required, so laboratories can effectively and quickly detect ASFV infection. The p30 protein is abundantly expressed early in cells and has excellent antigenicity. Therefore, this study aimed to produce and characterize p30 monoclonal antibodies with an ultimate goal of developing a monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for ASFV antibody detection. Three monoclonal antibodies against p30 protein that were expressed in E. coli were generated, and their characterizations were investigated. Furthermore, a blocking ELISA based on a monoclonal antibody was developed. To evaluate the performance of the assay, 186 sera samples (88 negative and 98 positive samples) were analyzed and a receiver-operating characteristic (ROC) analysis was applied to determine the cutoff value. Based on the ROC analysis, the area under the curve (AUC) was 0.997 (95% confidence interval: 99.2 to 100%). Besides, a diagnostic sensitivity of 97.96% (95% confidence interval: 92.82 to 99.75%) and a specificity of 98.96% (95% confidence interval: 93.83 to 99.97%) were achieved when the cutoff value was set to 38.38%. Moreover, the coefficients of inter- and intra-batches were <10%, indicating the good repeatability of the method. The maximum dilution of positive standard serum detected by this ELISA method was 1:512. The blocking ELISA was able to detect seroconversion in two out of five pigs at 10 Dpi and the p30 response increasing trend through the time course of the study (0–20 Dpi). In conclusion, the p30 mAb-based blocking ELISA developed in this study demonstrated a high repeatability with maximized diagnostic sensitivity and specificity. The assay could be a useful tool for field surveillance and epidemiological studies in swine herd.

Development and application of a colloidal-gold dual immunochromatography strip for detecting African swine fever virus antibodies

African swine fever (ASF) is an acute and highly contagious infectious disease caused by the African swine fever virus (ASFV). Currently, there is no vaccine against ASF worldwide, and no effective treatment measures are available. For this reason, developing a simple, rapid, specific, and sensitive serological detection method for ASFV antibodies is crucial for the prevention and control of ASF. In this study, a 1:1 mixture of gold-labeled p30 and p72 probes was used as the gold-labeled antigen. The p30 and p72 proteins and their monoclonal antibodies were coated on a nitrocellulose membrane (NC) as a test (T) line and control (C) line, respectively. A colloidal-gold dual immunochromatography strip (ICS) for ASFV p30 and p72 protein antibodies was established. The results showed that the colloidal-gold dual ICS could specifically detect ASFV antibodies within 5-10 min. There was no cross-reaction after testing healthy pig serum; porcine reproductive and respiratory syndrome virus (PRRSV), foot-and-mouth disease type A virus (FMDV-A), foot-and-mouth disease type O virus (FMDV-O), porcine circovirus type 2 (PCV-2), and classical swine fever virus (CSFV) positive sera. A positive result was obtained only for the positive control P1. The sensitivity of the test strips was 1:256, which was equivalent to that of commercially ELISA kits. Their coincidence rate with the two commercial ASFV ELISA antibodies detection kits was higher than 98%. The test strips were stably stored at 18-25 °C and 4 °C for 4 and 6 months, respectively. The colloidal-gold dual ICS prepared in this study had high sensitivity and specificity and were characterized by rapid detection, simple operation, and easy interpretation of results. Therefore, they are of great significance to diagnose, prevent, and control African swine fever. KEY POINTS: • We establish an antibody detection that is quick and can monitor an ASF infection. • We observe changes in two protein antibodies to dynamically monitor ASF infection. • We use diversified detection on a single test strip to detect both antibodies.

Development of Real-Time PCR Based on A137R Gene for the Detection of African Swine Fever Virus

African swine fever virus (ASFV) can infect domestic pigs and wild boars and causes huge economic losses in global swine industry. Therefore, early diagnosis of ASFV is important for the control and eradication of African swine fever (ASF). In this study, a SYBR Green-based real-time polymerase chain reaction (PCR) assay targeting the viral encoded A137R gene was established for the detection of ASFV infection. For the evaluation of the established real-time PCR, 34 clinical samples were assessed by both the A137R gene-based real-time PCR and OIE-recommended TaqMan PCR. The results showed that 85.29% (29/34) were detected by A137R gene-based real-time PCR, but only 79.41% (27/34) positive using OIE-recommended TaqMan PCR. Moreover, no cross-reaction with other common swine pathogens was found in the A137R gene-based real-time PCR. These results demonstrated that the established real-time PCR assay in this study showed better performance than the OIE-recommended method in detecting ASFV from clinical samples, which could be applied for control and eradication programs of ASF.

Evaluation of an automated insulated isothermal polymerase chain reaction system for rapid and reliable, on-site detection of African swine fever virus

OBJECTIVE

To evaluate the utility of an automated insulated isothermal PCR (iiPCR) system for rapid and reliable on-site detection of African swine fever virus (ASFV) in swine biological samples.

SAMPLE

Lymph node, tissue homogenate, whole blood, serum, spleen, and tonsil samples collected from swine in North and South Vietnam.

PROCEDURES

Analytic sensitivity of the iiPCR system was determined by serial dilution and analysis of 2 samples (swine tissue homogenate and blood) predetermined to be positive for ASFV. Analytic specificity was assessed by analysis of 2 samples predetermined to be negative for ASFV and positive or negative for other swine pathogens (classical swine fever virus, porcine reproductive and respiratory syndrome virus, foot-and-mouth disease virus, and porcine circovirus type 2). Diagnostic performance of the iiPCR system for detection of ASFV was determined by analysis of the various tissue sample types. For all tests, a real-time PCR assay was used as the reference method.

RESULTS

The iiPCR system was able to detect ASFV in swine blood or tissue homogenate at dilutions up to 10⁶, whereas the real-time PCR assay was able to detect dilutions of up to 10⁵ or 10⁶. The iiPCR system had high analytic specificity for detection of ASFV versus other swine pathogens. Between 97% and 100% agreement was found between results of the iiPCR system for the various tissue samples and results of real-time PCR assay.

CONCLUSIONS AND CLINICAL RELEVANCE

The evaluated iiPCR system was found to be a rapid, reliable, and sample-flexible method for ASFV detection and may be useful for disease surveillance and quarantine in national strategies for early ASF control.