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

Thoughts on African Swine Fever Vaccines

African swine fever (ASF) is an acute viral hemorrhagic disease of domestic swine with mortality rates approaching 100%. Devastating ASF outbreaks and continuing epidemics starting in the Caucasus region and now in the Russian Federation, Europe, China, and other parts of Southeast Asia (2007 to date) highlight its significance. ASF strain Georgia-07 and its derivatives are now endemic in extensive regions of Europe and Asia and are "out of Africa" forever, a situation that poses a grave if not an existential threat to the swine industry worldwide. While our current concern is Georgia-07, other emerging ASFV strains will threaten for the indefinite future. Economic analysis indicates that an ASF outbreak in the U.S. would result in approximately $15 billion USD in losses, assuming the disease is rapidly controlled and the U.S. is able to reenter export markets within two years. ASF's potential to spread and become endemic in new regions, its rapid and efficient transmission among pigs, and the relative stability of the causative agent ASF virus (ASFV) in the environment all provide significant challenges for disease control. Effective and robust methods, including vaccines for ASF response and recovery, are needed immediately.

Development of an indirect ELISA to specifically detect antibodies against African swine fever virus: bioinformatics approaches

BACKGROUND

African swine fever (ASF), characterized by acute, severe, and fast-spreading, is a highly lethal swine infectious disease caused by the African swine fever virus (ASFV), which has caused substantial economic losses to the pig industry worldwide in the past 100 years.

METHODS

This study started with bioinformatics methods and verified the epitope fusion protein method's reliability that does not rely on traditional epitope identification. Meanwhile, it will also express and purify the constructed genes through prokaryotic expression and establish antibody detection methods.

RESULTS

The results indicated that the protein had good reactivity and did not cross-react with other swine diseases. The receiver-operating characteristic analysis was performed to verify the determination. The area under the receiver-operating characteristic curve was 0.9991 (95% confidence interval 0.9973 to 1.001).

CONCLUSIONS

It was proved that the recombinant protein is feasible as a diagnostic antigen to distinguish ASFV and provides a new idea for ASFV antibody detection.

Development of a Potential Penside Colorimetric LAMP Assay Using Neutral Red for Detection of African Swine Fever Virus

African swine fever (ASF) has caused huge economic losses to the swine industry worldwide. Since there is no commercial ASF vaccine available, an early diagnosis is extremely important to prevent and control the disease. In this study, ASF virus (ASFV) capsid protein-encoding gene (p72) was selected and used to design primers for establishing a one-step visual loop-mediated isothermal amplification (LAMP) assay with neutral red, a pH-sensitive dye, as the color shift indicator. Neutral red exhibited a sharp contrast of color change from faint orange (negative) to pink (positive) during LAMP for detection of ASFV. The designed primer set targeting highly conserved region of the p72 gene was highly specific to ASFV and showed no cross-reactivity with other swine viruses. The detection limit for the one-step visual LAMP developed was 10 copies/reaction based on the recombinant plasmid containing the p72 gene of ASFV. More importantly, the developed one-step visual LAMP showed high consistency with the results of the real-time polymerase chain reaction (qPCR) method recommended by World Organization for Animal Health (OIE). Furthermore, the results demonstrate that the colorimetric detection with this LAMP assay could be directly applied for the whole blood and serum samples without requiring genome extraction. Based on our results, the developed one-step visual LAMP assay is a promising penside diagnostic tool for development of early and cost-effective ASF monitoring program that would greatly contribute to the prevention and control of ASF.

A Productive Expression Platform Derived from Host-Restricted Eilat Virus: Its Extensive Validation and Novel Strategy

Most alphaviruses are transmitted by mosquitoes and infect a wide range of insects and vertebrates. However, Eilat virus (EILV) is defective for infecting vertebrate cells at multiple levels of the viral life cycle. This host-restriction property renders EILV an attractive expression platform since it is not infectious for vertebrates and therefore provides a highly advantageous safety profile. Here, we investigated the feasibility of versatile EILV-based expression vectors. By replacing the structural genes of EILV with those of other alphaviruses, we generated seven different chimeras. These chimeras were readily rescued in the original mosquito cells and were able to reach high titers, suggesting that EILV is capable of packaging the structural proteins of different lineages. We also explored the ability of EILV to express authentic antigens via double subgenomic (SG) RNA vectors. Four foreign genetic materials of varied length were introduced into the EILV genome, and the expressed heterologous genetic materials were readily detected in the infected cells. By inserting an additional SG promoter into the chimera genome containing the structural genes of Chikungunya virus (CHIKV), we developed a bivalent vaccine candidate against CHIKV and Zika virus. These data demonstrate the outstanding compatibility of the EILV genome. The produced recombinants can be applied to vaccine and diagnostic tool development, but more investigations are required.

Application of portable real-time recombinase-aided amplification (rt-RAA) assay in the clinical diagnosis of ASFV and prospective DIVA diagnosis

African swine fever, a serious infectious disease, has been found in many countries around the world over the last nearly 100 years, and causes untold damage to the economy wherever it occurs. Diagnosis is currently performed by real-time PCR, which is highly sensitive but can only be carried out in a diagnostic laboratory environment with sophisticated equipment and expertise. A sensitive, rapid diagnostic method that can be implemented in agricultural settings is thus urgently needed for the detection and control of African swine fever virus (ASFV) infection. In this study, we developed an isothermal amplification technology to achieve molecular diagnosis of ASFV in clinical samples, using recombinase-aided amplification (RAA) assay combined with a portable instrument. This assay method avoids the limitations of traditional real-time PCR and offers detection times within 20 min, enabling detection of as few as 10 copies of ASFV DNA molecules per reaction without cross-reaction with other common swine viruses. We evaluated clinical performance using 200 clinical blood samples. The coincidence rate of the detection results between rt-RAA and RT-qPCR was 96.94% positive, 100% negative, and 97.50% total. We have also developed an rt-RAA system for the detection of ASFV targeting the EP402R gene, with detection of as few as 10 copies of DNA per reaction; this offers the possibility of DIVA (differentiating infected from vaccinated animals) diagnosis, because CD2V gene-deleted ASFV could soon be approved to be the leading candidate for live attenuated vaccine in China. The rt-RAA assay is a reliable, rapid, highly sensitive method, and it offers a reasonable alternative to RT-qPCR for point-of-care detection of ASFV. KEY POINTS: • The RT-RAA assay can detect as few as 10 copies of ASFV genome per reaction within 20 min. • The rt-RAA assay system targeting different genes can achieve differentiating infected from vaccinated diagnosis.

Development of Diagnostic Tests Provides Technical Support for the Control of African Swine Fever

African swine fever is a highly contagious global disease caused by the African swine fever virus. Since African swine fever (ASF) was introduced to Georgia in 2007, it has spread to many Eurasian countries at an extremely fast speed. It has recently spread to China and other major pig-producing countries in southeast Asia, threatening global pork production and food security. As there is no available vaccine at present, prevention and control must be carried out based on early detection and strict biosecurity measures. Early detection should be based on the rapid identification of the disease on the spot, followed by laboratory diagnosis, which is essential for disease control. In this review, we introduced the prevalence, transmission routes, eradication control strategies, and diagnostic methods of ASF. We reviewed the various methods of diagnosing ASF, focusing on their technical characteristics and clinical test results. Finally, we give some prospects for improving the diagnosis strategy in the future.

Development and characterization of monoclonal antibodies against the N-terminal domain of African swine fever virus structural protein, p54

African swine fever virus (ASFV), a re-emerging DNA virus, causes a highly contagious disease for domestic pigs. It is running rife worldwide and threatening the global swine industry. Protein p54 is an attractive candidate for ASFV diagnostic and vaccine design. In this work, we designed a peptide to mimic the N-terminal domain (NTD) of ASFV p54 and pretested it with sera from ASFV-infected pigs. The peptide could be well recognized by the sera, implying that the NTD of p54 contained some potential linear B cell epitopes. Then, the conjugates of the peptide with bovine serum albumin were used as the immunogen to generate monoclonal antibodies (mAbs). A total of six mAbs specific to the NTD of ASFV p54 protein were developed. Five of them well reacted with ASFV HLJ/18 strain and recognized a same linear B cell epitope ⁵FFQPV⁹. Furthermore, epitope ⁵FFQPV⁹ could be well recognized by ASFV-positive sera from natural infected pigs, suggesting that it was a natural linear B-cell epitope. Conservation analysis indicated that epitope ⁵FFQPV⁹ were highly conserved among ASFV epidemic isolates belonging to genotype I and II. Alanine-scanning mutagenesis further revealed that the residues (6F to 9V) of epitope ⁵FFQPV⁹ were the core binding sites for antibody recognition. This is the first research to characterize specific mAbs against NTD of p54 protein. These findings may help further understand the function of p54 protein and the improvement of ASFV diagnosis.

African swine fever: a New Zealand perspective on epidemiological risk factors for its occurrence

This article reviews key epidemiological and clinical features of African swine fever (ASF). We identify particular aspects of New Zealand's pig populations (commercial, non-commercial, and wild) that may affect the risk of disease entry or spread. Review of published literature is supplemented by analysis of demographic and spatial aspects of the New Zealand commercial, non-commercial, and feral pig populations to provide context around risk factors for the disease that are most relevant to New Zealand. The current Eurasian outbreak of ASF, including recent spread into Oceania, has increased the risk of an incursion of the disease into New Zealand. Large volumes of fresh pork importation (including from countries affected by ASF), large non-commercial pig populations with substantial spatial overlap with the country's commercial industry, limited monitoring of compliance with waste food feeding regulations, and lack of mandatory premises identification for non-commercial pig holdings would likely contribute to the risk of spread of ASF in the event of an incursion. Awareness amongst veterinarians of these risk factors will contribute to national biosecurity and disease preparedness efforts in New Zealand.

Characterization of Anti-p54 Monoclonal Antibodies and Their Potential Use for African Swine Fever Virus Diagnosis

African swine fever (ASF) is a highly lethal hemorrhagic viral disease of domestic pigs caused by African swine fever virus (ASFV). Although a good advance has been made to understand the virus, a safe and effective vaccine against ASFV is still lacking and its eradication solely depends on its early and accurate diagnosis. Thus, improving the available diagnostic assays and adding some validated techniques are useful for a range of serological investigations. The aim of this study was to produce and characterize p54 monoclonal antibodies with an ultimate goal of developing a monoclonal antibody-based enzyme-linked immunosorbent assay (ELISA) for ASFV antibody detection. Five monoclonal antibodies against p54 protein expressed in Escherichia coli was generated and their characterizations were investigated. Furthermore, a competitive enzyme-linked immunosorbent assay (cELISA) based on a monoclonal antibody designated as 2A7 was developed. To evaluate the performance of the assay, a total of 365 pig serum samples (178 negative and 187 positive samples) were tested and a receiver-operating characteristic (ROC) analysis was applied to determine the cut-off value. Based on the ROC analysis, the area under the curve (AUC) was 0.982 (95% confidence interval: 96.9% to 99.4%), besides a sensitivity of 92.5% and a specificity of 98.9% was achieved when the percent inhibition of 20% was selected as a threshold. Moreover, the result showed an excellent agreement when compared to other commercially available blocking ELISA (kappa value = 0.912) and showed no reaction to other swine pathogens. Overall, the newly developed cELISA method offers a promising approach for a rapid and convenient ASFV serodiagnosis, which could be used as alternative to other serological assays for screening possible ASFV infection.

Rapid and accurate detection of African swine fever virus by DNA endonuclease-targeted CRISPR trans reporter assay

The first case of African swine fever (ASF) outbreak in China was reported in a suburban pig farm in Shenyang in 2018. Since then, the rapid spread and extension of ASF has become the most serious threat for the swine industry. Therefore, rapid and accurate detection of African swine fever virus (ASFV) is essential to provide effective strategies to control the disease. In this study, we developed a rapid and accurate ASFV-detection method based on the DNA endonuclease-targeted CRISPR trans reporter (DETECTR) assay. By combining recombinase polymerase amplification with CRISPR-Cas12a proteins, the DETECTR assay demonstrated a minimum detection limit of eight copies with no cross reactivity with other swine viruses. Clinical blood samples were detected by DETECTR assay and showed 100% (30/30) agreement with real-time polymerase chain reaction assay. The rapid and accurate detection of ASFV may facilitate timely eradication measures and strict sanitary procedures to control and prevent the spread of ASF.

A CRISPR/Cas12a Based Universal Lateral Flow Biosensor for the Sensitive and Specific Detection of African Swine-Fever Viruses in Whole Blood

Cross-border pathogens such as the African swine fever virus (ASFV) still pose a socio-economic threat. Cheaper, faster, and accurate diagnostics are imperative for healthcare and food safety applications. Currently, the discovery of the Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR) has paved the way for the diagnostics based on Cas13 and Cas12/14 that exhibit collateral cleavage of target and single-stranded DNA (ssDNA) reporter. The reporter is fluorescently labeled to report the presence of a target. These methods are powerful; however, fluorescence-based approaches require expensive apparatuses, complicate results readout, and exhibit high-fluorescence background. Here, we present a new CRISPR-Cas-based approach that combines polymerase chain reaction (PCR) amplification, Cas12a, and a probe-based lateral flow biosensor (LFB) for the simultaneous detection of seven types of ASFV. In the presence of ASFVs, the LFB responded to reporter trans-cleavage by naked eyes and achieved a sensitivity of 2.5 × 10-15 M within 2 h, and unambiguously identified ASFV from swine blood. This system uses less time for PCR pre-amplification and requires cheaper devices; thus, it can be applied to virus monitoring and food samples detection.

Integrated Micropillar Polydimethylsiloxane Accurate CRISPR Detection System for Viral DNA Sensing

A fully Integrated Micropillar Polydimethylsiloxane Accurate CRISPR deTection (IMPACT) system is developed for viral DNA detection. This powerful system is patterned with high-aspect-ratio micropillars to enhance reporter probe binding. After surface modification and probe immobilization, the CRISPR-Cas12a/crRNA complex is injected into the fully enclosed microchannel. With the presence of a double-stranded DNA target, the CRISPR enzyme is activated and denatures the single-stranded DNA reporters from the micropillars. This collateral cleavage releases fluorescence reporters into the assay, and the intensity is linearly proportional to the target DNA concentration ranging from 0.1 to 10 nM. Importantly, this system does not rely on the traditional dye-quencher-labeled probe, thus reducing the fluorescence background presented in the assay. Furthermore, our one-step detection protocol is performed on-chip at isothermal conditions (37 °C) without using complicated and time-consuming off-chip probe hybridization and denaturation. This miniaturized and fully packed IMPACT chip demonstrates sensitive and accurate DNA detection within 120 min and paves ways to the next-generation point-of-care diagnostics, responding to emerging and deadly pathogen outbreaks.

Multiplex and visual detection of African Swine Fever Virus (ASFV) based on Hive-Chip and direct loop-mediated isothermal amplification

African swine fever is caused by African swine fever virus (ASFV), and has a mortality rate approaching 100%. It has already caused tremendous economy lost around the world. Without effective vaccine, rapid and accurate on-site detection plays an indispensable role in controlling outbreaks. Herein, by combining Hive-Chip and direct loop-mediated isothermal amplification (LAMP), we establish a multiplex and visual detection platform. LAMP primers targeting five ASFV genes (B646L, B962L, C717R, D1133L, and G1340L) were designed and pre-fixed in Hive-Chip. On-chip LAMP showed the limits of detection (LOD) of ASFV synthetic DNAs and mock samples are 30 and 50 copies per microliter, respectively, and there is no cross-reaction among the target genes. The overall performance of our platform is comparable to that of the commercial kits. From sample preparation to results readout, the entire process takes less than 70 min. Multiplex detection of real samples of ASFV and other swine viruses further demonstrates the high sensitivity and specificity of Hive-Chip. Overall, our platform provides a promising option for on-site, fast and accurate detection of ASFV.

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Hive-Chip firstly realized simultaneous detection of multiple genes of ASFV, largely avoiding false-negative results.

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Without nucleic acid extraction, direct LAMP was firstly incorporated into the Hive-Chip for visual detection.

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Because very little operation and no complicate instrument is required, on-site detection is possible for this platform.

Detection of African swine fever virus genotype XV in a sylvatic cycle in Saadani National Park, Tanzania

African swine fever (ASF) is a severe haemorrhagic disease of domestic pigs caused by ASF virus (ASFV). ASFV is transmitted by soft ticks (Ornithodoros moubata complex group) and by direct transmission. In Africa, ASF is maintained in transmission cycles of asymptomatic infection involving wild suids, mainly warthogs (Phacochoerus africanus). ASF outbreaks have been reported in many parts of Tanzania; however, active surveillance has been limited to pig farms in a few geographical locations. There is an information gap on whether and where the sylvatic cycle may occur independently of domestic pigs. To explore the existence of a sylvatic cycle in Saadani National Park in Tanzania, blood and serum samples were collected from 19 warthogs selected using convenience sampling along vehicle-accessible transects within the national park. The ticks were sampled from warthog burrows. Blood samples and ticks were subjected to ASFV molecular diagnosis (PCR) and genotyping, and warthog sera were subjected to serological (indirect ELISA) testing for ASFV antibody detection. All warthog blood samples were PCR-negative, but 16/19 (84%) of the warthog sera were seropositive by ELISA confirming exposure of warthogs to ASFV. Of the ticks sampled, 20/111 (18%) were positive for ASFV by conventional PCR. Sequencing of the p72 virus gene fragments showed that ASF viruses detected in ticks belonged to genotype XV. The results confirm the existence of a sylvatic cycle of ASFV in Saadani National Park, Tanzania, that involves ticks and warthogs independent of domestic pigs. Our findings suggest that genotype XV previously reported in 2008 in Tanzania is likely to be widely distributed and involved in both wild and domestic infection cycles. Whole-genome sequencing and analysis of the ASFV genotype XV circulating in Tanzania is recommended to determine the phylogeny of the viruses.

Genetic Characterisation of African Swine Fever Virus in Outbreaks in Ha Nam Province, Red River Delta Region of Vietnam, and Activity of Antimicrobial Products Against Virus Infection in Contaminated Feed

Introduction

African swine fever (ASF) was officially reported in Vietnam in February 2019 and spread across the whole country, affecting all 63 provinces and cities.

Material and Methods

In this study, ASF virus (ASFV) VN/Pig/HaNam/2019 (VN/Pig/HN/19) strain was isolated in primary porcine alveolar macrophage (PAM) cells from a sample originating from an outbreak farm in Vietnam’s Red River Delta region. The isolate was characterised using the haemadsorption (HAD) test, real-time PCR, and sequencing. The activity of antimicrobial feed products was evaluated via a contaminated ASFV feed assay.

Results

Phylogenetic analysis of the viral p72 and EP402R genes placed VN/Pig/HN/19 in genotype II and serogroup 8 and related it closely to Eastern European and Chinese strains. Infectious titres of the virus propagated in primary PAMs were 10⁶ HAD₅₀/ml. Our study reports the activity against ASFV VN/Pig/HN/19 strain of antimicrobial Sal CURB RM E Liquid, F2 Dry and K2 Liquid. Our feed assay findings suggest that the antimicrobial RM E Liquid has a strong effect against ASFV replication. These results suggest that among the Sal CURB products, the antimicrobial RM E Liquid may have the most potential as a mitigant feed additive for ASFV infection. Therefore, further studies on the use of antimicrobial Sal CURB RM E Liquid in vivo are required.

Conclusions

Our study demonstrates the threat of ASFV and emphasises the need to control and eradicate it in Vietnam by multiple measures.

Rapid and visual detection of African swine fever virus antibody by using fluorescent immunochromatography test strip

African swine fever virus (ASFV) is a large and complex DNA virus that causes a highly contagious and often lethal swine viral disease, for which no vaccine and effective treatments are available yet. Hence, ASFV presents significant economic consequences for the swine industry. A rapid and simple diagnostic method is urgently needed to monitor ASFV-specific antibodies for controlling the spread of ASFV. In this study, we chose the truncated p54 protein as an antigen and combined it with Eu-doped fluorescent microspheres as tracers to detect anti-ASFV antibodies specifically. Results showed that the truncated p54 protein had high specificity to ASFV antibody and had no cross-reactions with other swine virus antibodies. The results between our fluorescent immunochromatography test strip (FICTS) and commercial ELISA kits showed high consistency. The proposed FICTS offers a rapid, sensitive, specific, and visual method for ASFV antibody detection and shows great potential for ASF epidemic surveillance and control.

African Swine Fever Virus: An Emerging DNA Arbovirus

African swine fever virus (ASFV) is the sole member of the family Asfarviridae, and the only known DNA arbovirus. Since its identification in Kenya in 1921, ASFV has remained endemic in Africa, maintained in a sylvatic cycle between Ornithodoros soft ticks and warthogs (Phacochoerus africanus) which do not develop clinical disease with ASFV infection. However, ASFV causes a devastating and economically significant disease of domestic (Sus scrofa domesticus) and feral (Sus scrofa ferus) swine. There is no ASFV vaccine available, and current control measures consist of strict animal quarantine and culling procedures. The virus is highly stable and easily spreads by infected swine, contaminated pork products and fomites, or via transmission by the Ornithodoros vector. Competent Ornithodoros argasid soft tick vectors are known to exist not only in Africa, but also in parts of Europe and the Americas. Once ASFV is established in the argasid soft tick vector, eradication can be difficult due to the long lifespan of Ornithodoros ticks and their proclivity to inhabit the burrows of warthogs or pens and shelters of domestic pigs. Establishment of endemic ASFV infections in wild boar populations further complicates the control of ASF. Between the late 1950s and early 1980s, ASFV emerged in Europe, Russia and South America, but was mostly eradicated by the mid-1990s. In 2007, a highly virulent genotype II ASFV strain emerged in the Caucasus region and subsequently spread into the Russian Federation and Europe, where it has continued to circulate and spread. Most recently, ASFV emerged in China and has now spread to several neighboring countries in Southeast Asia. The high morbidity and mortality associated with ASFV, the lack of an efficacious vaccine, and the complex makeup of the ASFV virion and genome as well as its lifecycle, make this pathogen a serious threat to the global swine industry and national economies. Topics covered by this review include factors important for ASFV infection, replication, maintenance, and transmission, with attention to the role of the argasid tick vector and the sylvatic transmission cycle, current and future control strategies for ASF, and knowledge gaps regarding the virus itself, its vector and host species.

Establishment and characterization of the pig tonsil epithelial (PT) cell line as a new model for persist infection of Japanese Encephalitis Virus

Japanese encephalitis virus (JEV) causes a serious zoonotic disease worldwide, pig is the reservoir and amplifying host of JEV. JEV can persist infect tonsil in pig, but the relation between persist infection in tonsil and reservoir are not clear until now. A stable pig tonsil cell line is necessary for JEV persist infection research. In this study, we established a continuous epithelial cell line, named PT cell, from the pig tonsil. This cell is susceptible to JEV. We determined the growth characteristics, molecular properties, microstructure profiles of PT cell. JEV is easy to enter PT cell which may partly explain the reason of persist infection. We further determined that LMAN2L, a mannose lectin proteins, is the primary viral receptors for JEV entry in PT cell. IFITM3, an cellular surface antiviral factor, is underexpression in PT cell after JEV infection. All these results provide solid evidence that PT cell will promote additional research on JEV persist infection in pig tonsil.

An improvement of real-time polymerase chain reaction system based on probe modification is required for accurate detection of African swine fever virus in clinical samples in Vietnam

Objective

The rapid and reliable detection of the African swine fever virus (ASFV) plays an important role in emergency control and preventive measures of ASF. Some methods have been recommended by FAO/OIE to detect ASFV in clinical samples, including real-time polymerase chain reaction (PCR). However, mismatches in primer and probe binding regions may cause a false-negative result. Here, a slight modification in probe sequence has been conducted to improve the qualification of real-time PCR based on World Organization for Animal Health (OIE) protocol for accurate detection of ASFV in field samples in Vietnam.

Methods

Seven positive confirmed samples (four samples have no mismatch, and three samples contained one mutation in probe binding sites) were used to establish novel real-time PCR with slightly modified probe (Y = C or T) in comparison with original probe recommended by OIE.

Results

Both real-time PCRs using the OIE-recommended probe and novel modified probe can detect ASFV in clinical samples without mismatch in probe binding site. A high correlation of cycle quantification (Cq) values was observed in which Cq values obtained from both probes arranged from 22 to 25, suggesting that modified probe sequence does not impede the qualification of real-time PCR to detect ASFV in clinical samples. However, the samples with one mutation in probe binding sites were ASFV negative with OIE recommended probe but positive with our modified probe (Cq value ranked between 33.12–35.78).

Conclusion

We demonstrated for the first time that a mismatch in probe binding regions caused a false negative result by OIE recommended real-time PCR, and a slightly modified probe is required to enhance the sensitivity and obtain an ASF accurate diagnosis in field samples in Vietnam.

Bead-Based Multiplex Assay for the Simultaneous Detection of Antibodies to African Swine Fever Virus and Classical Swine Fever Virus

African swine fever (ASF) and Classical swine fever (CSF) are both highly contagious diseases of domestic pigs and wild boar. In the last years, several cases of both diseases have been reported in the Caucasus, Russian Federation and Eastern Europe. Thus, the probability of encountering these two viruses in the same area is increasing. Since differentiation by clinical or post-mortem examination is not possible, laboratory tools for differential diagnosis are required. In the present work, we have developed a triplex bead-based assay using some of the most immunogenic antigens of each virus, for the simultaneous detection of antibodies; i.e. the VP72 and VP30 of ASF virus (ASFV) and the E2 protein of CSF virus (CSFV). The assay was firstly set up and optimized using well characterized reference serum samples specific for each pathogen. Then, a panel of 352 sera from experimentally infected animals with either ASFV or CSFV were analyzed in the multiplex assay. A collection of 253 field negative sera was also included in the study. The results of the multiplex analysis were compared to those obtained by two commercially available ELISAs for detection of antibodies against ASFV or CSFV, and considered in this study as the reference techniques. The data obtained showed values of 97.3% sensitivity and 98.3% specificity for detection of antibodies to ASFV and 95.7% of sensitivity and 99.8% specificity for detection of antibodies to CSFV. This multiplex assay allows the simultaneous and differential detection of antibodies against ASFV and CSFV, providing a valuable tool for surveillance studies. Moreover, this method is rather versatile, offering the possibility of increasing the panel of antigens from other swine diseases that could be of interest for a differential diagnosis along with ASF and CSF.

Roles of African Swine Fever Virus Structural Proteins in Viral Infection

African swine fever virus (ASFV) is a large, double-stranded DNA virus and the sole member of the Asfarviridae family. ASFV infects domestic pigs, wild boars, warthogs, and bush pigs, as well as soft ticks (Ornithodoros erraticus), which likely act as a vector. The major target is swine monocyte-macrophage cells. The virus can cause high fever, haemorrhagic lesions, cyanosis, anorexia, and even fatalities in domestic pigs. Currently, there is no vaccine and effective disease control strategies against its spread are culling infected pigs and maintaining high biosecurity standards. African swine fever (ASF) spread to Europe from Africa in the middle of the 20^th century, and later also to South America and the Caribbean. Since then, ASF has spread more widely and thus is still a great challenge for swine breeding. The genome of ASFV ranges in length from about 170 to 193 kbp depending on the isolate and contains between 150 and 167 open reading frames (ORFs). The ASFV genome encodes 150 to 200 proteins, around 50 of them structural. The roles of virus structural proteins in viral infection have been described. These proteins, such as pp220, pp62, p72, p54, p30, and CD2v, serve as the major component of virus particles and have roles in attachment, entry, and replication. All studies on ASFV proteins lay a good foundation upon which to clarify the infection mechanism and develop vaccines and diagnosis methods. In this paper, the roles of ASFV structural proteins in viral infection are reviewed.

Piloting the effectiveness of pig health education in combination with oxfendazole treatment on prevention and/or control of porcine cysticercosis, gastrointestinal parasites, African swine fever and ectoparasites in Angónia District, Mozambique

A community-based intervention combining health education (HE) and treatment of pigs for control of porcine cysticercosis (PC), gastrointestinal (GI) helminths, African swine fever (ASF) and external parasites was tested involving six villages of resource-poor smallholder pig farmers. Farmers and pigs of six rural villages were randomly allocated into group 1 (HE), which served as controls, and group 2 (HE + OFZ) pigs received a single oral dose of 30 mg/kg OFZ. Farmers were trained in pig health, housing and feeding. The proportion of farmers with confined pigs, the adoption rate of the introduced pig pen, the sero-prevalence of PC and ASF, the prevalence and intensity of GI nematodes and the prevalence of ectoparasites were measured at 9, 15 and 24 months after initiation and compared to a baseline survey to seek the effectiveness of the interventions. There was no clear effect of the intervention on the sero-prevalence of PC, but analysis of the rate of change in prevalence between the two groups showed significant effect with the rate of change to lower prevalence in the HE + OFZ group compared to the HE group. Although HE managed to improve the farmer's knowledge in the control and prevention of ASF and ectoparasites, there was no significant reduction in the sero-prevalence of ASF and the prevalence of ectoparasites throughout the two-year period. The reported ineffectiveness of the intervention in this study suggested that more research is needed to develop more effective methods for controlling PC, ASF and pig parasites.

African swine fever virus infection in Classical swine fever subclinically infected wild boars

BACKGROUND

Recently moderate-virulence classical swine fever virus (CSFV) strains have been proven capable of generating postnatal persistent infection (PI), defined by the maintenance of viremia and the inability to generate CSFV-specific immune responses in animals. These animals also showed a type I interferon blockade in the absence of clinical signs. In this study, we assessed the infection generated in 7-week-old CSFV PI wild boars after infection with the African swine fever virus (ASFV). The wild boars were divided in two groups and were infected with ASFV. Group A comprised boars who were CSFV PI in a subclinical form and Group B comprised pestivirus-free wild boars. Some relevant parameters related to CSFV replication and the immune response of CSFV PI animals were studied. Additionally, serum soluble factors such as IFN-α, TNF-α, IL-6, IL-10, IFN-γ and sCD163 were analysed before and after ASFV infection to assess their role in disease progression.

RESULTS

After ASFV infection, only the CSFV PI wild boars showed progressive acute haemorrhagic disease; however, the survival rates following ASFV infection was similar in both experimental groups. Notwithstanding, the CSFV RNA load of CSFV PI animals remained unaltered over the study; likewise, the ASFV DNA load detected after infection was similar between groups. Interestingly, systemic type I FN-α and IL-10 levels in sera were almost undetectable in CSFV PI animals, yet detectable in Group B, while detectable levels of IFN-γ were found in both groups. Finally, the flow cytometry analysis showed an increase in myelomonocytic cells (CD172a⁺) and a decrease in CD4⁺ T cells in the PBMCs from CSFV PI animals after ASFV infection.

CONCLUSIONS

Our results showed that the immune response plays a role in the progression of disease in CSFV subclinically infected wild boars after ASFV infection, and the immune response comprised the systemic type I interferon blockade. ASFV does not produce any interference with CSFV replication, or vice versa. ASFV infection could be a trigger factor for the disease progression in CSFV PI animals, as their survival after ASFV was similar to that of the pestivirus-free ASFV-infected group. This fact suggests a high resistance in CSFV PI animals even against a virus like ASFV; this may mean that there are relevant implications for CSF control in endemic countries. The diagnosis of ASFV and CSFV co-infection in endemic countries cannot be ruled out and need to be studied in greater depth.

African Swine Fever Virus: A Review

African swine fever (ASF) is a highly contagious viral disease of swine which causes high mortality, approaching 100%, in domestic pigs. ASF is caused by a large, double stranded DNA virus, ASF virus (ASFV), which replicates predominantly in the cytoplasm of macrophages and is the only member of the Asfarviridae family, genus Asfivirus. The natural hosts of this virus include wild suids and arthropod vectors of the Ornithodoros genus. The infection of ASFV in its reservoir hosts is usually asymptomatic and develops a persistent infection. In contrast, infection of domestic pigs leads to a lethal hemorrhagic fever for which there is no effective vaccine. Identification of ASFV genes involved in virulence and the characterization of mechanisms used by the virus to evade the immune response of the host are recognized as critical steps in the development of a vaccine. Moreover, the interplay of the viral products with host pathways, which are relevant for virus replication, provides the basic information needed for the identification of potential targets for the development of intervention strategies against this disease.

Challenges for African swine fever vaccine development-"… perhaps the end of the beginning."

African swine fever (ASF), an acute, viral hemorrhagic disease in domestic swine with mortality rates approaching 100%, is arguably the most significant emerging disease threat for the swine industry worldwide. Devastating ASF outbreaks and continuing epidemic in the Caucasus region and Russia (2007-to date) highlight significance of this disease threat. There is no vaccine for ASF, thus leaving animal slaughter the only effective disease control option. It is clear, however, that vaccination is possible since protection against reinfection with the homologous strain of African swine fever virus (ASFV) has been clearly demonstrated. Vaccine development has been hindered by large gaps in knowledge concerning ASFV infection and immunity, the extent of ASFV strain variation in nature and the identification of viral proteins (protective antigens) responsible for inducing protective immune responses in the pig. This review focuses on the challenges surrounding ASF vaccine design and development, with an emphasis on existing knowledge gaps.

Detection of African Swine Fever Virus Antibodies in Serum and Oral Fluid Specimens Using a Recombinant Protein 30 (p30) Dual Matrix Indirect ELISA

In the absence of effective vaccine(s), control of African swine fever caused by African swine fever virus (ASFV) must be based on early, efficient, cost-effective detection and strict control and elimination strategies. For this purpose, we developed an indirect ELISA capable of detecting ASFV antibodies in either serum or oral fluid specimens. The recombinant protein used in the ELISA was selected by comparing the early serum antibody response of ASFV-infected pigs (NHV-p68 isolate) to three major recombinant polypeptides (p30, p54, p72) using a multiplex fluorescent microbead-based immunoassay (FMIA). Non-hazardous (non-infectious) antibody-positive serum for use as plate positive controls and for the calculation of sample-to-positive (S:P) ratios was produced by inoculating pigs with a replicon particle (RP) vaccine expressing the ASFV p30 gene. The optimized ELISA detected anti-p30 antibodies in serum and/or oral fluid samples from pigs inoculated with ASFV under experimental conditions beginning 8 to 12 days post inoculation. Tests on serum (n = 200) and oral fluid (n = 200) field samples from an ASFV-free population demonstrated that the assay was highly diagnostically specific. The convenience and diagnostic utility of oral fluid sampling combined with the flexibility to test either serum or oral fluid on the same platform suggests that this assay will be highly useful under the conditions for which OIE recommends ASFV antibody surveillance, i.e., in ASFV-endemic areas and for the detection of infections with ASFV isolates of low virulence.

Evidence for the presence of African swine fever virus in an endemic region of Western Kenya in the absence of any reported outbreak

BACKGROUND

African swine fever (ASF), caused by African swine fever virus (ASFV), is a severe haemorrhagic disease of pigs, outbreaks of which can have a devastating impact upon commercial and small-holder pig production. Pig production in western Kenya is characterised by low-input, free-range systems practised by poor farmers keeping between two and ten pigs. These farmers are particularly vulnerable to the catastrophic loss of livestock assets experienced in an ASF outbreak. This study wished to expand our understanding of ASFV epidemiology during a period when no outbreaks were reported.

RESULTS

Two hundred and seventy six whole blood samples were analysed using two independent conventional and real time PCR assays to detect ASFV. Despite no recorded outbreak of clinical ASF during this time, virus was detected in 90/277 samples analysed by conventional PCR and 142/209 samples analysed by qPCR. Genotyping of a sub-set of these samples indicated that the viruses associated with the positive samples were classified within genotype IX and that these strains were therefore genetically similar to the virus associated with the 2006/2007 ASF outbreaks in Kenya.

CONCLUSION

The detection of ASFV viral DNA in a relatively high number of pigs delivered for slaughter during a period with no reported outbreaks provides support for two hypotheses, which are not mutually exclusive: (1) that virus prevalence may be over-estimated by slaughter-slab sampling, relative to that prevailing in the wider pig population; (2) that sub-clinical, chronically infected or recovered pigs may be responsible for persistence of the virus in endemic areas.

Recombinant Protein p30 for Serological Diagnosis of African Swine Fever by Immunoblotting Assay

This article is devoted to the development and evaluation of the immunoblotting test system for serological diagnosis of African swine fever (ASF), based on the highly purified recombinant p30 of ASF virus (ASFV) strain Stavropol 01/08 (Stavropol 2008), representative of the ASFV currently circulating in the Russian Federation. The main project stages are as follows: (i) cloning of the central hydrophilic region of the ASFV gene CP204L (p30) into a prokaryotic vector; (ii) expression and chromatographic purification of the recombinant product p30 with thioredoxin and poly-histidine site (p30e1_TrxA_6xHis); (iii) development of the immunoblotting test system (Rec p30-IB) using the highly purified recombinant p30; and (iv) evaluation of Rec p30-IB using sera and organ samples from domestic pigs and wild boars experimentally or naturally infected by ASFV. Testing of the Rec p30-IB showed the diagnostic specificity and sensitivity of the assay to be 98.75% and 100.00%, respectively. High sensitivity of the Rec p30-IB allowed the detection of ASFV-specific antibodies in samples of organs of the immune system and blood sera, collected from domestic pigs and wild boars, starting from 6 to 8 days post-infection, regardless of virus virulence, seroimmunotype and geographic origin of the samples (East Europe, South Europe, West Europe, Central and south-east Africa).

African swine fever virus serotype-specific proteins are significant protective antigens for African swine fever

African swine fever (ASF) is an emerging disease threat for the swine industry worldwide. No ASF vaccine is available and progress is hindered by lack of knowledge concerning the extent of ASFV strain diversity and the viral antigens conferring type-specific protective immunity in pigs. Available data from vaccination/challenge experiments in pigs indicate that ASF protective immunity may be haemadsorption inhibition (HAI) serotype-specific. Recently, we have shown that two ASFV proteins, CD2v (EP402R) and C-type lectin (EP153R), are necessary and sufficient for mediating HAI serological specificity (Malogolovkin et al., 2015).. Here, using ASFV inter-serotypic chimeric viruses and vaccination/challenge experiments in pigs, we demonstrate that serotype-specific CD2v and/or C-type lectin proteins are important for protection against homologous ASFV infection. Thus, these viral proteins represent significant protective antigens for ASFV that should be targeted in future vaccine design and development. Additionally, these data support the concept of HAI serotype-specific protective immunity.

Assessment of African Swine Fever Diagnostic Techniques as a Response to the Epidemic Outbreaks in Eastern European Union Countries: How To Improve Surveillance and Control Programs

This study represents a complete comparative analysis of the most widely used African swine fever (ASF) diagnostic techniques in the European Union (EU) using field and experimental samples from animals infected with genotype II ASF virus (ASFV) isolates circulating in Europe. To detect ASFV, three different PCRs were evaluated in parallel using 785 field and experimental samples. The results showed almost perfect agreement between the Universal ProbeLibrary (UPL-PCR) and the real-time (κ = 0.94 [95% confidence interval {CI}, 0.91 to 0.97]) and conventional (κ = 0.88 [95% CI, 0.83 to 0.92]) World Organisation for Animal Health (OIE)-prescribed PCRs. The UPL-PCR had greater diagnostic sensitivity for detecting survivors and allows earlier detection of the disease. Compared to the commercial antigen enzyme-linked immunosorbent assay (ELISA), good-to-moderate agreement (κ = 0.67 [95% CI, 0.58 to 0.76]) was obtained, with a sensitivity of 77.2% in the commercial test. For ASF antibody detection, five serological methods were tested, including three commercial ELISAs, the OIE-ELISA, and the confirmatory immunoperoxidase test (IPT). Greater sensitivity was obtained with the IPT than with the ELISAs, since the IPT was able to detect ASF antibodies at an earlier point in the serological response, when few antibodies are present. The analysis of the exudate tissues from dead wild boars showed that IPT might be a useful serological tool for determining whether or not animals had been exposed to virus infection, regardless of whether antibodies were present. In conclusion, the UPL-PCR in combination with the IPT was the most trustworthy method for detecting ASF during the epidemic outbreaks affecting EU countries in 2014. The use of the most appropriate diagnostic tools is critical when implementing effective control programs.

A longitudinal survey of African swine fever in Uganda reveals high apparent disease incidence rates in domestic pigs, but absence of detectable persistent virus infections in blood and serum

BACKGROUND

African swine fever (ASF) is a fatal, haemorrhagic disease of domestic pigs, that poses a serious threat to pig farmers and is currently endemic in domestic pigs in most of sub-Saharan Africa. To obtain insight into the factors related to ASF outbreaks at the farm-level, a longitudinal study was performed in one of the major pig producing areas in central Uganda. Potential risk factors associated with outbreaks of ASF were investigated including the possible presence of apparently healthy ASF-virus (ASFV) infected pigs, which could act as long-term carriers of the virus. Blood and serum were sampled from 715 pigs (241 farms) and 649 pigs (233 farms) to investigate presence of ASFV and antibodies, during the periods of June-October 2010 and March-June 2011, respectively. To determine the potential contribution of different risks to ASF spread, a questionnaire-based survey was administered to farmers to assess the association between ASF outbreaks during the study period and the risk factors.

RESULTS

Fifty-one (21 %) and 13 (5.6 %) farms reported an ASF outbreak on their farms in the previous one to two years and during the study period, respectively. The incidence rate for ASF prior to the study period was estimated at 14.1 per 100 pig farm-years and 5.6 per 100 pig farm-years during the study. Three pigs tested positive for ASFV using real-time PCR, but none tested positive for ASFV specific antibodies using two different commercial ELISA tests.

CONCLUSIONS

There was no evidence for existence of pigs that were long-term carriers for the virus based on the analysis of blood and serum as there were no seropositive pigs and the only three ASFV DNA positive pigs were acutely infected and were linked to outbreaks reported by farmers during the study. Potential ASF risk factors were present on both small and medium-scale pig farms, although small scale farms exhibited a higher proportion with multiple potential risk factors (like borrowing boars for sows mating, buying replacement from neighboring farms without ascertaining health status, etc) and did not implement any biosecurity measures. However, no risk factors were significantly associated with ASF reports during the study.

African swine fever virus CD2v and C-type lectin gene loci mediate serological specificity

African swine fever (ASF) is an emerging disease threat for the swine industry worldwide. No ASF vaccine is available and progress is hindered by lack of knowledge concerning the extent of ASF virus (ASFV) strain diversity and the viral antigens responsible for protection in the pig. Available data from vaccination/challenge experiments in pigs indicate ASF protective immunity is haemadsorption inhibition (HAI) serotype-specific. A better understanding of ASFV HAI serological groups and their diversity in nature, as well as improved methods to serotype ASFV isolates, is needed. Here, we demonstrated that the genetic locus encoding ASFV CD2v and C-type lectin proteins mediates HAI serological specificity and that CD2v/C-type lectin genotyping provides a simple method to group ASFVs by serotype, thus facilitating study of ASFV strain diversity in nature, and providing information necessary for eventual vaccine design, development and efficacious use.

Detection of African Swine Fever Antibodies in Experimental and Field Samples from the Russian Federation: Implications for Control

African swine fever (ASF) re-entered in Europe in 2007 by Georgia rapidly affecting neighbouring countries. Since then, ASF has caused severe problems to the Russian Federation (RF) and spread to Northern and Western regions, including Ukraine (2012 and 2014) and Belarus (2013). At the beginning of 2014, dead wild boars were found in Lithuania and Poland. Several outbreaks have been later notified in the European Union(EU), affecting domestic pigs and wild boar of Latvia, Lithuania and Poland, and also wild boar in Estonia, causing major problems for the EU pig sector. Some studies have been performed with this ASFV isolate, revealing that it belongs to genotype II and causes an acute form of the disease. However, few data are available about the presence of antibodies in field and experimental samples from the affected area. This study analysed samples from experimental infections with ASFV isolated from the RF in 2013 (74 sera and 3 tissue exudates), and field samples from the RF from 2013 to 2014 (266 samples, including 32 and 7 tissue exudates from domestic pigs and wild boar, respectively). All samples were tested by a commercial ELISA and, some of them (79), also by immunochromatographic tests. Positive and doubtful samples were confirmed by immunoblotting test. Positive results were found in experimental and field samples, which confirm the presence of antibodies against ASFV in the RF. Antibodies were detected in animals inoculated with three different ASFV isolates, with some differences found among them. Only a small percentage of field samples was positive for ASF antibodies (3.7%), in agreement with other observations that reported a high virulence for the ASFV isolates in the area. These results confirm the potential presence of survivor animals that should be considered in affected areas to help design effective control and eradication plans against ASF.

Prevalence of African swine fever virus in apparently healthy domestic pigs in Uganda

BACKGROUND

African swine fever (ASF) is a contagious viral disease which can cause up to 100% mortality among domestic pigs leading to serious socio-economic impact on people's livelihoods. ASF is endemic in Uganda and there is paucity of information on the epidemiology of the disease. The major aim of this study was to determine the seroprevalence and prevalence of African swine fever virus (ASFV) in apparently healthy slaughter pigs at Wambizi slaughterhouse in Kampala city, Uganda. We also estimated the presence of ASFV antibodies and circulating viral antigens in pigs from selected districts of Uganda during targeted surveillance. We analysed 540 and 181 blood samples collected from slaughter pigs and pigs from targeted surveillance districts respectively.

RESULTS

The prevalence of ASFV in slaughter pigs was 52.96% (95% CI, 48.75-57.14) and 11.5% (95% CI, 9.06-14.45) by ELISA and PCR respectively. In surveillance districts, the proportion of ASFV positive pigs was 53.59% (95% CI, 46.33-60.71) and 0.55% (95% CI, 0.1-3.06) by ELISA and PCR respectively.

CONCLUSION

The study has found out a high seroprevalence of ASFV antibodies in apparently healthy slaughter pigs and also a high proportion of ASFV antibody seropositive pigs in surveyed districts in Uganda indicating exposure to ASFV. However, there was a lower prevalence of ASFV infection implying that there could be low virulent strains of ASFV circulating in domestic pigs in Uganda which requires further investigation.