To My Quyen, Truong Minh Hieu, Tran Duy Thanh, Pham Trang Thanh Nguyen,
Nguyen Khanh Thuan, Nguyen Phuc Khanh, Nguyen Thanh Lam*
1. Introduction
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 (Galindo and Alonso, 2017). ASF is endemic in countries in sub-Saharan Africa, but since its introduction to the Caucasus region in 2007, a highly virulent strain of ASFV has continued to circulate and spread into Eastern Europe and Russia, and most recently into Western Europe, China, and various countries of Southeast Asia (Karger et al., 2019). The clinical signs and pathological features of the first outbreaks on ASF in Vietnam in 2019, caused by an isolate with 100% similarity to the genotype II (p72) isolates from Georgia in 2007 and China in 2018. The disease onset with a peracute to acute clinical course with high mortality (Nga et al., 2020a).
The recent spread of ASF in the People's Republic of China and neighboring countries in Asia has had significant economic consequences with an estimated direct cost of $55–$130 billion. This pandemic has devastated the swine industry in large geographical areas of Southeast Asia with 14 countries reporting ASF outbreaks since the first documented case was confirmed in the city of Shenyang, Liaoning Province, China, on 3 August 2018. In the absence of any available vaccines, the control of ASF relies on the detection and culling of infected animals (Tran et al., 2021d).
2. African swine fever in Vietnam
Que et al., 2020 confirmed that ASF outbreaks pose adverse impacts on national pork supply and demand, especially in the traditional sector. The national pig supply falls by nearly 27.8% in the traditional sector with a 5% negative demand shock and by 33.2% with a 20% negative demand shock in the simulated scenarios compared to the non-outbreak scenarios. The impacts are differentiated by region and show the Red River Delta and Southeast suffer the highest losses. The modern sector is less likely to be affected and even benefits from the ASF outbreak. National pig sector income from the modern sector increases by 16.9% with a 5% negative demand shock and by 14% with a 20% negative demand shock in the simulated scenarios compared to the non-outbreak scenarios. The results are driven by the modern sector’s strict biosecurity practices and high technology growth. ASF outbreaks tend to accelerate the restructuring process of the pig industry towards faster expansion of the commercial and modern pig sectors and shrinking of the traditional sector. Creation of employment opportunities for smallholders who are squeezed out as a result of ASF should be considered to ensure that livelihoods are not compromised as a result of disease (Que et al., 2020).
2.1. Viruses
Le et al., 2019 confirmed the 2019 outbreak in Vietnam by real-time PCR. The causative strain belonged to p72 genotype II and was 100% identical with viruses isolated in China (2018) and Georgia (2007). International prevention and control collaboration is needed (Le et al., 2019).
Tran et al., conducted further investigation of the intergenic region of ASFVs isolated in the Capital Hanoi region showed two different variants, IGR I and IGR II, which were located between the I73R and I329L genes of the p72 genotype II ASFV strains. This finding suggests co-circulation of two ASFV variants in the domestic pig population in Vietnam (Tran et al., 2021b).
Figure 1. (a) Phylogenetic analysis of ASFV isolates from the Capital Hanoi of Vietnam based on its partial p72 genes. The sequences of the p72 of representative ASF virus were downloaded from the NCBI database. The neighbour-joining method was used to construct phylogenetic trees using MEGA 7 software. Numbers along branches indicate bootstrap values > 80% (1,000 replicates. The red triangle indicates the ASFV isolates from this study. Scale bar indicates nucleotide substitutions per site; (b) alignment of the partial nucleotide sequence of the intergenic region located between 173R and 1329L of ASFV isolates from the Capital Hanoi region of Vietnam with reference ASFV strains. The mutation that results in the insertion of a single nucleotide internal repeat sequence IGGAATATATA in the ASFVS from the Capital Hanoi region of Vietnam is indicated by grey shading (Tran et al., 2021b).
Tran et al., showed that the phylogenetic analysis of ASFVs isolated in the North Central region of Vietnam belong to genotype II and serotype 8. Additionally, tandem repeat sequence (TRS) studies indicated that these ASFVs are very close to ASFV strains detected in China and Belgium, 2018, and differ from ASFV isolated in Georgia in 2007 (Tran et al., 2021c).
Mai et al., 2021 collected 26 ASFV isolates from organs and blood samples from domestic pigs from 23 different provinces of northern, central and southern Vietnam during 2019-2020 ASF outbreaks were genetically characterized. Nucleotide sequences were determined for a portion of the B646L (p72) gene, the complete E183L (p54) gene, the variable region of EP402R (CD2v), the central variable region (CVR) of pB602L, and a tandem repeat sequence (TRS) between the I73R and I329L genes. Analysis of the partial B646L (p72) and EP402R (CD2v) gene sequences and the full-length E183L (p54) gene sequence showed that all 26 ASFV isolates belonged to genotype II and serotype VIII and that they were identical to the strain Georgia/2007/1 and all ASFV strains sequenced in China. The TRS between the I73R and I329L genes contained a 10-nucleotide insertion that was observed in the Chinese ASFV strain CN201801 isolated from domestic pigs in 2018, but not in the Georgia/2007/1 and China/Jilin/2018/boar strains isolated from wild boar in China (Mai et al., 2021).
Tran et al., have pointed out that 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. Their study reports the activity against ASFV VN/Pig/HN/19 strain of antimicrobial Sal CURB RM E Liquid, F2 Dry and K2 Liquid. Their 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 (Tran et al., 2020b)
Figure 2. (A, B) The combination of feed with the Sal CURB RM E Liquid, Sal CURB F2 Dry or Sal CURB K2 Liquid and 1×105/mL HAD50 of VN/Pig/HN/19 strain one, three, and seven days post inoculation. Values represent mean and standard deviation results from three independent experiments. Significant differences compared to control are denoted by * for P < 0.05 and ** for P < 0.01 (Tran et al., 2020b).
2.1. Pathology
Nga et al., 2020 analyzed that the clinical signs and pathological features of the first outbreaks on ASF in Vietnam in 2019, caused by an isolate with 100% similarity to the genotype II (p72) isolates from Georgia in 2007 and China in 2018. The disease onset with a peracute to acute clinical course with high mortality. Some animals showed very unspecific clinical signs with other showing severe hyperthermia, respiratory distress, diarrhea, or vomit. Hemorrhagic splenomegaly and lymphadenitis were the main lesions observed at post mortem examination, with histopathological changes confirming the lymphoid depletion and multi-organ hemorrhages. Monocyte-macrophages were identified by means of immunohistochemical methods as the main target cell for the ASF virus in tissue sections (Nga et al., 2020b).
Izzati et al., 2021 analyzed samples from eight ASFV-infected farms. Histopathological results revealed the characteristic lesions of the acute to the subacute clinical form of ASF. Immunohistochemical results showed ASFV viral antigen distribution in mononuclear cells/macrophage in various organs, hepatocytes and renal tubular epithelium. Molecular analysis of partial capsid protein 72 gene revealed that ASFV strain from the eight separate outbreaks belonged to genotype II (Izzati et al., 2021b).
Figure 3. Gross lesions in ASFV infected pigs in Vietnam, 2019 (1a–f). Marble-like haemorrhage in the superficial inguinal lymph nodes, case 18 (a). Diffuse haemorrhage in the mesenteric lymph nodes, case 74 (b). Petechial haemorrhage on the renal capsule, renal cortex and diffuse haemorrhage in the renal pelvis, case 18 (c). Hyperaemic splenomegaly in relation to the size of the stomach. Gastrohepatic lymph nodes showed diffuse haemorrhage, case 59 (d). Ecchymoses of the epicardium with serosanguinous pericardial fluid, case 58 (e). Ecchymoses of the myocardium, case 58 (f)(Izzati et al., 2021b).
Figure 4. HE staining of tissues from ASF-infected pigs (3a–i). There is sinus haemorrhage in the lymph node tissue of case 58 (a). Interstitial pneumonia and an increased number of pulmonary intravascular macrophages are present in the lung tissue of case 58 (b). Three to four layers of mononuclear cell infiltrations in the meninges and perivascular cuffing are present in the cerebral tissue of case 20. The inset shows swollen endothelial cells and apoptotic bodies in the perivascular cuffing lesion (c). Epicardial and myocardial haemorrhage with infiltration of mononuclear cells is present in the heart tissue of case 18 (d). Multifocal haemorrhages are seen in the superficial and deep lamina propria in the gastric epithelial tissue of case 58 (e). Multifocal haemorrhages are present in the outer longitudinal muscularis layer in the small intestinal tissue of case 18 (f). The large intestinal tissue of case 58 shows multifocal haemorrhages in the deep lamina propria and dilated capillaries filled with erythrocytes in the submucosa layer with extensive oedema beneath (g). There is extensive haemorrhage in the pelvis region of the renal tissue in case 20 (h). Multifocal haemorrhages are found in the cortical region, sometimes extended into the capsular layer of the renal tissue in case 19 (i) (Izzati et al., 2021b).
Oh et al., 2021 determined age-related viral loads in 5 major organs (lung, liver, spleen, kidney, and lymph node) by immunohistochemistry as well as in the blood by real-time polymerase chain reaction (PCR). Age-related systemic pathological lesions were analyzed in the listed organs among three age groups. Weaned pigs had significantly (p < 0.05) higher levels of viral loads in their lung, liver, lymph nodes and blood than in those of fattening pigs and sows. Fattening pigs had significantly (p < 0.05) higher scores of macroscopic lung and lymphoid lesions, and microscopic liver lesions compared with those of weaned pigs and sows. The results of this study demonstrated that viral loads were age-related in acute naturally occurring ASF but the severity of pathological lesions was not correlated with the level of viral loads in the five major organs (Oh et al., 2021a).
Pornthummawat et al., 2021 recognized that histopathological evidence could benefit from further insights into the status and role of the surviving animals; therefore, they performed a histopathological study on four pigs from farms with a history of ASF outbreak. They 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 (Pornthummawat et al., 2021).
2.3. Epidemiology
Lee et al,. 2021 affirmed that the indirect contact exhibited an important role in transmitting the ASF virus. In order to minimize ASF transmission between farms, they found that movement restriction needed to reach a certain level (approximately between 50% and 75%) and that the restriction had to be applied in a timely manner. This study offers valuable insight into how ASF virus can be transmitted via direct and indirect contact and controlled among farms under the various simulation scenarios. Their results suggest that the enforcement of movement restriction was an effective control measure as soon as the outbreaks were reported. In addition, this study provided evidence that high standards of biosecurity can contribute to the reduction of disease spread (Lee et al., 2021b).
Additionally, Lee et al,. 2021 found that the timing of culling at 16, 12, 8, and 6 weeks had resulted in a reduction of the number of median infected farms by 81.92%, 91.63%, 100%, and 100%, respectively. Finally, their evaluation of the implication of stability of ties between farms indicated that if the farms were to have the same trading partners for at least six months could significantly reduce the median number of infected farms to two (95th percentile: 413) than in the basic model. The study showed that pig movements among farms had a significant influence on the transmission dynamics of ASF virus. In addition, they found that the either timing of culling, reduction in the number of trading partners each farm had, or decreased mean contact rate during the outbreaks were essential to prevent or stop further outbreaks (Lee et al., 2021c).
Pham et al., 2021 proposed a novel architectural framework for simultaneously deploying any epidemic simulation program both on premises and on the cloud to improve performance and scalability. They also conducted some experiments to evaluate the proposed architectural framework on some aspects when applying it to simulate the spread of ASF in Vietnam (Pham et al., 2021).
Oh et al., 2021 conducted a serological follow-up for more than 14 months with 14 convalescent gilts and their offspring. All convalescent animals had long lasting high serum antibody levels without persistent viremia. They also did not excrete virus via nasal discharge post-recovery. These convalescent pigs could partially perform as replacement gilts despite the fact that ASF affected reproductive performance. Here, they confirmed that there were neither the carriers of nor recurrence of disease in the convalescent pigs and their offspring following the outbreak of acute ASF (Oh et al., 2021b).
2.4. Diagnosis
Lee at el., 2021 inoculated intramuscularly ten pigs with an ASFV strain from Vietnam (titer, 103.5 HAD50/mL), and their temperature, clinical signs, and virus excretion patterns were recorded. In addition, herd and environmental samples were collected daily. The pigs died 5–8 days-post-inoculation (dpi), and the incubation period was 3.7 ± 0.5 dpi. ASFV genome was first detected in the blood (2.2 ± 0.8) and then in rectal (3.1 ± 0.7), nasal (3.2 ± 0.4), and oral (3.6 ± 0.7 dpi) swab samples. ASFV was detected in oral fluid samples collected using a chewed rope from 3 dpi. The liver showed the highest viral loads, and ear tissue also exhibited high viral loads among 11 tissues obtained from dead pigs. Overall, ASFV from Vietnam was classified as peracute to acute form. The rope-based oral fluid collection method could be useful for early ASFV detection and allows successful ASF surveillance in large pig farms. Furthermore, ear tissue samples might be a simple alternative specimen for diagnosing ASF infection in dead pigs (Lee et al., 2021a).
Tran et al., 2021 conducted a slight modification in probe sequence 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 (Tran et al., 2020a).
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. 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 their modified probe (Cq value ranked between 33.12-35.78) (Tran et al., 2020a).
Tran at el., 2021 modified a colorimetric loop-mediated isothermal amplification (LAMP) assay and evaluated for ASF virus detection using crude serum samples collected from domestic pigs in Vietnam during the 2019 outbreak. The LAMP results can be readily visualized to the naked eye within 30 min without the requirement of DNA extraction and sophisticated equipment. The sensitivity, specificity and limit of detection of direct colorimetric LAMP assay were comparable to a commercial diagnostic real-time PCR kit. Results strongly indicate that the adapted colorimetric LAMP assay has a remarkable potential for the in-field diagnosis of ASF (Tran et al., 2021a).
Figure 6. Virus isolation is combined with the HAD test. No RBC, Mock-non infected cells without red blood cell at 72 h; RBC, M
ock-non infected cells with red blood cell observed at 72 h; 1–7, hemadsorption in the culture of PAM cells infected with seven ASFV positive samples (Original magnification, 400X). HAD, haemadsorption; RBC, red blood cells; PAM, porcine alveolar macrophages; ASFV, African swine fever virus (Tran et al., 2020a).
2.5. Vaccine, control and prevention
Tran et al., 2021 demonstrated that ASFV-G-ΔI177L is able to protect pigs against the virulent ASFV isolate currently circulating and producing disease in Vietnam with similar efficacy as reported against the Georgia strain. Comparative studies performed using a large number of pigs of European and Vietnamese origin demonstrated that a minimum protective dose of 102 HAD50 of ASFV-G-ΔI177L equally protects animals of both breeds. In concurrence with those results, the onset of immunity in these animal breed showed appearance of protection in approximately one-third of the animals by the second week post vaccination, with full protection achieved by the fourth week post vaccination. Therefore, results presented here demonstrated that ASFV-G-ΔI177L is able to induce protection against virulent Vietnamese ASFV field strains and is effective in protecting local breeds of pigs as efficiently as previously shown for European cross-bred pigs (Tran et al., 2021d).
Dung et al., reported the antiviral ability of SNPs against ASFV. The microbial contamination in the pig house was significantly reduced by spraying the SNP solution 25 ppm. SNP solution with the concentration of 0.78 ppm does not show any toxicity to porcine alveolar macrophage cells, while completely inhibits ASFV at the titer of 103 HAD50. This study confirms that SNPs have a highly antiviral ability against ASFV and is a promising disinfectant that can be used to prevent the ASFV transmission (Thi Ngoc Dung et al., 2020).
3. In Asia
Mason-D’Croz at el., 2020 applied two linked global economic models to explore the consequences of different scales of the epidemic on pork prices and on the prices of other food types and animal feeds. The models project global pork prices increasing by 17–85% and unmet demand driving price increases of other meats. This price rise reduces the quantity of pork demanded but also spurs production in other parts of the world, and imports make up half the Chinese losses. Demand for, and prices of, food types such as beef and poultry rise, while prices for maize and soybean used in feed decline. There is a slight decline in average per capita calorie availability in China, indicating the importance of assuring the dietary needs of low-income populations. Outside China, projections for calorie availability are mixed, reflecting the direct and indirect effects of the ASF epidemic on food and feed markets (Mason-D’Croz et al., 2020).
Mighell E and Ward MP., 2021 used data collated from reports of confirmed cases and described spatial-temporal distribution of ASF throughout Asia during its early phase from 1 August 2018 (reported start date) to 31 December 2019 to provide an overview and comparative analysis. Analysis revealed a propagating epidemic of ASFV throughout Asia, with peaks corresponding to increased reports from China, Vietnam and Laos. Two clusters of reported outbreaks were found. During the epidemic, ASFV primarily spread from the North-East to the South-East: A larger, secondary cluster in the North-East represented earlier reports, while the smaller, primary cluster in the South-East was characterized by later reports. Significant differences in country-specific epidemics, morbidity, mortality and unit types were discovered. The initial number of outbreaks and enterprise size are likely predictors of the speed of spread and the effectiveness of ASFV stamping out procedures. Biosecurity methods, wild boar populations and the transportation of pigs and movement of infected fomites are discussed as likely risk
factors for facilitating ASFV spread across Asia (Mighell and Ward, 2021)
Figure 7. Distribution of ASF outbreak sites in Asia 2018–2019. Sites have been shaded green to red by epidemic day (1 to 507). Day 1 = 1 August 2018, Day 507 = 20 December 2019. Primary (22/02/2019-22/09/2019) and secondary (14/08/2018-15/10/2018) clusters have been overlayed, with observed/expected values being 12.87 and 6.21, respectively. Data was extracted from the World Organisation for Animal Health (Mighell and Ward, 2021).
Wu et al., 2020 recapitulated the epidemic situation of ASF in China as of July 2020 and analyzed the influencing factors during its transmission. Since the situation facing the prevention, control, and eradication of ASF in China is not optimistic, safe and effective vaccines are urgently needed. In light of the continuous development of ASF vaccines in the world, the current scenarios and evolving trends of ASF vaccines are emphatically analyzed in the latter part of the review. The latest research outcomes showed that attempts on ASF gene-deleted vaccines and virus-vectored vaccines have proven to provide complete homologous protection with promising efficacy. Moreover, gaps and future research directions of ASF vaccine are also discussed (Wu et al., 2020).
3.1. Viruses
Ge et al., 2019 showed that ASF causative strain contained two tandem repeat sequence insertions in the intergenic region between the I73R and the I329L genes, and was different from previously reported strains in China and other countries (Ge et al., 2019).
Sun et al., 2021 reported on their 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 (Sun et al., 2021).
Ju et al., 2021 suggested that ASFV genes expression demonstrated a time-depended pattern and ASFV early genes were involved in antagonizing host innate immunity. Moreover, viral small RNA (vsRNA) was generated as well. Meanwhile, transcriptome analysis of host genes suggested a strong inhibition host immunity-related genes by ASFV infection in PAMs, while enhanced chemokine-mediated signaling pathways and neutrophil chemotaxis were observed in ASFV infected PAMs. Furthermore, ASFV infection also down-regulated host microRNAs (miRNAs) that putatively targeted viral genes, while also triggering dysregulation of host metabolism that promoted virus replication at transcription level. Most importantly, infection of PAMs with ASFV induced a different transcriptome pattern from that of highly pathogenic porcine reproductive and respiratory syndrome virus (HP-PRRSV), which is known to trigger a host cytokine storm. In conclusion, their transcriptome data implied that ASFV infection in PAMs appeared to be associated with strong inhibition of host immune responses, dysregulation of host chemokine axis and metabolic pathways (Ju et al., 2021).
3.2. Pathology
Izzati et al., 2021 conducted to characterize the pathology of natural cases of CSF in northern Vietnam in 2018 and their genetic prevalence. A total of 10 representative pigs were collected from four provinces (Hung Yen, Ha Noi, Quang Ninh and Thai Binh) during five outbreaks and examined pathologically. The gross and histopathological findings showed the disease was expressed as the acute or the subacute to chronic form of CSF, depending on the age of the animals. The most consistently observed lesions associated with infection by the classical swine fever virus (CSFV) included lymphoid depletions in tonsils, lymph node and spleen; histiocytic hyperplasia in spleen; cerebral haemorrhage; perivascular cuffing in the brain; renal erythrodiapedesis; urothelial vacuolation and degeneration and interstitial pneumonia. The immunohistochemical findings showed a ubiquitous CSFV antigen mainly in the monocytes/macrophages and in the epithelial and endothelial cells in various organs. CSFV neurotropism was also found in the small neurons of the cerebrum and the ganglia of the myenteric plexus. Analysis of the full-length envelope protein (E2) genome sequence showed that all strains were genetically clustered into subgenotype 2.5, sharing a nucleotide identity of 94.0%–100.00%. Based on the results of this study, the strain was categorized as a moderately virulent CSFV (Izzati et al., 2021a)
Vlasove et al., 2020 isolated caused the death of pigs manifesting, as a rule, signs of an acute or subacute form of the disease when using various methods of infection including intramuscular, direct contact, intranasal and oral routes. The virus was hemadsorbing, belonging to serotype 8 and genotype II, and accumulated in the blood with a titer of 6.5 to 7.5 lg HAU50/cm3. The ASFV isolates circulating in the central region of Russia were found to have an insertion of 10 base pairs in the intergenic region I73R/I329L. However, the ASFV isolated in the Irkutsk region and South Ossetia, as well as Georgia 2007/1 (FR682468.1), lacked this insertion (Vlasov et al., 2020)
Figure 8. Acute form of ASF infection in pigs: a) slight ear cyanosis; b) splenomegaly; c) enlarged mesenteric lymph nodes; d) hepatomegaly; e) stomach with enlarged and hemorrhagic gastric lymph nodes; f) hemorrhages on the kidneys (Vlasov et al., 2020).
3.3. Epidemiology
Denstedt et al., 2021 found an extensive overlap between wild boar habitat and domestic pig ranging areas around villages bordering forests in all three countries, creating a high-risk interface for viral spillover between domestic pig and wild boar populations. Fifteen and three wild boar carcasses were detected through passive reporting in Laos and Viet Nam, respectively, in 2019 and early 2020. Four of five carcasses screened in Laos and two of three in Viet Nam were confirmed positive for ASFV using real-time PCR. There were no confirmed reports of wild boar carcasses in Cambodia. This is the first confirmation of ASF in wild boar in Southeast Asia, the result of a probable viral spillover from domestic pigs, which highlights the importance of early reporting and monitoring of ASF in wild boar to enable the implementation of appropriate biosecurity measures (Denstedt et al., 2021).
Figure 9. The interface between domestic pigs and wild boar at Namsat Village, Houaphanh Province in Laos. Locations of farms with domestic pig mortalities, domestic pig carcass disposal sites, and detected wild boar hoofprints and carcasses are indicated both within the village and within Nam Et - Phou Louey National Park boundaries (Denstedt et al., 2021).
Figure 10. The timeline of ASF detection in domestic pigs in Laos, Cambodia, and Vietnam, and subsequently wild boar in Laos and Vietnam (Denstedt et al., 2021).
You et al., 2021 showed that the total economic loss accounts for 0.78% of China’s gross domestic product in 2019, with impacts experienced in almost all economic sectors through links to the pork industry and a substantial decrease in consumer surplus. Scenario analyses demonstrate that the worst cases of pig production reduction and price increase would trigger 1.4% and 2.07% declines in gross domestic product, respectively. These findings demonstrate an urgent need for rapid ASF containment and prevention measures to avoid future outbreaks and economic declines (You et al., 2021).
Gao et al,. 2021 suggested that the risk of ASFV transmission though the legal live-pig trade is highest in the southeastern regions of China. Vulnerable regions centred around Zhejiang, Jiangsu and Anhui provinces, especially throughout the months of January and December. Liaoning province contributes most to transmission risk with 46.7% of the overall annual risk. This study quantified the risk of ASFV spread in China related to the legal trade of pigs and provides detailed and new information for the development of ASFV monitoring and control plans in China and other countries who also face the challenge of ASFV (Gao et al., 2021).
Yang et al., 2021 indicated the first occurrence of ASFV has not been purely dependent on the geographical distance from existing infected regions. Instead, the pork supply–demand patterns have played an important role. Predictions based on a new distance measure achieve better performance in predicting ASFV spread among Chinese provinces and thus have the potential to enable the design of more effective control interventions (Yang et al., 2021).
Tuan et al., 2021 confirmed that there is a high risk of ASF transmission from traders in the pig value chain, including collectors, slaughterers and retailers at the provincial, district and commune levels. These actors all participate in the sale and purchase of pigs that could be infected. Further, live pigs and pig products are not subject to quarantine within the province and their transport is not controlled. In addition, when an epidemic occurs, it takes a long time to identify the disease due to the lack of resources and capacity among local animal husbandry actors and the inability to analyse samples locally to detect ASF. This allows time for pathogens to spread through transport and sales of sick pigs. The results of this study indicate that farms that had no cases of ASF now tend to apply more biosecurity measures in pig production than those that had pigs with ASF. Officers at provincial and district levels have limited resources and capacity for monitoring and surveillance of ASF. There is a need for compliance by all pig producers and other actors in the pig value chain to adopt biosecurity practices. Therefore, awareness, knowledge and understanding of infection and risks of ASF need to be improved. Veterinary officials at the provincial and district levels need to improve capacity and resources to perform rapid tests for ASF and need to coordinate with local actors on the control and prevention of ASF in the community (Tuan, 2021).
3.4. Diagnosis
Wang et al., 2020 established a novel quantitative real-time polymerase chain reaction (qPCR) assay with lyophilized powder reagents (LPR), targeting the major structural protein p72 gene. This assay had many advantages, such as saving time and money, good sensitivity and repeatability. The sensitivity of this assay was 100 copies/μl of ASFV plasmid templates, and the assay showed 10-fold greater sensitivity than a qPCR assay recommended by OIE. Furthermore, specificity analysis showed that qPCR with LPR for ASFV had no cross-reactivity with other important swine pathogens. In clinical diagnoses of 218 blood samples of domestic pigs in China, the positive rate of the diagnosis of ASFV by qPCR with the LPR and commercial kit reached 80.73% (176/218) and 76.61% (167/218) respectively. The coincidence rate between the two assays is 92.20% (201/218), and kappa value is 0.768 (p < .0001) by SPSS analysis. The overall agreement between the two assays was 95.87% (209/218). Further Pearson correlation and linear regression analysis showed a significant correlation between the two assays with an R2 value of 0.9438. The entire procedure, from specimen processing to result reporting, can be completed within 2 hr. Our results demonstrated that the qPCR-LPR assay is a good laboratory diagnostic tool for sensitive and efficient detection of ASFV (Wang et al., 2020).
Matsumoto et al., 2020 evaluated an ASF antigen detection RDT from Shenzhen Lvshiyuan Biotechnology Co. Ltd by using clinical field samples submitted to the National Animal Health Laboratory (NAHL) from ASF suspect cases between June and December 2019 in Lao PDR. Positive (n = 57) and negative (n = 50) samples of whole blood, serum and haemolysed serum were assessed by RDT and PCR, with the latter used as the gold standard reference comparator. Overall the RDT had a diagnostic sensitivity (DSe) of 65 %, 95 % CI [51–77] and diagnostic specificity (DSp) of 76 %, 95 % CI [62–87]. The RDT demonstrated improved performance on samples with lower PCR cycle threshold (ct) values with each additional cycle reducing the odds of the RDT returning a positive by 17 % relative to the previous cycle, 95 % CI [8 %–28 %] (P < 0.01). While this test shows promise for field application, complete validation of diagnostic accuracy requires a larger sample size (Matsumoto et al., 2020).
3.5. Vaccine, control and prevention
Li et al., 2021 reported that deletion of the QP509L and QP383R genes (ASFV-ΔQP509L/QP383R) from the highly virulent ASFV CN/GS/2018 strain results in complete viral attenuation in swine. Animals inoculated with ASFV-ΔQP509L/QP383R at a 104 50% hemadsorbing dose (HAD50) remained clinically normal during the 17-day observational period. All ASFV-ΔQP509L/QP383R-infected animals had low viremia titers and developed a low-level p30-specific antibody response. However, ASFV-ΔQP509L/QP383R did not induce protection against challenge with the virulent parental ASFV CN/GS/2018 isolate. RNA-sequencing analysis revealed that innate immune-related genes (Ifnb, Traf2, Cxcl10, Isg15, Rantes, and Mx1) were significantly lower in ASFV-ΔQP509L/QP383R-infected than in ASFV-infected porcine alveolar macrophages. In addition, ASFV-ΔQP509L/QP383R-infected pigs had low levels of IFN-β based on ELISA. These data suggest that deletion of ASFV QP509L/383R reduces virulence but does not induce protection against lethal ASFV challenge (Li et al., 2021).
4. African swine fever in global situation
4.1. Viruses
Niederwerder et al., 2021 determined the minimum and median infectious doses of the Georgia 2007 strain of ASFV through oral exposure during natural drinking and feeding behaviors. The minimum infectious dose of ASFV in liquid was 100 50% tissue culture infectious dose (TCID50), compared with 104 TCID50 in feed. The median infectious dose was 101.0 TCID50 for liquid and 106.8 TCID50 for feed. Their findings demonstrate that ASFV Georgia 2007 can easily be transmitted orally, although higher doses are required for infection in plant-based feed. These data provide important information that can be incorporated into risk models for ASFV transmission (Niederwerder et al., 2019).
Wang et al., 2021 performed experiments in domestic pigs to analyze the kinetics of representative circulating interferons (IFNs), interleukins (ILs), growth factors, tumor necrosis factors (TNFs), and chemokines induced by infection of type II virulent ASFV SY18. Pigs infected with this Chinese prototypical isolate developed severe clinical manifestations mostly from 3 days post inoculation (dpi) and died from 7 to 8 dpi. Serum analysis revealed a trend of robust and sustained elevation of pro-inflammatory cytokines including TNF-α, IFN-α, IL-1β, IL-6, IL-8, IL-12, IL-18, RANTES (regulated upon activation, normal T cell expressed and secreted), and IFN-γ-induced protein 10 (IP-10) from 3 dpi, but not the anti-inflammatory cytokines IL-10 and transforming growth factor-β (TGF-β). Moreover, secondary drastic increase of the levels of TNF-α, IL-1β, IL-6, and IL-8, as well as elevated IL-10, was observed at the terminal phase of infection. This pattern of cytokine secretion clearly drew an image inflammatory cytokine and imbalanced pro- and anti-inflammatory response, which paved a way for further of a typical cytokine storm characterized by delayed and dysregulated initiation of the secretion of pro- understanding of the molecular basis of ASFV pathogenesis (Wang et al., 2021).
Figure 11. Kinetics of serum interferons. Sera were collected daily both before and after virus inoculation. The concentrations of interferon (IFN)-α in each pig included for analysis were measured via quantitative ELISA. IFN-γ was undetectable and thus not shown. Data of different individuals are distinguished by color and are shown as mean (SD). Asterisks indicate statistical significance (Wang et al., 2021).
Njau et al., 2021 reported that the complete genome of a Tanzanian genotype II isolate, Tanzania/Rukwa/2017/1, collected in 2017 and determined using an Illumina short read strategy. The Tanzania/Rukwa/2017/1 sequence is 183,186 bp in length (in a single contig) and contains 188 open reading frames. Considering only un-gapped sites in the pairwise alignments, the new sequence has 99.961% identity with the updated Georgia 2007/1 reference isolate (FR682468.2), 99.960% identity with Polish isolate Pol16_29413_o23 (MG939586) and 99.957% identity with Chinese isolate ASFV-wbBS01 (MK645909.1). This represents 73 single nucleotide polymorphisms (SNPs) relative to the Polish isolate and 78 SNPs with the Chinese genome. Phylogenetic analysis indicated that Tanzania/Rukwa/2017/1 clusters most closely with Georgia 2007/1. The majority of the differences between Tanzania/Rukwa/2017/1 and Georgia 2007/1 genotype II genomes are insertions/deletions (indels) as is typical for ASFV. The indels included differences in the length and copy number of the terminal multicopy gene families, MGF 360 and 110. The Rukwa2017/1 sequence is the first complete genotype II genome from a precisely mapped locality in Africa, since the exact origin of Georgia2007/1 is unknown. It, therefore, provides baseline information for future analyses of the diversity and phylogeography of this globally important genetic sub-group of ASF viruses (Njau et al., 2021).
Cacketett et al., 2020 determined total RNA abundance, transcription start sites, and transcription termination sites at single-nucleotide resolution. This allowed us to characterize DNA consensus motifs of early and late ASFV core promoters, as well as a polythymidylate sequence determinant for transcription termination. Their results demonstrate that ASFV utilizes alternative transcription start sites between early and late stages of infection and that ASFV RNA polymerase (RNAP) undergoes promoter-proximal transcript slippage at 5′ ends of transcription units, adding quasitemplated AU- and AUAU-5′ extensions to mRNAs. Here, they present the first much-needed genome-wide transcriptome study that provides unique insight into ASFV transcription and serves as a resource to aid future functional analyses of ASFV genes which are essential to combat this devastating disease (Cackett et al., 2020).
Jaing et al., 2017 used a whole transcriptomic RNA-Seq method to characterize differentially expressed genes in pigs infected with a low pathogenic ASFV isolate, OUR T88/3 (OURT), or the highly pathogenic Georgia 2007/1 (GRG). After infection, pigs infected with OURT showed no or few clinical signs; whereas, GRG produced clinical signs consistent with acute ASF. RNA-Seq detected the expression of ASFV genes from the whole blood of the GRG, but not the OURT pigs, consistent with the pathotypes of these strains and the replication of GRG in circulating monocytes. Even though GRG and OURT possess different pathogenic properties, there was significant overlap in the most upregulated host genes. A small number of differentially expressed microRNAs were also detected in GRG and OURT pigs. These data confirm previous studies describing the response of macrophages and lymphocytes to ASFV infection, as well as reveal unique gene pathways upregulated in response to infection with GRG (Jaing et al., 2017).
Figure 12. Clinical scores for GRG- and OURT-infected pigs, and virus shedding for GRG-infected pigs. Panel a shows clinical scores for GRG (solid squares) and OURT (open circles) pigs. The results show the mean and standard deviation for six pigs infected with OURT and four pigs infected with GRG. The GRG pigs were terminated between 7 and 10 days after infection. Panel b shows virus shedding in the GRG pigs. Shedding was determined by measuring virus in oral, nasal and fecal samples. Viral nucleic acid was not detected in samples from the OURT pigs (Jaing et al., 2017).
Gallardo et al., 2021 compared the infection dynamics of three genotype II 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 trans-mission 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 (Gallardo et al., 2021).
Figure13: Identification of the ASFV isolates and design of the animal experiment (Gallardo et al., 2021).
*Dpi (days post-infection); dpe (days post-exposure).
4.2. Pathology
Salguero, 2020 provided that ASF can follow clinical courses from peracute to chronic in domestic pigs (Sus scrofa) depending on a variety of factors, including the immune status of the animals and the virulence of the ASFV strain. The key features of the pathogenesis of the disease in domestic swine are a) a severe lymphoid depletion including lymphopenia and a state of immunodeficiency, and b) hemorrhages. However, African wild swine like bushpigs (Potamochoerus larvatus), red river hogs (Potamochoerus porcus), and warthogs (Phacochoerus africanus) can be infected by ASFV showing no clinical signs of disease and acting as natural reservoir hosts. In this article they review the key features of the gross and microscopic pathology together with a description of the pathogenesis of ASFV infection in domestic pigs following the different clinical courses. The pathogenesis of ASF in wild and domestic swine is also described, what can provide important information for the design of control strategies, such as vaccines (Salguero, 2020).
Figure 14. (A) Toluidine blue stained semithin (1 μm) section showing a macrophage with margination of the nuclear chromatin and a juxtanuclear clear intracytoplasmic inclusion body (arrowhead) in the spleen from a pig experimentally infected with acute ASF (3 dpi). (B) Transmission electron microscopy image of the nucleus (n) and cytoplasm (c) of a macrophage in the spleen from a pig infected with ASFV showing margination of the nuclear chromatin and a viral factory within the cytoplasm (arrow). (C) Apoptosis of lymphocytes (arrows) in the spleen of from a pig experimentally infected with acute ASF (5 dpi) (Salguero, 2020).
Figure 15. (A) Lethargic animal in acute ASF. The animal show cyanosis ion the ears abdomen and limbs. (B) Severe cyanosis in an animal suffering from acute ASF, associated to very high hyperthermia (41–42°C). (C) Cyanosis in the snout and lips in acute ASF. (D) Cyanosis in the limbs in acute ASF. (E) Multifocal petechiae and ecchymosis in the skin in acute ASF. (F) Blood-stained perianal area in a pig affected by subacute ASF. (G) Severe hydropericardium (arrow) in subacute ASF. (H) Moderate to severe ascites (arrow) in subacute ASF (Salguero, 2020).
4.3. Epidemiology
O’Neil et al., 2020 reported that the mathematical model results provide insight into the key processes that drive the ASF dynamics and show that environmental transmission is a key mechanism determining the severity of an infectious outbreak and that direct frequency dependent transmission and transmission from individuals that survive initial ASF infection but eventually succumb to the disease are key for the long-term persistence of the virus. By considering scenarios representative of Estonia and Spain they show that faster degradation of carcasses in Spain, due to elevated temperature and abundant obligate scavengers, may reduce the severity of the infectious outbreak. Their results also suggest that the higher underlying host density and longer breeding season associated with supplementary feeding leads to a more pronounced epidemic outbreak and persistence of the disease in the long-term. The model is used to assess disease control measures and suggests that a combination of culling and infected carcass removal is the most effective method to eradicate the virus without also eradicating the host population, and that early implementation of these control measures will reduce infection levels whilst maintaining a higher host population density and in some situations prevent ASF from establishing in a population (O’Neill et al., 2020).
4.4. Diagnosis
Khanal et al., 2021 investigated that the use of feed dust collected from experimentally inoculated feed as a novel diagnostic sample type for ASFV detection. Moist swabs were used to collect dust from creep feeders after natural consumption of feed inoculated with 3.1–5.4 log10 TCID50/g ASFV Georgia 2007 in the presence and absence of antimicrobial feed additives. Results validate the potential use of feed dust swabs as a novel diagnostic surveillance tool for detection and quantification of viral nucleic acid and infectious virus titre in ASFV-contaminated feed (Khanal et al., 2021).
Szeredi et al., 2020 confirmed that two sensitive tests were developed for the detection of the p72 major capsid protein of ASFV both in cell culture with an immunocytochemical (IC) and in tissue samples with an immunohistochemical (IHC) method using a commercially available mouse monoclonal antibody (clone 1BC11). The IC test was able to detect the virus at high virus dilutions in cell culture and the IHC test indicated the presence of ASFV in all formalin-fixed and paraffin-embedded tissue samples collected from two wild boars. The reported IC and IHC methods were found to be useful ancillary laboratory tests for research purposes and for the diagnosis of acute ASF (Szeredi et al., 2020).
4.5. Vaccine, control and prevention
Borca et al., 2020 reported the discovery that the deletion of a previously uncharacterized gene, I177L, from the highly virulent ASFV-G produces complete virus attenuation in swine. Animals inoculated intramuscularly with the virus lacking the I177L gene, ASFV-G-ΔI177L, at a dose range of 102 to 106 50% hemadsorbing doses (HAD50), remained clinically normal during the 28-day observational period. All ASFV-G-ΔI177L-infected animals had low viremia titers, showed no virus shedding, and developed a strong virus-specific antibody response; importantly, they were protected when challenged with the virulent parental strain ASFV-G. ASFV-G-ΔI177L is one of the few experimental vaccine candidate virus strains reported to be able to induce protection against the ASFV Georgia isolate, and it is the first vaccine capable of inducing sterile immunity against the current ASFV strain responsible for recent outbreaks (Borca et al., 2020).
Barasona et al., 2019 demonstrated that oral immunization of wild boar with a non-hemadsorbing, attenuated ASF virus of genotype II isolated in Latvia in 2017 (Lv17/WB/Rie1) conferred 92% protection against challenge with a virulent ASF virus isolate (Arm07). This is the first report of a promising vaccine against ASF virus in wild boar by oral administration. Further studies should assess the safety of repeated administration and overdose, characterize long-term shedding and verify the genetic stability of the vaccine virus to confirm if Lv17/WB/Rie1 could be used for free-ranging wild boar in ASF control programs (Barasona et al., 2019b).
Niederwerder et al., 2021 investigated the efficacy of medium-chain fatty acid and formaldehyde-based feed additives in inactivating ASFV. Feed additives were tested in cell culture and in feed ingredients under a transoceanic shipment model. Both chemical additives reduced ASFV infectivity in a dose-dependent manner. This study provides evidence that chemical feed additives may potentially serve as mitigants for reducing the risk of ASFV introduction and transmission through feed (Niederwerder et al., 2021).
Figure 16. Titers of antibody against ASFV in wild boar orally vaccinated with Lv17/WB/Rie1 (gray) and wild boar exposed through contact with vaccinated animals (blue). The latter animals were exposed through contact starting from 0 days (animal ID7), 7 days (ID10), and 15 days (ID17) after vaccination. Titers were determined using the indirect immunoperoxidase test (Barasona et al., 2019a).
5. References
Barasona, J.A., Gallardo, C., Cadenas-Fernández, E., Jurado, C., Rivera, B., Rodríguez-Bertos, A., Arias, M., Sánchez-Vizcaíno, J.M., 2019a. First Oral Vaccination of Eurasian Wild Boar Against African Swine Fever Virus Genotype II. Frontiers in Veterinary Science 6.
Barasona, J.A., Gallardo, C., Cadenas-Fernández, E., Jurado, C., Rivera, B., Rodríguez-Bertos, A., Arias, M., Sánchez-Vizcaíno, J.M., 2019b. First Oral Vaccination of Eurasian Wild Boar Against African Swine Fever Virus Genotype II. Frontiers in Veterinary Science 6, 137.
Borca, M.V., Ramirez-Medina, E., Silva, E., Vuono, E., Rai, A., Pruitt, S., Holinka, L.G., Velazquez-Salinas, L., Zhu, J., Gladue, D.P., 2020. Development of a Highly Effective African Swine Fever Virus Vaccine by Deletion of the I177L Gene Results in Sterile Immunity against the Current Epidemic Eurasia Strain. Journal of virology 94, e02017-02019.
Cackett, G., Matelska, D., Sýkora, M., Portugal, R., Malecki, M., Bähler, J., Dixon, L., Werner, F., Shisler, J.L., 2020. The African Swine Fever Virus Transcriptome. Journal of Virology 94, e00119-00120.
Denstedt, E., Porco, A., Hwang, J., Nga, N.T.T., Ngoc, P.T.B., Chea, S., Khammavong, K., Milavong, P., Sours, S., Osbjer, K., Tum, S., Douangngeun, B., Theppanya, W., Van Long, N., Thanh Phuong, N., Tin Vinh Quang, L., Van Hung, V., Hoa, N.T., Le Anh, D., Fine, A., Pruvot, M., 2021. Detection of African swine fever virus in free-ranging wild boar in Southeast Asia. Transboundary and Emerging Diseases 68, 2669-2675.
Galindo, I., Alonso, C., 2017. African Swine Fever Virus: A Review. Viruses 9, 103.
Gallardo, C., Soler, A., Nurmoja, I., Cano-Gómez, C., Cvetkova, S., Frant, M., Woźniakowski, G., Simón, A., Pérez, C., Nieto, R., Arias, M., 2021. Dynamics of African swine fever virus (ASFV) infection in domestic pigs infected with virulent, moderate virulent and attenuated genotype II ASFV European isolates. Transboundary and Emerging Diseases 68, 2826-2841.
Gao, X., Liu, T., Liu, Y., Xiao, J., Wang, H., 2021. Transmission of African swine fever in China Through Legal Trade of Live Pigs. Transboundary and Emerging Diseases 68, 355-360.
Ge, S., Liu, Y., Li, L., Wang, Q., Li, J., Ren, W., Liu, C., Bao, J., Wu, X., Wang, Z., 2019. An extra insertion of tandem repeat sequence in African swine fever virus, China, 2019. Virus Genes 55, 843-847.
Izzati, U.Z., Hoa, N.T., Lan, N.T., Diep, N.V., Fuke, N., Hirai, T., Yamaguchi, R., 2021a. Pathology of the outbreak of subgenotype 2.5 classical swine fever virus in northern Vietnam. Veterinary Medicine and Science 7, 164-174.
Izzati, U.Z., Inanaga, M., Hoa, N.T., Nueangphuet, P., Myint, O., Truong, Q.L., Lan, N.T., Norimine, J., Hirai, T., Yamaguchi, R., 2021b. Pathological investigation and viral antigen distribution of emerging African swine fever in Vietnam. Transboundary and Emerging Diseases 68, 2039-2050.
Jaing, C., Rowland, R.R.R., Allen, J.E., Certoma, A., Thissen, J.B., Bingham, J., Rowe, B., White, J.R., Wynne, J.W., Johnson, D., Gaudreault, N.N., Williams, D.T., 2017. Gene expression analysis of whole blood RNA from pigs infected with low and high pathogenic African swine fever viruses. Scientific Reports 7, 10115.
Ju, X., Li, F., Li, J., Wu, C., Xiang, G., Zhao, X., Nan, Y., Zhao, D., Ding, Q., 2021. Genome-wide transcriptomic analysis of highly virulent African swine fever virus infection reveals complex and unique virus host interaction. Veterinary Microbiology 261, 109211.
Karger, A., Pérez-Núñez, D., Urquiza, J., Hinojar, P., Alonso, C., Freitas, F.B., Revilla, Y., Le Potier, M.-F., Montoya, M., 2019. An Update on African Swine Fever Virology. Viruses 11, 864.
Khanal, P., Olcha, M., Niederwerder, M.C., 2021. Detection of African swine fever virus in feed dust collected from experimentally inoculated complete feed using quantitative PCR and virus titration assays. Transboundary and Emerging Diseases n/a.
Le, V.P., Jeong, D.G., Yoon, S.-W., Kwon, H.-M., Trinh, T.B.N., Nguyen, T.L., Bui, T.T.N., Oh, J., Kim, J.B., Cheong, K.M., Van Tuyen, N., Bae, E., Vu, T.T.H., Yeom, M., Na, W., Song, D., 2019. Outbreak of African Swine Fever, Vietnam, 2019. Emerg Infect Dis 25, 1433-1435.
Lee, H.S., Bui, V.N., Dao, D.T., Bui, N.A., Le, T.D., Kieu, M.A., Nguyen, Q.H., Tran, L.H., Roh, J.-H., So, K.-M., Hur, T.-Y., Oh, S.-I., 2021a. Pathogenicity of an African swine fever virus strain isolated in Vietnam and alternative diagnostic specimens for early detection of viral infection. Porcine Health Management 7, 36.
Lee, H.S., Thakur, K.K., Bui, V.N., Pham, T.L., Bui, A.N., Dao, T.D., Thanh, V.T., Wieland, B., 2021b. A stochastic simulation model of African swine fever transmission in domestic pig farms in the Red River Delta region in Vietnam. Transboundary and Emerging Diseases 68, 1384-1391.
Lee, H.S., Thakur, K.K., Pham-Thanh, L., Dao, T.D., Bui, A.N., Bui, V.N., Quang, H.N., 2021c. A stochastic network-based model to simulate farm-level transmission of African swine fever virus in Vietnam. Plos one 16, e0247770.
Li, D., Wu, P., Liu, H., Feng, T., Yang, W., Ru, Y., Li, P., Qi, X., Shi, Z., Zheng, H., 2021. A QP509L/QP383R-deleted African swine fever virus is highly attenuated in swine but does not confer protection against parental virus challenge. Journal of Virology 0, JVI.01500-01521.
Mai, N.T.A., Vu, X.D., Nguyen, T.T.H., Nguyen, V.T., Trinh, T.B.N., Kim, Y.J., Kim, H.-J., Cho, K.-H., Nguyen, T.L., Bui, T.T.N., Jeong, D.G., Yoon, S.-W., Truong, T., Ambagala, A., Song, D., Le, V.P., 2021. Molecular profile of African swine fever virus (ASFV) circulating in Vietnam during 2019-2020 outbreaks. Archives of Virology 166, 885-890.
Mason-D’Croz, D., Bogard, J.R., Herrero, M., Robinson, S., Sulser, T.B., Wiebe, K., Willenbockel, D., Godfray, H.C.J., 2020. Modelling the global economic consequences of a major African swine fever outbreak in China. Nature Food 1, 221-228.
Matsumoto, N., Siengsanan-Lamont, J., Gleeson, L.J., Douangngeun, B., Theppangna, W., Khounsy, S., Phommachanh, P., Halasa, T., Bush, R.D., Blacksell, S.D., 2020. Evaluation of the diagnostic accuracy of an affordable rapid diagnostic test for African Swine Fever antigen detection in Lao People’s Democratic Republic. Journal of Virological Methods 286, 113975.
Mighell, E., Ward, M.P., 2021. African Swine Fever spread across Asia, 2018–2019. Transboundary and Emerging Diseases 68, 2722-2732.
Niederwerder, M.C., Dee, S., Diel, D.G., Stoian, A.M.M., Constance, L.A., Olcha, M., Petrovan, V., Patterson, G., Cino-Ozuna, A.G., Rowland, R.R.R., 2021. Mitigating the risk of African swine fever virus in feed with anti-viral chemical additives. Transboundary and Emerging Diseases 68, 477-486.
Niederwerder, M.C., Stoian, A.M.M., Rowland, R.R.R., Dritz, S.S., Petrovan, V., Constance, L.A., Gebhardt, J.T., Olcha, M., Jones, C.K., Woodworth, J.C., Fang, Y., Liang, J., Hefley, T.J., 2019. Infectious Dose of African Swine Fever Virus When Consumed Naturally in Liquid or Feed. Emerg Infect Dis 25, 891-897.
Njau, E.P., Domelevo Entfellner, J.-B., Machuka, E.M., Bochere, E.N., Cleaveland, S., Shirima, G.M., Kusiluka, L.J., Upton, C., Bishop, R.P., Pelle, R., Okoth, E.A., 2021. The first genotype II African swine fever virus isolated in Africa provides insight into the current Eurasian pandemic. Scientific Reports 11, 13081.
Nga, B.T.T., Tran Anh Dao, B., Nguyen Thi, L., Osaki, M., Kawashima, K., Song, D., Salguero, F.J., Le, V.P., 2020a. Clinical and Pathological Study of the First Outbreak Cases of African Swine Fever in Vietnam, 2019. Frontiers in Veterinary Science 7.
Nga, B.T.T., Tran Anh Dao, B., Nguyen Thi, L., Osaki, M., Kawashima, K., Song, D., Salguero, F.J., Le, V.P., 2020b. Clinical and Pathological Study of the First Outbreak Cases of African Swine Fever in Vietnam, 2019. Frontiers in veterinary science 7, 392-392.
O’Neill, X., White, A., Ruiz-Fons, F., Gortázar, C., 2020. Modelling the transmission and persistence of African swine fever in wild boar in contrasting European scenarios. Scientific Reports 10, 5895.
Oh, T., Do, D.T., Lai, D.C., Nguyen, T.C., Vo, H.V., Chae, C., 2021a. Age-related viral load and severity of systemic pathological lesions in acute naturally occurring African swine fever virus genotype II infections. Comparative Immunology, Microbiology and Infectious Diseases 79, 101709.
Oh, T., Nguyen, T.M., Ngo, T.T.N., Thinh, D., Nguyen, T.T.P., Do, L.D., Do, D.T., 2021b. Long-term follow-up of convalescent pigs and their offspring after an outbreak of acute African swine fever in Vietnam. Transboundary and Emerging Diseases n/a.
Pornthummawat, A., Truong, Q.L., Hoa, N.T., Lan, N.T., Izzati, U.Z., Suwanruengsri, M., Nueangphuet, P., Hirai, T., Yamaguchi, R., 2021. Pathological lesions and presence of viral antigens in four surviving pigs in African swine fever outbreak farms in Vietnam. Journal of Veterinary Medical Science advpub.
Pham, L.M., Parlavantzas, N., Le, H.-H., Bui, Q.H., 2021. Towards a Framework for High-Performance Simulation of Livestock Disease Outbreak: A Case Study of Spread of African Swine Fever in Vietnam. Animals 11, 2743.
Que, N.N., Linh, P.T.N., Thang, T.C., Thuy, N.T., Thinh, N.T., Rich, K.M., Nguyen-Viet, H., 2020. Economic impacts of African swine fever in Vietnam.
Salguero, F.J., 2020. Comparative Pathology and Pathogenesis of African Swine Fever Infection in Swine. Frontiers in Veterinary Science 7, 282.
Sun, E., Zhang, Z., Wang, Z., He, X., Zhang, X., Wang, L., Wang, W., Huang, L., Xi, F., Huangfu, H., Tsegay, G., Huo, H., Sun, J., Tian, Z., Xia, W., Yu, X., Li, F., Liu, R., Guan, Y., Zhao, D., Bu, Z., 2021. Emergence and prevalence of naturally occurring lower virulent African swine fever viruses in domestic pigs in China in 2020. Science China Life Sciences 64, 752-765.
Szeredi, L., Bakcsa, E., Zádori, Z., Mészáros, I., Olasz, F., Bálint, Á., Locsmándi, G., Erdélyi, K., 2020. Detection of African swine fever virus in cell culture and wild boar tissues using a commercially available monoclonal antibody. Journal of Virological Methods 282, 113886.
Tuan, H.A., 2021. Identification of risk factors for African swine fever (ASF) along pig value chain in Lào Cai Province.
Thi Ngoc Dung, T., Nang Nam, V., Thi Nhan, T., Ngoc, T.T.B., Minh, L.Q., Nga, B.T.T., Phan Le, V., Viet Quang, D., 2020. Silver nanoparticles as potential antiviral agents against African swine fever virus. Materials Research Express 6, 1250g1259.
Tran, D.H., Tran, H.T., Le, U.P., Vu, X.D., Trinh, T.B.N., Do, H.D.K., Than, V.T., Bui, L.M., Vu, V.V., Nguyen, T.L., Phung, H.T.T., Le, V.P., 2021a. Direct colorimetric LAMP assay for rapid detection of African swine fever virus: A validation study during an outbreak in Vietnam. Transboundary and Emerging Diseases 68, 2595-2602.
Tran, H.T.T., Dang, A.K., Ly, D.V., Vu, H.T., Hoang, T.V., Nguyen, C.T., Chu, N.T., Nguyen, V.T., Nguyen, H.T., Truong, A.D., Pham, N.T., Dang, H.V., 2020a. 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. Asian-Australas J Anim Sci 33, 1683-1690.
Tran, H.T.T., Truong, A.D., Dang, A.K., Ly, D.V., Nguyen, C.T., Chu, N.T., Hoang, T.V., Nguyen, H.T., Dang, H.V., 2021b. Circulation of two different variants of intergenic region (IGR) located between the I73R and I329L genes of African swine fever virus strains in Vietnam. Transboundary and Emerging Diseases 68, 2693-2695.
Tran, H.T.T., Truong, A.D., Dang, A.K., Ly, D.V., Nguyen, C.T., Chu, N.T., Nguyen, H.T., Dang, H.V., 2021c. Genetic characterization of African swine fever viruses circulating in North Central region of Vietnam. Transboundary and Emerging Diseases 68, 1697-1699.
Tran, H.T.T., Truong, A.D., Ly, D.V., Vu, T.H., Hoang, V.T., Nguyen, T.C., Chu, T.N., Nguyen, T.H., Pham, N.T., Nguyen, T., Yersin, A.G., Dang, H.V., 2020b. 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. J Vet Res 64, 207-213.
Tran, X.H., Le, T.T.P., Nguyen, Q.H., Do, T.T., Nguyen, V.D., Gay, C.G., Borca, M.V., Gladue, D.P., 2021d. African swine fever virus vaccine candidate ASFV-G-ΔI177L efficiently protects European and native pig breeds against circulating Vietnamese field strain. Transboundary and Emerging Diseases n/a.
Vlasov, M., Imatdinov, A., Titov, I., Vasković, N., Lyska, V., Sevskikh, T., Sybgatullova, A., Pivova, E., Morgunov, S., Balyshev, V., 2020. Characteristics of African Swine Fever Virus Isolated from Domestic Pigs and Wild Boars in the Russian Federation and South Ossetia. Acta Veterinaria-Beograd 70, 58-70.
Wang, A., Jia, R., Liu, Y., Zhou, J., Qi, Y., Chen, Y., Liu, D., Zhao, J., Shi, H., Zhang, J., Zhang, G., 2020. Development of a novel quantitative real-time PCR assay with lyophilized powder reagent to detect African swine fever virus in blood samples of domestic pigs in China. Transboundary and Emerging Diseases 67, 284-297.
Wang, S., Zhang, J., Zhang, Y., Yang, J., Wang, L., Qi, Y., Han, X., Zhou, X., Miao, F., Chen, T., Wang, Y., Zhang, F., Zhang, S., Hu, R., 2021. Cytokine Storm in Domestic Pigs Induced by Infection of Virulent African Swine Fever Virus. Frontiers in Veterinary Science 7, 601641.
Wu, K., Liu, J., Wang, L., Fan, S., Li, Z., Li, Y., Yi, L., Ding, H., Zhao, M., Chen, J., 2020. Current State of Global African Swine Fever Vaccine Development under the Prevalence and Transmission of ASF in China. Vaccines 8, 531.
Yang, J., Tang, K., Cao, Z., Pfeiffer, D.U., Zhao, K., Zhang, Q., Zeng, D.D., 2021. Demand-driven spreading patterns of African swine fever in China. Chaos: An Interdisciplinary Journal of Nonlinear Science 31, 061102.
You, S., Liu, T., Zhang, M., Zhao, X., Dong, Y., Wu, B., Wang, Y., Li, J., Wei, X., Shi, B., 2021. African swine fever outbreaks in China led to gross domestic product and economic losses. Nature Food 2, 802-808.
