Biological characterization of African swine fever virus genotype II strains from north-eastern Estonia in European wild boar

Virulent African swine fever virus infection of porcine monocytes causes SLA I subversion due to loss of proper ER structure/function

African swine fever virus (ASFV) is a large DNA virus of the Asfarviridae family that causes a fatal hemorrhagic disease in domestic swine and wild boar. Infections with moderately virulent strains predominantly result in a milder clinical course and lower lethality. As target cells of ASFV, monocytes play a crucial role in triggering T-cell-mediated immune defense and ASF pathogenesis. We compared the effect of the highly virulent "Armenia2008" (ASFV-A) virus strain with that of the naturally attenuated "Estonia2014" (ASFV-E) on cellular immune activation in vivo and on primary monocytes ex vivo. Specifically, we asked whether antigen presentation of porcine monocytes is impaired upon ASFV-A infection. ASFV-A-infected monocytes are characterized by lower levels of swine leukocyte antigen (SLA) class I on the cell surface than ASFV-E-infected and uninfected monocytes. Despite stable steady-state SLA I mRNA/protein levels and expression of critical components of the antigen processing machinery, a marked decrease in maturation and reduced surface transport of SLA I were observed in ASFV-A-infected monocytes. The intracellular maturation block of SLA I was accompanied by a loss of functional rough ER structures and a pronounced formation of ER-associated aggresomes. This unsolved cellular stress resulted in a shutdown of overall host cell protein translation, mitochondrial dysfunction, and caspase-3-mediated apoptosis. In contrast, no such cellular subversion phenomenon was found in ASFV-E-infected monocytes. Our findings suggest that in domestic pigs infected with highly virulent ASFV-A, sequential subversion events occur in infected monocytes, likely leading to compromised T-cell activation and impaired downstream responses against ASFV.

Habitat suitability mapping and landscape connectivity analysis to predict African swine fever spread in wild boar populations: A focus on Northern Italy

African swine fever (ASF) is a highly contagious disease affecting wild and domestic pigs, characterised by severe haemorrhagic symptoms and high mortality rates. Originally confined to Sub-Saharan Africa, ASF virus genotype II has spread to Europe since 2014, mainly affecting Eastern Europe, and progressing through wild boar migrations and human action. In January 2022, the first case of ASF, due to genotype II, was reported in North-western Italy, in a wild boar carcass. Thereafter, numerous positive wild boars were identified, indicating an expanding wild epidemic, severely threatening Italian pig farming and trade. This study focused on the mapping of the suitable habitats for wild boars and their potential dispersal corridors in Northern Italy, using species distribution models and landscape connectivity analysis. The resulting maps identified areas with higher likelihood of wild boar presence, highlighting their preferential pathways crossing Northern Italy. The distribution of ASF positive wild boars along the major corridors predicted by the model suggests the obtained maps as valuable support to decision-makers to improve ASF surveillance and carcass early detection, aiming for eradication. The applied framework can be easily replicated in other regions and countries.

Genome-Wide Approach Identifies Natural Large-Fragment Deletion in ASFV Strains Circulating in Italy During 2023

African swine fever (ASF), characterized by high mortality rates in infected animals, remains a significant global veterinary and economic concern, due to the widespread distribution of ASF virus (ASFV) genotype II across five continents. In this study, ASFV strains collected in Italy during 2022-2023 from two geographical clusters, North-West (Alessandria) and Calabria, were fully sequenced. In addition, an in vivo experiment in pigs was performed. Complete genomic sequencing of 30 strains revealed large-fragment deletions and translocations. In Alessandria, five samples showed two different deletions in the 5' genomic region: a ~4340 bp deletion (positions ~9020-13,356 in Georgia 2007/1) in four samples and a 2162 bp deletion (positions 17,837-19,998) in one sample. Another strain showed a truncation of 1950 bp at the 3' end. In Calabria, strains showed a 5137 bp deletion (positions 10,755-15,891) and ~2 kb truncations in the 3' region. Two strains showed a translocation from the region 1-2244 to positions 188,631-190,584. In vivo characterization of the deleted strain 22489.4_2312/RC/2023 revealed identical disease progression to the wild-type strain, with severe ASF symptoms in inoculated pigs. This study is the first to report natural deleted strains of ASFV in Italy, revealing unique genomic deletions distinct from those in previously known strains.

African Swine Fever in Saxony-Disease Dynamics

African swine fever (ASF) emerged in Germany in 2020. A few weeks after the initial occurrence, infected wild boar were detected in Saxony. In this study, data from wild boar surveillance in Saxony were analyzed. The analysis focused on the eastern districts of the state, where the disease likely spread through infected wild boar. Additionally, data from Meissen, located approximately 65 km further west, were examined. In Meissen, the disease emerged one year later, and a human-mediated introduction was suspected. To evaluate the progression of ASF in the two study areas over time, data from active and passive surveillance were analyzed both descriptively and using a Bayesian space-time model. Prevalence estimates were calculated for wild boar testing positive for the ASF virus and for ASF-specific antibodies. Higher prevalence estimates were observed in the eastern districts, indicating a stronger viral load, consistent with patterns seen in other European regions. Over time, seroprevalence increased, suggesting an accumulation of surviving wild boar. The findings provide important insights into the epidemiology of ASF in wild boar over time. They complement the existing knowledge and support targeted ASF control measures. This is particularly significant, as ASF continues to spread across Europe rather than being successfully eradicated.

Genome-wide characterization of structure variations in the Xiang pig for genetic resistance to African swine fever

African swine fever (ASF) is a rapidly fatal viral haemorrhagic fever in Chinese domestic pigs. Although very high mortality is observed in pig farms after an ASF outbreak, clinically healthy and antibody-positive pigs are found in those farms, and viral detection is rare from these pigs. The ability of pigs to resist ASF viral infection may be modulated by host genetic variations. However, the genetic basis of the resistance of domestic pigs against ASF remains unclear. We generated a comprehensive set of structural variations (SVs) in a Chinese indigenous Xiang pig with ASF-resistant (Xiang-R) and ASF-susceptible (Xiang-S) phenotypes using whole-genome resequencing method. A total of 53,589 nonredundant SVs were identified, with an average of 25,656 SVs per individual in the Xiang pig genome, including insertion, deletion, inversion and duplication variations. The Xiang-R group harboured more SVs than the Xiang-S group. The F-statistics (FST) was carried out to reveal genetic differences between two populations using the resequencing data at each SV locus. We identified 2,414 population-stratified SVs and annotated 1,152 Ensembl genes (including 986 protein-coding genes), in which 1,326 SVs might disturb the structure and expression of the Ensembl genes. Those protein-coding genes were mainly enriched in the Wnt, Hippo, and calcium signalling pathways. Other important pathways associated with the ASF viral infection were also identified, such as the endocytosis, apoptosis, focal adhesion, Fc gamma R-mediated phagocytosis, junction, NOD-like receptor, PI3K-Akt, and c-type lectin receptor signalling pathways. Finally, we identified 135 candidate adaptive genes overlapping 166 SVs that were involved in the virus entry and virus-host cell interactions. The fact that some of population-stratified SVs regions detected as selective sweep signals gave another support for the genetic variations affecting pig resistance against ASF. The research indicates that SVs play an important role in the evolutionary processes of Xiang pig adaptation to ASF infection.

Ancestry and genome-wide association study of domestic pigs that survive African swine fever in Uganda

African swine fever (ASF) is endemic to Uganda and causes annual outbreaks. Some pigs survive these outbreaks and remain asymptomatic but are African swine fever virus (ASFV) positive. The potential heritability and genetic disparities in disease susceptibility among Ugandan pigs are not fully understood. In a 12-year study, whole blood and tissue samples were collected from 212 pigs across 19 districts in Uganda. Polymerase chain reaction (PCR) assays were used to determine ASFV infection status and genotyping was completed using a commercial porcine array. The point prevalence of ASF was calculated for each district, and breed composition origins were quantified for the sampled pigs by implementing established ancestry analyses. Genome-wide associated studies (GWAS) were conducted using all available domestic swine samples (full study population; n = 206) as well as a reduced dataset (farm-level study population; n = 129). This study revealed a greater number of ASFV-positive pigs in border districts than in non-border districts, a high level of admixture among domestic pigs sampled from Ugandan smallholder farms, and 48 loci that were associated with ASFV infection status. The discovery of 48 significant SNPs and 28 putative candidate genes may imply the possibility of heritability for resistance to ASFV. However, additional investigations in ASFV-endemic regions are required to fully elucidate the heritability of ASFV susceptibility among surviving pigs in Uganda.

Comparative evaluation of disease dynamics in wild boar and domestic pigs experimentally inoculated intranasally with the European highly virulent African swine fever virus genotype II strain "Armenia 2007"

Since the reintroduction of African swine fever virus (ASFV) in Europe in 2007 and its subsequent spread to Asia, wild boar has played a crucial role in maintaining and disseminating the virus. There are significant gaps in the knowledge regarding infection dynamics and disease pathogenesis in domestic pigs and wild boar, particularly at the early infection stage. We aimed to compare domestic pigs and wild boar infected intranasally to mimic natural infection with one of the original highly virulent genotype II ASFV isolates (Armenia 2007). The study involved euthanising three domestic pigs and three wild boar on days 1, 2, 3, and 5 post-infection, while four domestic pigs and four wild boar were monitored until they reached a humane endpoint. The parameters assessed included clinical signs, macroscopic lesions, viremia levels, tissue viral load, and virus shedding in nasal and rectal swabs from day 1 post-infection. Compared with domestic pigs, wild boar were more susceptible to ASFV, with a shorter incubation period and earlier onset of clinical signs. While wild boar reached a humane endpoint earlier than domestic pigs did, the macroscopic lesions were comparatively less severe. In addition, wild boar had earlier viremia, and the virus was also detected earlier in tissues. The medial retropharyngeal lymph nodes were identified as key portals for ASFV infection in both subspecies. No viral genome was detected in nasal or rectal swabs until shortly before reaching the humane endpoint in both domestic pigs and wild boar, suggesting limited virus shedding in acute infections.

Genetic Variations of African Swine Fever Virus: Major Challenges and Prospects

African swine fever (ASF) is a contagious viral disease affecting pigs and wild boars. It typically presents as a hemorrhagic fever but can also manifest in various forms, ranging from acute to asymptomatic. ASF has spread extensively globally, significantly impacting the swine industry. The complex and highly variable character of the ASFV genome makes vaccine development and disease surveillance extremely difficult. The overall trend in ASFV evolution is towards decreased virulence and increased transmissibility. Factors such as gene mutation, viral recombination, and the strain-specificity of virulence-associated genes facilitate viral variations. This review deeply discusses the influence of these factors on viral immune evasion, pathogenicity, and the ensuing complexities encountered in vaccine development, disease detection, and surveillance. The ultimate goal of this review is to thoroughly explore the genetic evolution patterns and variation mechanisms of ASFV, providing a theoretical foundation for advancement in vaccine and diagnostic technologies.

When ASFV Infection Meets the cGAS-STING Signaling Pathway

The African swine fever virus (ASFV) has the ability to infect both wild boars and domestic pigs, regardless of their breeds or ages, often resulting in a mortality rate of 100%. Host innate immunity is the most important defense weapon against invasion of pathogenic microbial infection. cGAS-STING signaling pathway is one of the greatest discoveries of the twenty-first century, which is crucial in host's innate immune response. Recent studies have found that the interaction between cGAS/STING pathway and ASFV plays a key role during ASFV infection. At the same time, ASFV has also evolved different strategies to evade the killing of the host cGAS/STING pathway and promote its survival. Here, we review the latest progress in the interaction between ASFV infection, cGAS/STING pathways, and their related molecular mechanisms, aiming to provide new ideas for further research on the pathogenesis of ASFV, the development of vaccines and therapeutic drugs.

Overview of Modern Commercial Kits for Laboratory Diagnosis of African Swine Fever and Swine Influenza A Viruses

Rapid and early detection of infectious diseases in pigs is important, especially for the implementation of control measures in suspected cases of African swine fever (ASF), as an effective and safe vaccine is not yet available in most of the affected countries. Additionally, analysis for swine influenza is of significance due to its high morbidity rate (up to 100%) despite a lower mortality rate compared to ASF. The wide distribution of swine influenza A virus (SwIAV) across various countries, the emergence of constantly new recombinant strains, and the danger of human infection underscore the need for rapid and accurate diagnosis. Several diagnostic approaches and commercial methods should be applied depending on the scenario, type of sample and the objective of the studies being implemented. At the early diagnosis of an outbreak, virus genome detection using a variety of PCR assays proves to be the most sensitive and specific technique. As the disease evolves, serology gains diagnostic value, as specific antibodies appear later in the course of the disease (after 7-10 days post-infection (DPI) for ASF and between 10-21 DPI for SwIAV). The ongoing development of commercial kits with enhanced sensitivity and specificity is evident. This review aims to analyse recent advances and current commercial kits utilised for the diagnosis of ASF and SwIAV.

A sensitive luciferase reporter assay for the detection of infectious African swine fever virus

African swine fever virus (ASFV) is a complex DNA virus causing severe hemorrhagic disease in domestic pigs and wild boar. The disease has spread worldwide, with important socio-economic consequences. Early virus detection and control measures are crucial as there are no effective vaccines nor antivirals on the market. While the diagnosis of ASFV is fast and based primarily on qPCR, the detection of infectious ASFV is a labor-intensive process requiring susceptible macrophages and subsequent antibody-based staining or hemadsorption. The latter cannot detect ASFV isolates devoid of functional CD2v (EP402R) expression. Here, we report the development of a plasmid-based reporter assay (RA) for the sensitive detection and titration of infectious ASFV. To this end, we constructed a plasmid for secreted NanoLuc luciferase (secNluc) expression driven by the ASFV DNA polymerase gene G1211R promoter. Infection of plasmid-transfected immortalized porcine kidney macrophages (IPKM) followed by measurement of secNluc from cell culture supernatants allowed reliable automated quantification of infectious ASFV. The RA-based titers matched the titers determined by conventional p72-staining or hemadsorption protocols. The novel assay is specific for ASFV as it does not detect classical swine fever virus nor porcine reproductive and respiratory syndrome virus. It is applicable to ASFV of different genotypes, virulence, and sources, including ASFV from sera and whole blood from infected pigs as well as non-hemadsorbing ASFV.

Predicting Disparity between ASF-Managed Areas and Wild Boar Habitats: A Case of South Korea

African swine fever (ASF) is a highly contagious viral disease affecting both domestic and wild boars. Since its first outbreak in South Korea in 2019, substantial efforts have been made to prevent ASF transmission by reducing the wild boar population and eliminating infected carcasses; however, the persistence of ASF transmission has posed challenges to these efforts. To improve ASF management strategies, the limitations of current management strategies must be identified by considering disparities between wild boar habitats and ASF-managed areas with environmental and anthropogenic characteristics of wild boars and their management strategies. Here, ensemble species distribution models were used to estimate wild boar habitats and potential ASF-managed areas, with elevation, distance to urban areas, and Normalized Difference Vegetation Index as important variables. Binary maps of wild boar habitats and potential ASF-managed areas were generated using the maxSSS as the threshold criterion. Disparity areas of ASF management were identified by overlying regions evaluated as wild boar habitats with those not classified as ASF-managed areas. Dense forests near urban regions like Chungcheongbuk-do, Gyeongsangbuk-do, and Gyeongsangnam-do were evaluated as disparity areas having high risk of ASF transmission. These findings hold significant potential for refining ASF management strategies and establishing proactive control measures.

Epidemiological impacts of attenuated African swine fever virus circulating in wild boar populations

African swine fever virus (ASFV) genotype II has been present in wild boar in the European Union since 2014. Control measures have reduced the incidence of the ASF, but highly virulent as well as attenuated ASFV strains continue to circulate. We present the intraherd epidemiological parameters of low and highly virulent ASFV in wild boar from experimental data, and for the first time, evaluate the impact of attenuated strain circulation through unique deterministic compartmental model simulations under various potential scenarios and hypotheses. Using an estimated PCR infectious threshold of TPCR = 36.4, we obtained several transmission parameters, like an Rx (experimental intraherd R0) value of 4.5. We also introduce two novel epidemiological parameters: infectious power and resistance power, which indicate the ability of animals to transmit the infection and the reduction in infectiousness after successive exposures to varying virulence strains, respectively. The presence of ASFV attenuated strains results in 4-17% of animals either remaining in a carrier state or becoming susceptible again when exposed to highly virulent ASFV for more than two years. The timing between exposures to viruses of different virulence also influences the percentage of animals that die or remain susceptible. The findings of this study can be utilized in epidemiological modelling and provide insight into important risk situations that should be considered for surveillance and future potential ASF vaccination strategies in wild boar.

Low transmission risk of African swine fever virus between wild boar infected by an attenuated isolate and susceptible domestic pigs

African swine fever (ASF) is a lethal infectious disease that affects domestic and wild pigs. This complex virus has already affected five continents and more than 70 countries and is considered to be the main threat to the global swine industry. The disease can potentially be transmitted directly through contact with infectious animals, or indirectly by means of contaminated feed or environments. Nevertheless, the knowledge regarding the transmission patterns of different ASF virus isolates at the wildlife-livestock interface is still limited. We have, therefore, assessed the potential transmission of an attenuated ASF virus isolate between infectious wild boar and directly exposed domestic pig. We registered 3,369 interspecific interactions between animals, which were brief and mostly initiated by wild boar. The major patterns observed during the study were head-to-head contact owing to sniffing, thus suggesting a high probability of pathogen transmission. However, only one of the five domestic pigs had a short period of viremia and became serologically positive for ASF virus antibodies. It was additionally discovered that the wild boar did not transmit the virulent virus isolate to the domestic pigs, which suggests that the presence of attenuated ASF virus isolates in affected areas may control the spreading of other more virulent isolates. These outcomes may help make decisions related to large-scale targeted management actions against ASF in field conditions.

Differentiation of African Swine Fever Virus Strains Isolated in Estonia by Multiple Genetic Markers

The African swine fever virus (ASFV) was first detected in Estonia, in September 2014. In the subsequent three years, the virus spread explosively all over the country. Only one county, the island of Hiiumaa, remained free of the disease. Due to the drastic decrease in the wild boar population in the period of 2015-2018, the number of ASFV-positive cases among wild boar decreased substantially. From the beginning of 2019 to the autumn of 2020, no ASFV-positive wild boar or domestic pigs were detected in Estonia. A new occurrence of ASFV was detected in August 2020, and by the end of 2022, ASFV had been confirmed in seven counties in Estonia. Investigations into proven molecular markers, such as IGR I73R/I329L, MGF505-5R, K145R, O174L, and B602L, were performed with the aim of clarifying whether these cases of ASFV were new entries or remnants of previous epidemics. The sequences from the period of 2014-2022 were compared to the Georgia 2007/1 reference sequence and the variant strains present in Europe. The results indicated that not all the molecular markers of the virus successfully used in other geographical regions were suitable for tracing the spread of ASFV in Estonia. Only the B602L-gene analysis enabled us to place the ASFV isolates spreading in 2020-2022 into two epidemiologically different clusters.

Disease threats to tigers and their prey

The contraction of the global tiger population over the last 100 years into small, often isolated subpopulations has made them increasingly vulnerable to the impact of disease. Despite this, the health of wild tigers continues to be insufficiently funded and explored. For example, canine distemper virus (CDV), has been associated with localized declines and increased risk of extinction, and yet has received little research attention in most tiger range countries. The emergence of new pathogenic threats has posed fresh challenges, including African swine fever virus (ASFV), which has the potential to devastate wild boar populations, and severe acute respiratory syndrome coronavirus (SARS-CoV2) with implications for tiger conservation that remain unknown. The objective of this review is to synthesize current research on the health of tigers and their prey that impacts the conservation of tigers in the wild. Published sources are interpreted based on three mechanisms through which disease can affect the viability of tiger populations: (1) by reducing the survival of adult tigers, (2) by reducing breeding productivity, and (3) by reducing the carrying capacity of tiger habitat through decreased prey abundance. Examples of CDV, SARS-CoV2, carnivore protoparvovirus 1 and ASFV are used to illustrate these processes and inform discussion of research and mitigation priorities.

Design of a Replicative-Competent MGF110 (1L-5-6L) Deleted African Swine Fever Virus (Genotype II)

Viral individual genes functions and their role in the interaction with the host cells remain the main area in the study of African swine fever virus (ASFV) biology. The extreme heterogeneity of the ASFV makes it difficult to develop vaccines against this pathogen. In this work, we generated the ASFV deletion mutant virus Volgograd/D(1L-5-6L) with the six genes deletion in multigenic family 110 (MGF110) (1L-5-6L) and studied its characteristics in vitro. The homologous recombination method was used for the deletion in ASFV parental strain Volgograd/14с. A series of six passages was carried out in the COS-1 cell culture using the limiting dilution method. The recombinant strain Volgograd/D(1L-5-6L) MGF110 was selected by the plaque formation method. Performed study of viral replication showed no changes in viral growth kinetics in comparison with the parental strain. The ASFV Volgograd/D(1L-5-6L) MGF110 is a great tool available to test the importance of MGF110 for virus virulence and vaccine development.

Genetic variation and evolution of attenuated African swine fever virus strain isolated in the field: A review

It has been reported that there were several "mutant isolated in the field " of African swine fever virus (ASFV) since ASFV was reported, which may be the result of the continuous adaptation and evolution of ASFV. The emergence of ASFV field mutants may lead to chronic or asymptomatic "atypical clinical symptoms" in pigs and hinder the development of porcine industry. Here we analyzed the published ASFV "field attenuated strain" gene sequences and reviewed the genetic differences between field attenuated and virulent ASFV strains, hoping for providing a reference for the scientific prevention and control of ASF and the development of new vaccines. In this study we found the deletion of EP153R and EP402R occurred in 4 field attenuated strains, and all the differential genes of field attenuated strains mainly range in regions with low GC content. The evolution of MGF110 family genes was identified by analysis of two field attenuated ASFV strains from Portugal. We also found that some tandem repeat sequence plays an important role in the evolution of strains of NH/P68 and OURT 88/3 but not in strains Estonia 2014, HuB20 and Pig/Heilongjiang/HRB1/2020.

[Alternative approaches to the diagnosis of African swine fever in the Russian Federation in 2017-2021]

INTRODUCTION

Prevention and control of African swine fever (ASF) transmission on the territory of the Russian Federation requires monitoring based on testing of samples from pigs and wild boars. Specific anti-ASFV antibodies are rarely detected in samples during routine serological diagnostics. Although, ASF isolates with weakened virulence were confirmed in Russia and neighboring countries.The aim of this work was to determine the possibility of using alternative samples for ASF diagnosis and evaluate the effectiveness of the diagnostic methods used on the territory of Russia.

MATERIALS AND METHODS

Biological materials obtained from experimentally infected animals and samples collected in the "field" conditions were used in this study.

RESULTS

Complex testing (RT-PCR and ELISA) is a more effective approach to diagnose chronic and asymptomatic forms of ASF compared to the separate use of these techniques. The possibility and efficiency of using alternative samples in diagnostics are demonstrated. It was confirmed that IPT method overcomes ELISA by high diagnostic sensitivity and detection of antibodies on earlier stages in extended range of samples. Anti-ASFV antibodies were detected in domestic and wild pigs in five regions of Russia. Samples from infected pigs that are negative in RT-PCR can be positive for anti-ASFV antibodies. The detection of antibodies in samples from shot wild boars (negative or uncertain in RT-PCR test) suggests the existence of animals surviving ASF infection.

CONCLUSION

The data obtained suggest a revision of the ASF surveillance strategy, by introducing complex diagnostic methods aimed at detection of both the virus genome and anti-ASFV antibodies simultaneously.

Targeting the search of African swine fever-infected wild boar carcasses: A tool for early detection

This study analyses the temporal and spatial distribution of found dead African swine fever (ASF)-positive wild boar carcasses from 2017 to January 2021 in affected European countries: Bulgaria, Estonia, Germany, Hungary, Latvia, Lithuania, Romania, Poland, Serbia and Slovakia. During this period, a total of 21,785 cases were confirmed in 19,071 unique locations. The temporal analysis of aggregated cases per month evidenced that most countries located in southern latitudes showed a higher number of cases between January and April, whereas in northern latitudes there was no clear temporal pattern. The space-time K-function evidenced a space-time clustering in the ASF-positive wild boar carcasses, which was most prominent within distances of 2 km and within 1 week. A Bayesian hierarchical spatial model was calibrated to evaluate the association between the probability of finding ASF-positive wild boar carcasses and landscape factors (i.e. the presence of a path and paved road), land use and wild boar abundance. Results showed the highest likelihood of finding ASF-positive wild boar carcasses in areas of transition between woodland and shrub, green urban areas and mixed forests. The presence of a path and a higher abundance of wild boar also increased slightly the odds of finding an ASF-positive dead wild boar. In summary, this paper aims to provide recommendations to design a search strategy to find ASF-infected wild boar carcasses, which is a crucial activity in the management of the disease, not just for surveillance purposes (i.e. the early detection of an introduction and the regular monitoring to understand the epidemiology and dynamics), but also for control, namely the disposal of infected carcasses as a virus source.

The baseline immunological and hygienic status of pigs impact disease severity of African swine fever

African Swine Fever virus (ASFV) is a large double-enveloped DNA virus of the Asfarviridae family that causes a lethal hemorrhagic disease in domestic pigs and wild boars. Since 2007, a highly virulent genotype II strain has emerged and spread in Europe and South-East Asia, where millions of animals succumbed to the disease. Field- and laboratory-attenuated strains of ASFV cause highly variable clinical disease severity and survival, and mechanisms remain unclear. We hypothesized that the immunological and hygienic status of pigs is a determinant of ASF disease course. Here we compared the immunological profile at baseline and in response to ASFV infection in specific pathogen-free (SPF) and farm-raised Large White domestic pigs. At steady state, SPF pigs showed lower white blood cell counts and a lower basal inflammatory and antiviral transcriptomic profile compared to farm pigs, associated with profound differences in gut microbiome composition. After inoculation with a highly virulent ASFV genotype II strain (Armenia 2008), severe clinical signs, viremia and pro-inflammatory cytokines appeared sooner in SPF pigs, indicating a reduced capacity to control early virus replication. In contrast, during infection with an attenuated field isolate (Estonia 2014), SPF pigs presented a milder and shorter clinical disease with full recovery, whereas farm pigs presented severe protracted disease with 50% lethality. Interestingly, farm pigs showed higher production of inflammatory cytokines, whereas SPF pigs produced more anti-inflammatory IL-1ra early after infection and presented a stronger expansion of leukocytes in the recovery phase. Altogether, our data indicate that the hygiene-dependent innate immune status has a double-edge sword impact on immune responses in ASF pathogenesis. While the higher baseline innate immune activity helps the host in reducing initial virus replication, it promotes immunopathological cytokine responses, and delays lymphocyte proliferation after infection with an attenuated strain. Such effects should be considered for live vaccine development and vigilance.

African Swine Fever Virus: A Review

African swine fever (ASF) is a viral disease with a high fatality rate in both domestic pigs and wild boars. ASF has greatly challenged pig-raising countries and also negatively impacted regional and national trade of pork products. To date, ASF has spread throughout Africa, Europe, and Asia. The development of safe and effective ASF vaccines is urgently required for the control of ASF outbreaks. The ASF virus (ASFV), the causative agent of ASF, has a large genome and a complex structure. The functions of nearly half of its viral genes still remain to be explored. Knowledge on the structure and function of ASFV proteins, the mechanism underlying ASFV infection and immunity, and the identification of major immunogenicity genes will contribute to the development of an ASF vaccine. In this context, this paper reviews the available knowledge on the structure, replication, protein function, virulence genes, immune evasion, inactivation, vaccines, control, and diagnosis of ASFV.

The study of antigen carrying and lesions observed in pigs that survived post African swine fever virus infection

African swine fever (ASF) is a dangerous infectious disease of domestic pigs and wild boar caused by African swine fever virus (ASFV). In Vietnam, the ASF epidemic is gradually turning into an endemic status with several recovered pigs post infection, but there were not many studies evaluating the role of these pigs in the epidemiological context in Vietnam. The aim of this study was to evaluate the viral antigen distribution and lesions in recovered pigs post ASFV infection. Ten pigs recovered from ASF at 6 weeks of age were monitored and assessed for anti-ASFV antibodies and viremia until slaughter. The five major organs (lung, liver, spleen, kidney, and lymph nodes) of these pigs were evaluated for microscopic lesions and viral antigen distribution. Anti-ASFV antibody was consistently observed to be high (S/P% ≥ 80) until slaughter, while viremia levels were very high (7 log₁₀ copies/mL) at 6 weeks of age and gradually decreased to undetectable levels at 12 weeks of age (6th week post-infection). At slaughter, the ASFV-associated lesions in the organs of these pigs were mild and nonspecific. Seven out of ten pigs recovering from ASF still carried the virus in surveyed organ tissues, although not in the serum. These findings suggest that ASF-recovered pigs may be potential carriers of the virus, contributing to the increased complexity in the current endemic status in Vietnam.

African Swine Fever Control and Prevention: An Update on Vaccine Development

African swine fever (ASF) is a lethal and highly contagious viral disease of domestic and wild pigs, listed as a notifiable disease reported to the World Organization for Animal Health (OIE). Despite its limited host range and absent zoonotic potential, the socio-economic and environmental impact of ASF is very high, representing a serious threat to the global swine industry and the many stakeholders involved. Currently, only control and eradication measures based mainly on early detection and strict stamping-out policies are available, however, the rapid spread of the disease in new countries, and in new regions in countries already affected, show these strategies to be lacking. In this review, we discuss approaches to ASF vaccinology, with emphasis on the advances made over the last decade, including the development of virulence-associated gene deleted strains such as the very promising ASFV-G-ΔI177L/ΔLVR, that replicates efficiently in a stable porcine epithelial cell line, and the cross-protecting BA71ΔCD2 capable of stably growing in the commercial COS-1 cell line, or the naturally attenuated Lv17/WB/Rie1 which shows solid protection in wild boar. We also consider the key constraints involved in the scale-up and commercialization of promising live attenuated and virus-vectored vaccine candidates, namely cross-protection, safety, lack of suitable animal models, compatibility with wildlife immunization, availability of established and licensed cell lines, and differentiating infected from vaccinated animals (DIVA) strategy.

Vaccination With a Gamma Irradiation-Inactivated African Swine Fever Virus Is Safe But Does Not Protect Against a Challenge

African swine fever (ASF) is among the most devastating viral diseases of pigs and wild boar worldwide. In recent years, the disease has spread alarmingly. Despite intensive research activities, a commercialized vaccine is still not available, and efficacious live attenuated vaccine candidates raise safety concerns. From a safety perspective, inactivated preparations would be most favourable. However, both historical and more recent trials with chemical inactivation did not show an appreciable protective effect. Under the assumption that the integrity of viral particles could enhance presentation of antigens, we used gamma irradiation for inactivation. To this means, gamma irradiated ASFV “Estonia 2014” was adjuvanted with either Polygen™ or Montanide™ ISA 201 VG, respectively. Subsequently, five weaner pigs per preparation were immunized twice with a three-week interval. Six weeks after the first immunization, all animals were challenged with the highly virulent ASFV strain “Armenia 2008”. Although ASFV p72-specific IgG antibodies were detectable in all vaccinated animals prior challenge, no protection could be observed. All animals developed an acute lethal course of ASF and had to be euthanized at a moderate humane endpoint within six days. Indeed, the vaccinated pigs showed even higher clinical scores and a higher inner body temperature than the control group. However, significantly lower viral loads were detectable in spleen and liver of immunized animals at the time point of euthanasia. This phenomenon suggests an immune mediated disease enhancement that needs further investigation.

Development of a Blocking ELISA Kit for Detection of ASFV Antibody Based on a Monoclonal Antibody against Full Length p72

BACKGROUND

African swine fever virus (ASFV) is the etiologic agent of African swine fever (ASF), a disease of highly contagious and significant threat to pork production. At present, the sensitive detection methods are the keys to the disease control.

OBJECTIVE

Full length p72 is produced by eukaryotic system and its monoclonal antibody (mAb) 34C10 is subsequently recovered. A blocking ELISA kit for detection of ASFV antibody is developed based on p72 trimers and 34C10.

METHODS

Full length p72 is expressed and is used as immunogen to prepare a panel of monoclonal antibodies. The mAb 34C10 is verified by immunofluorescent and tested by ELISAs with positive serums. The constant affinity of 34C10 is then confirmed. A blocking ELISA kit is further developed and is compared with two commercial kits.

RESULTS

The mAb 34C10 is specifically bound to p72 protein, and it exhibits blocking affection to positive serum. IFA experiment shows that 34C10 could bind to p72 expressed by baculoviruses and the binding affinity of 34C10 is found to be as high as 1.85 × 1011 L/mol. The blocking ELISA kit shows high coincidence with a commercial ELISA kit. The sensitivity between these two kits is 97.6% (95%, CI: 90.65-99.58) and the specificity between them is 100% (95%, CI: 98.34-100).

CONCLUSION

The blocking ELISA developed in this study may have great potential for diagnosis of ASF. The structure of the antigen p72 is found to be a key factor for the performance of the kit.

HIGHLIGHTS

For the first time, the eukaryotic expressed full-length p72 protein is utilized to recover the monoclonal antibody and it is coated as antigen during the development of the blocking ELISA kit. This study sheds new light on the development of the blocking ELISA kits, especially for the development of diagnostic kit for the contagious virus with bio-safety problems.

Adaptive Cellular Immunity against African Swine Fever Virus Infections

African swine fever virus (ASFV) remains a threat to global pig populations. Infections with ASFV lead to a hemorrhagic disease with up to 100% lethality in Eurasian domestic and wild pigs. Although myeloid cells are the main target cells for ASFV, T cell responses are impacted by the infection as well. The complex responses remain not well understood, and, consequently, there is no commercially available vaccine. Here, we review the current knowledge about the induction of antiviral T cell responses by cells of the myeloid lineage, as well as T cell responses in infected animals, recent efforts in vaccine research, and T cell epitopes present in ASFV.

A systematic review of genotypes and serogroups of African swine fever virus

African swine fever virus (ASFV) is the causative agent of African swine fever (ASF). The virus causes an acute highly hemorrhagic disease in domestic pigs, with high mortality. Although the overall genome mutation rate of ASFV, a large DNA virus, is relatively low, ASFV exhibits genetic and antigenic diversity. ASFV can be classified into 24 genotypes on the basis of the B646L gene. Cross-protected ASFV strains can be divided into eight serogroups on the basis of antibody-mediated hemadsorption inhibition. Here, we review research progress on ASFV genotyping and serogrouping, and explain how this information assists in the rapid identification of virus origin during ASF outbreaks and will aid in the development of ASF vaccines.

Integrating digital and field surveillance as complementary efforts to manage epidemic diseases of livestock: African swine fever as a case study

SARS-CoV-2 has clearly shown that efficient management of infectious diseases requires a top-down approach which must be complemented with a bottom-up response to be effective. Here we investigate a novel approach to surveillance for transboundary animal diseases using African Swine (ASF) fever as a model. We collected data both at a population level and at the local level on information-seeking behavior respectively through digital data and targeted questionnaire-based surveys to relevant stakeholders such as pig farmers and veterinary authorities. Our study shows how information-seeking behavior and resulting public attention during an epidemic, can be identified through novel data streams from digital platforms such as Wikipedia. Leveraging attention in a critical moment can be key to providing the correct information at the right moment, especially to an interested cohort of people. We also bring evidence on how field surveys aimed at local workers and veterinary authorities remain a crucial tool to assess more in-depth preparedness and awareness among front-line actors. We conclude that these two tools should be used in combination to maximize the outcome of surveillance and prevention activities for selected transboundary animal diseases such as ASF.

Pathological lesions and presence of viral antigens in four surviving pigs in African swine fever outbreak farms in Vietnam

Investigation of the role of animals that have recovered and survived from African swine fever (ASF) in carrying the ASF virus is currently intense and ongoing. However, no clear definition of the carrier stage has been established. The aim of the present study was to establish criteria to elucidate a clear status of survival in naturally ASF-infected domestic pigs in Vietnam. Seroconversion from previous infection was confirmed by serological assay, and the absence of the viral genome in various organs was also assured by molecular analysis of a partial p72 gene. We recognized that histopathological evidence could benefit from further insights into the status and role of the surviving animals; therefore, we performed a histopathological study on four pigs from farms with a history of ASF outbreak. We found fibrotic changes in the reparative process as the main finding in all four pigs. Immunohistochemical detection of viral protein revealed an interesting result. Despite the negative result from viral genome detection, the p30 protein gave a positive signal in the tonsils, lung, and stomach. This raises the possibility of stress-induced viral reactivation in long-term survivors and the risk of further outbreaks from human handling of contaminated carcasses.

Genotype I African swine fever viruses emerged in domestic pigs in China and caused chronic infection

The Georgia-07-like genotype II African swine fever virus (ASFV) with high virulence has been prevalent in China since 2018. Here, we report that genotype I ASFVs have now also emerged in China. Two non-haemadsorbing genotype I ASFVs, HeN/ZZ-P1/21 and SD/DY-I/21, were isolated from pig farms in Henan and Shandong province, respectively. Phylogenetic analysis of the whole genome sequences suggested that both isolates share high similarity with NH/P68 and OURT88/3, two genotype I ASFVs isolated in Portugal in the last century. Animal challenge testing revealed that SD/DY-I/21 shows low virulence and efficient transmissibility in pigs, and causes mild onset of infection and chronic disease. SD/DY-I/21 was found to cause necrotic skin lesions and joint swelling. The emergence of genotype I ASFVs will present more problems and challenges for the control and prevention of African swine fever in China.

African Swine Fever Re-Emerging in Estonia: The Role of Seropositive Wild Boar from an Epidemiological Perspective

African swine fever (ASF) emerged in Estonia in 2014. From February 2019 to August 2020, no pigs or wild boar tested positive for ASF virus (ASFV), only ASFV-specific antibodies could be detected in shot wild boar. However, ASF recently re-emerged in wild boar. We tested three hypotheses that might explain the current situation: (i) ASFV may have been present throughout, but at a prevalence below the detection limit; (ii) seropositive wild boar may have remained infectious (i.e., virus-carriers) and kept the epidemic going; or (iii) ASF was gone for 1.5 years, but was recently re-introduced. Using Estonian surveillance data, the sensitivity of the surveillance system and the confidence in freedom from ASF were estimated. Furthermore, the detection probability was determined and cluster analyses were performed to investigate the role of serological positive wild boar. The results suggest that the surveillance system was not able to detect virus circulation at a design prevalence below 1%. With respect to the confidence in freedom from ASF, the results indicate that circulating virus should have been detected over time, if the prevalence was ≥2%. However, the decreasing wild boar population density and ongoing surveillance activities made ASFV circulation at a low prevalence unlikely. Cluster analyses provided no evidence for a significant accumulation of serologically positive wild boar in temporal connection to the re-emergence of ASFV. Further targeted research, such as long-term experimental studies and molecular epidemiology, is necessary to improve our knowledge on the epidemiology of ASF and to control the disease more effectively.

African Swine Fever in Wild Boar in Europe-A Review

The introduction of genotype II African swine fever (ASF) virus, presumably from Africa into Georgia in 2007, and its continuous spread through Europe and Asia as a panzootic disease of suids, continues to have a huge socio-economic impact. ASF is characterized by hemorrhagic fever leading to a high case/fatality ratio in pigs. In Europe, wild boar are especially affected. This review summarizes the currently available knowledge on ASF in wild boar in Europe. The current ASF panzootic is characterized by self-sustaining cycles of infection in the wild boar population. Spill-over and spill-back events occur from wild boar to domestic pigs and vice versa. The social structure of wild boar populations and the spatial behavior of the animals, a variety of ASF virus (ASFV) transmission mechanisms and persistence in the environment complicate the modeling of the disease. Control measures focus on the detection and removal of wild boar carcasses, in which ASFV can remain infectious for months. Further measures include the reduction in wild boar density and the limitation of wild boar movements through fences. Using these measures, the Czech Republic and Belgium succeeded in eliminating ASF in their territories, while the disease spread in others. So far, no vaccine is available to protect wild boar or domestic pigs reliably against ASF.

Dynamics of African swine fever virus (ASFV) infection in domestic pigs infected with virulent, moderate virulent and attenuated genotype II ASFV European isolates

This study aimed to compare the infection dynamics of three genotype II African swine fever viruses (ASFV) circulating in Europe. Eighteen domestic pigs divided into three groups were infected intramuscularly or by direct contact with two haemadsorbent ASFVs (HAD) from Poland (Pol16/DP/ OUT21) and Estonia (Est16/WB/Viru8), and with the Latvian non-HAD ASFV (Lv17/WB/Rie1). Parameters, such as symptoms, pathogenicity, and distribution of the virus in tissues, humoral immune response, and dissemination of the virus by blood, oropharyngeal and rectal routes, were investigated. The Polish ASFV caused a case of rapidly developing fatal acute disease, while the Estonian ASFV caused acute to sub-acute infections and two animals survived. In contrast, animals infected with the ASFV from Latvia developed a more subtle, mild, or even subclinical disease. Oral excretion was sporadic or even absent in the attenuated group, whereas in animals that developed an acute or sub-acute form of ASF, oral excretion began at the same time the ASFV was detected in the blood, or even 3 days earlier, and persisted up to 22 days. Regardless of virulence, blood was the main route of transmission of ASFV and infectious virus was isolated from persistently infected animals for at least 19 days in the attenuated group and up to 44 days in the group of moderate virulence. Rectal excretion was limited to the acute phase of infection. In terms of diagnostics, the ASFV genome was detected in contact pigs from oropharyngeal samples earlier than in blood, independently of virulence. Together with blood, both samples could allow to detect ASFV infection for a longer period. The results presented here provide quantitative data on the spread and excretion of ASFV strains of different virulence among domestic pigs that can help to better focus surveillance activities and, thus, increase the ability to detect ASF introductions earlier.

African swine fever endemic persistence in wild boar populations: Key mechanisms explored through modelling

African swine fever (ASF) is a serious global concern from an ecological and economic point of view. While it is well established that its main transmission routes comprise contact between infected and susceptible animals and transmission through contaminated carcasses, the specific mechanism leading to its long‐term persistence is still not clear. Among others, a proposed mechanism involves the potential role of convalescent individuals, which would be able to shed the virus after the end of the acute infection. Using a spatially explicit, stochastic, individual‐based model, we tested: (1) if ASF can persist when transmission occurs only through infected wild boars and infected carcasses; (2) if the animals that survive ASF can play a relevant role in increasing ASF persistence chances; (3) how hunting pressure can affect the ASF probability to persist. The scenario in which only direct and carcass‐mediated transmission were contemplated had 52% probability of virus persistence 10 years after the initial outbreak. The inclusion of survivor‐mediated transmission corresponded to slightly higher persistence probabilities (57%). ASF prevalence during the endemic phase was generally low, ranging 0.1–0.2%. The proportion of seropositive individuals gradually decreased with time and ranged 4.5–6.6%. Our results indicate that direct and carcass‐mediated infection routes are sufficient to explain and justify the long‐term persistence of ASF at low wild boar density and the ongoing geographic expansion of the disease front in the European continent. During the initial years of an ASF outbreak, hunting should be carefully evaluated as a management tool, in terms of potential benefits and negative side‐effects, and combined with an intensive effort for the detection and removal of wild boar carcasses. During the endemic phase, further increasing hunting effort should not be considered as an effective strategy. Additional effort should be dedicated to finding and removing as many wild boar carcasses as possible.

African Swine Fever and Its Epidemiological Course in Lithuanian Wild Boar

African swine fever (ASF) has been present in Lithuania since 2014. It is mainly the wild boar population that is affected. Currently, little is known about the epidemiological course of ASF in Lithuania. In the present study, ASF surveillance data from 2016-2021 were analyzed. The numbers of samples taken from hunted wild boar and wild boar found dead per year and month were recorded and the prevalence was estimated for each study month and administrative unit. A Bayesian space-time model was used to calculate the temporal trend of the prevalence estimates. In addition, population data were analyzed on a yearly basis. Most samples were investigated in 2016 and 2017 and originated from hunted animals. Prevalence estimates of ASF virus-positive wild boar decreased from May 2019 onwards. Seroprevalence estimates showed a slight decrease at the same time, but they increased again at the end of the study period. A significant decrease in the population density was observed over time. The results of the study show that ASF is still present in the Lithuanian wild boar population. A joint interdisciplinary effort is needed to identify weaknesses in the control of ASF in Lithuania and to combat the disease more successfully.

New perspectives for evaluating relative risks of African swine fever virus contamination in global feed ingredient supply chains

There are no published reports indicating that the African swine fever virus (ASFV) has been detected in feed ingredients or complete feed. This is primarily because there are only a few laboratories in the world that have the biosecurity and analytical capabilities of detecting ASFV in feed. Several in vitro studies have been conducted to evaluate ASFV concentration, viability and inactivation when ASFV was added to various feed ingredients and complete feed. These inoculation studies have shown that some feed matrices support virus survival longer than others and the reasons for this are unknown. Current analytical methodologies have significant limitations in sensitivity, repeatability, ability to detect viable virus particles and association with infectivity. As a result, interpretation of findings using various measures may lead to misleading conclusions. Because of analytical and technical challenges, as well as the lack of ASFV contamination data in feed supply chains, quantitative risk assessments have not been conducted. A few qualitative risk assessments have been conducted, but they have not considered differences in potential scenarios for ASFV contamination between various types of feed ingredient supply chains. Therefore, the purpose of this review is to provide a more holistic understanding of the relative potential risks of ASFV contamination in various global feed ingredient supply chains and provide recommendations for addressing the challenges identified.

The Use of Colistin in Food-Producing Animals in Estonia-Vaccination as an Effective Alternative to Consumption of Critically Important Antimicrobials in Pigs

Reducing the use of critically important antimicrobials in veterinary medicine is increasingly important to avoid the development and spread of antimicrobial resistance. The aim of this study was to analyse ten-year trends of colistin consumption in Estonia and to ascertain the possible association between Escherichia (E.) coli vaccination and colistin consumption in pig production. Colistin sales data (2010-2019) were collected from the wholesalers, allowing differentiation of target species. In Estonia, the amount of sold colistin increased constantly and almost doubled from 2010 to 2013, and decreased from 2013 to 2019 by 92.5% in total. On average across a ten-year study period, 89.7% of colistin was used in pig production. The number of sold doses of E. coli vaccines for pigs was very low before 2014 (<2000) and increased drastically to 2019 (362,000). According to linear time-series model with autoregressive integrated moving average (ARIMA) the consumption of colistin in pigs decreased on average by 0.23 mg/PCU for every 10,000 E. coli vaccine doses (95% CI -0.39, -0.06; p = 0.006) over ten years. This study revealed that in pig production, vaccination against E. coli strains contributes to the expected downward trend in colistin consumption.

T-cell responses in domestic pigs and wild boar upon infection with the moderately virulent African swine fever virus strain 'Estonia2014'

Infection with African swine fever virus (ASFV) causes a highly lethal haemorrhagic disease in domestic and Eurasian wild pigs. Thus, it is a major threat to pig populations worldwide and a cause of substantial economic losses. Recently, less virulent ASFV strains emerged naturally, which showed higher experimental virulence in wild boar than in domestic pigs. The reason for this difference in disease progression and outcome is unclear but likely involves different immunological responses. Unfortunately, besides the importance of CD8α⁺ lymphocytes, little is known about the immune responses against ASFV in suids. Against this background, we used a multicolour flow cytometry platform to investigate the T-cell responses in wild boar and domestic pigs after infection with the moderately virulent ASFV strain 'Estonia2014' in two independent trials. CD4^- /CD8α⁺ and CD4⁺ /CD8α⁺ αβ T-cell frequencies increased in both subspecies in various tissues, but CD8α⁺ γδ T cells differentiated and responded in wild boar only. Proliferation in CD8α⁺ T cells was found 10 days post infectionem only. Frequencies of T-bet⁺ T cells increased in wild boar but not in domestic pigs. Of note, we found a considerable loss of perforin expression in cytotoxic T cells, 5 and 7 dpi. Both subspecies established a regulatory T-cell response 10 dpi. In domestic pigs, we show increasing levels of ICOS⁺ and CD8α⁺ invariant Natural Killer T cells. These disparities in T-cell responses might explain some of the differences in disease progression in wild boar and domestic pigs and should pave the way for future studies.

ASF Exit Strategy: Providing cumulative evidence of the absence of African swine fever virus circulation in wild boar populations using standard surveillance measures

EFSA assessed the role of seropositive wild boar in African swine fever (ASF) persistence. Surveillance data from Estonia and Latvia investigated with a generalised equation method demonstrated a significantly slower decline in seroprevalence in adult animals compared with subadults. The seroprevalence in adults, taking more than 24 months to approach zero after the last detection of ASFV circulation, would be a poor indicator to demonstrate the absence of virus circulation. A narrative literature review updated the knowledge on the mortality rate, the duration of protective immunity and maternal antibodies and transmission parameters. In addition, parameters potentially leading to prolonged virus circulation (persistence) in wild boar populations were reviewed. A stochastic explicit model was used to evaluate the dynamics of virus prevalence, seroprevalence and the number of carcasses attributed to ASF. Secondly, the impact of four scenarios on the duration of ASF virus (ASFV) persistence was evaluated with the model, namely a: (1) prolonged, lifelong infectious period, (2) reduction in the case-fatality rate and prolonged transient infectiousness; (3) change in duration of protective immunity and (4) change in the duration of protection from maternal antibodies. Only the lifelong infectious period scenario had an important prolonging effect on the persistence of ASF. Finally, the model tested the performance of different proposed surveillance strategies to provide evidence of the absence of virus circulation (Exit Strategy). A two-phase approach (Screening Phase, Confirmation Phase) was suggested for the Exit Strategy. The accuracy of the Exit Strategy increases with increasing numbers of carcasses collected and tested. The inclusion of active surveillance based on hunting has limited impact on the performance of the Exit Strategy compared with lengthening of the monitoring period. This performance improvement should be reasonably balanced against an unnecessary prolonged 'time free' with only a marginal gain in performance. Recommendations are provided for minimum monitoring periods leading to minimal failure rates of the Exit Strategy. The proposed Exit Strategy would fail with the presence of lifelong infectious wild boar. That said, it should be emphasised that the existence of such animals is speculative, based on current knowledge.

Emergence and prevalence of naturally occurring lower virulent African swine fever viruses in domestic pigs in China in 2020

African swine fever virus (ASFV) has been circulating in China for more than two years, and it is not clear whether the biological properties of the virus have changed. Here, we report on our surveillance of ASFVs in seven provinces of China, from June to December, 2020. A total of 22 viruses were isolated and characterized as genotype II ASFVs, with mutations, deletions, insertions, or short-fragment replacement occurring in all isolates compared with Pig/HLJ/2018 (HLJ/18), the earliest isolate in China. Eleven isolates had four different types of natural mutations or deletion in the EP402R gene and displayed a non-hemadsorbing (non-HAD) phenotype. Four isolates were tested for virulence in pigs; two were found to be as highly lethal as HLJ/18. However, two non-HAD isolates showed lower virulence but were highly transmissible; infection with 10⁶ TCID₅₀ dose was partially lethal and caused acute or sub-acute disease, whereas 10³ TCID₅₀ dose caused non-lethal, sub-acute or chronic disease, and persistent infection. The emergence of lower virulent natural mutants brings greater difficulty to the early diagnosis of ASF and creates new challenges for ASFV control.

African Swine Fever Laboratory Diagnosis-Lessons Learned from Recent Animal Trials

African swine fever virus (ASFV) causes a hemorrhagic disease in pigs with high socio-economic consequences. To lower the impact of disease incursions, early detection is crucial. In the context of experimental animal trials, we evaluated diagnostic workflows for a high sample throughput in active surveillance, alternative sample matrices for passive surveillance, and lateral flow devices (LFD) for rapid testing. We could demonstrate that EDTA blood is significantly better suited for early ASFV detection than serum. Tissues recommended by the respective diagnostic manuals were in general comparable in their performance, with spleen samples giving best results. Superficial lymph nodes, ear punches, and different blood swabs were also evaluated as potential alternatives. In summary, all matrices yielded positive results at the peak of clinical signs and could be fit for purpose in passive surveillance. However, weaknesses were discovered for some matrices when it comes to the early phase of infection or recovery. The antigen LFD showed variable results with best performance in the clinical phase. The antibody LFD was quite comparable with ELISA systems. Concluding, alternative approaches are feasible but have to be embedded in control strategies selecting test methods and sample materials following a “fit-for-purpose” approach.

With or without a Vaccine-A Review of Complementary and Alternative Approaches to Managing African Swine Fever in Resource-Constrained Smallholder Settings

The spectacular recent spread of African swine fever (ASF) in Eastern Europe and Asia has been strongly associated, as it is in the endemic areas in Africa, with free-ranging pig populations and low-biosecurity backyard pig farming. Managing the disease in wild boar populations and in circumstances where the disease in domestic pigs is largely driven by poverty is particularly challenging and may remain so even in the presence of effective vaccines. The only option currently available to prevent ASF is strict biosecurity. Among small-scale pig farmers biosecurity measures are often considered unaffordable or impossible to implement. However, as outbreaks of ASF are also unaffordable, the adoption of basic biosecurity measures is imperative to achieve control and prevent losses. Biosecurity measures can be adapted to fit smallholder contexts, culture and costs. A longer-term approach that could prove valuable particularly for free-ranging pig populations would be exploitation of innate resistance to the virus, which is fully effective in wild African suids and has been observed in some domestic pig populations in areas of prolonged endemicity. We explore available options for preventing ASF in terms of feasibility, practicality and affordability among domestic pig populations that are at greatest risk of exposure to ASF.

African Swine Fever Epidemiology and Control

African swine fever is a devastating disease that can result in death in almost all infected pigs. The continuing spread of African swine fever from Africa to Europe and recently to the high-pig production countries of China and others in Southeast Asia threatens global pork production and food security. The African swine fever virus is an unusual complex DNA virus and is not related to other viruses. This has presented challenges for vaccine development, and currently none is available. The virus is extremely well adapted to replicate in its hosts in the sylvatic cycle in East and South Africa. Its spread to other regions, with different wildlife hosts, climatic conditions, and pig production systems, has revealed unexpected epidemiological scenarios and different challenges for control. Here we review the epidemiology of African swine fever in these different scenarios and methods used for control. We also discuss progress toward vaccine development and research priorities to better understand this complex disease and improve control.

African Swine Fever Circulation among Free-Ranging Pigs in Sardinia: Data from the Eradication Program

African swine fever virus (ASFV), the cause of a devastating disease affecting domestic and wild pigs, has been present in Sardinia since 1978. In the framework of the regional ASF eradication plan, 4484 illegal pigs were culled between December 2017 and February 2020. The highest disease prevalence was observed in the municipality with the highest free-ranging pig density, and culling actions drastically reduced ASFV circulation among these animals. ASFV-antibody were detected in 36.7% of tested animals, which were apparently healthy, thus, the circulation of low-virulence ASFV isolates was hypothesized. ASFV genome was detected in 53 out of 2726 tested animals, and virus isolation was achieved in two distinct culling actions. Two ASFV haemadsorbing strains were isolated from antibody-positive apparently healthy pigs: 55234/18 and 103917/18. Typing analysis revealed that both isolates belong to p72 genotype I, B602L subgroup X; phylogenetic analysis based on whole genome sequencing data showed that they were closely related to Sardinian ASFV strains collected since 2010, especially 22653/Ca/2014. Our data suggested the absence of immune-escaped ASFV variants circulating among free-ranging pigs, indicating that other elements contributed to virus circulation among these animals. Understanding factors behind disease persistence in endemic settings might contribute to developing effective countermeasures against this disease.

Advanced "lab-on-a-chip" to detect viruses - Current challenges and future perspectives

Massive viral outbreaks draw attention to viruses that have not been thoroughly studied or understood. In recent decades, microfluidic chips, known as "lab-on-a-chip", appears as a promising tool for the detection of viruses. Here, we review the development of microfluidic chips that could be used in response to viral detection, specifically for viruses involved in more recent outbreaks. The advantages as well as the disadvantages of microfluidic systems are discussed and analyzed. We also propose ideas for future development of these microfluidic chips and we expect this advanced technology to be used in the future for viral outbreaks.