On October 21, 2025, a team from Chungnam National University, Jeonbuk National University, and the National Institute of Veterinary Research (NIVR) of Vietnam published their latest research in the prestigious journal *Virulence*.
The team isolated the rASFV I/II strain ASF/HD 231005 from Hai Duong Province, Vietnam, and found that this strain exhibits extremely high pathogenicity, rapid onset of disease, short incubation period, and high tissue invasiveness. This research serves as a wake-up call for the prevention and control of novel ASFV variants in Asia.
Research Highlights
* **First-ever identification of the unique genomic structure of recombinant type I/II African swine fever virus:** The B646L gene originates from type I, the E183L gene from type II, and the EP402R gene belongs to serogroup 8, confirming its cross-genotype recombination characteristics.
* **Rapid lethality:** All patients died on average within 5.5 days after challenge, 1.5 days shorter than the classic type II genotype.
• Extreme heat stress: Fever exceeding 40.5°C was observed on day 2.6, with peak viremia 10 times higher than normal.
• Vaccine off-target effect: The genotype II candidate vaccine was completely unable to neutralize the virus, indicating a gap in "cross-genotype protection".
This study is the first to systematically report the pathogenicity and pathological characteristics of a highly pathogenic recombinant African swine fever virus (rASFV I/II) discovered in Vietnam.
The study indicates that this novel recombinant virus combines characteristics of genotypes I and II, resulting in faster disease progression in infected pigs: an average incubation period of only 3 days, high fever appearing as early as 2.6 days post-infection, and all infected pigs dying within 5–7 days.
Pathological analysis showed that the virus replicated more effectively in various organs, particularly in the lungs and kidneys, where viral loads were significantly higher than those of previous genotype II strains. More notably, infected pigs exhibited pronounced jaundice, suggesting that the virus may cause severe hemolysis, which is relatively rare in previous African swine fever infections.
Introduction
African swine fever (ASF) still lacks a safe and effective vaccine and remains one of the most devastating diseases facing the global pig farming industry. In recent years, several Asian countries have successively discovered novel ASFV strains (rASFV I/II) with genotypes I and II recombinant, and their lethality (>90%), transmissibility, and vaccine susceptibility have attracted high attention.
Research Results
1. Viral Genome Characteristics:
The rASFV I/II strain (ASF/HD 231005) isolated from Hai Duong City, Vietnam, exhibits typical recombination characteristics in its genome. It is highly homologous to recombinant strains reported in China and Vietnam, but contains 39 unique variant sites.
Figure 1. Phylogenetic tree of African swine fever virus (ASFV) constructed based on the nucleotide sequences of (A) B646L (p72), (B) E183L (p54), and (C) EP402R (CD2v) genes.
Red boxes represent isolates from this study, yellow asterisks indicate previously reported recombinant strains (rASFV I/II); asterisks (*) mark type II genotype strains reported for the first time in various countries. Genotypes (G) are determined based on the B646L (p72) sequence, and serogroups (SG) are determined based on the phylogenetic relationship of EP402R (CD2v). The scale bar shows the number of nucleotide substitutions (phylogenetic distance) at each locus. The phylogenetic tree was constructed using the maximum likelihood method, and the bootstrap support was calculated after 1000 replicates.
2. Clinical characteristics:
This strain is characterized by a short incubation period, rapid onset, and extremely rapid disease progression. Persistent high fever (>40.5℃) appeared on the second day after infection. Clinical symptom scores increased significantly day by day from the first day, entering the severe stage on the fourth day and reaching the end of exhaustion on the sixth day. All infected pigs died within 5–7 days, with 60% dying on the fifth day. The average survival time was only 5.5 days, and the mortality rate was 100%, showing that it has extremely high lethality and rapid pathogenicity.
Figure 2. Survival rate over time (%) of animals inoculated with genotype I/II recombinant African swine fever virus (rASFV I/II) (red line) and the negative control group (blue line).
Figure 3. (A) Individual rectal temperatures of pigs in the rASFV I/II infection group and the negative control group. Red and blue dots represent the body temperature of each pig, and horizontal lines represent the average temperature of the corresponding groups. The red and blue short lines show the average temperature of the two groups at each time point. *p<0.05. (B) Clinical scores of pigs in the rASFV I/II infection group (red line) and the negative control group (blue line).
3. Pathological characteristics:
This recombinant African swine fever virus (rASFV I/II) infection can cause systemic, severe hemorrhagic pathological damage, the most prominent feature of which is early widespread vascular endothelial damage accompanied by hemolysis. Infected pigs all showed severe congestive splenomegaly and hemorrhagic enlargement of lymph nodes in multiple sites throughout the body (e.g., mesenteric lymph nodes up to 100%), and significant hemorrhage was also seen in the lungs (70%) and kidneys (40%). Ninety percent of infected individuals developed severe jaundice. The pathological mechanism was related to virus-induced acute hemolysis and hemoglobin metabolism disorders, while the hepatocyte structure remained relatively intact, suggesting that the jaundice was hemolytic rather than hepatic. Histological comparison showed that this strain caused extensive microthrombus formation and endothelial damage as early as day 3 post-infection (3/3), significantly earlier and more severe than genotype II strains (1/3), accompanied by early necrosis of the spleen and lymph nodes and lymphocyte depletion. Early renal tubular degeneration was also observed in the kidneys.
These pathological features indicate that rASFV I/II has stronger vascular invasiveness and a faster tissue-destructive ability, and the hemolytic jaundice it causes can serve as an important pathological marker distinguishing it from conventional circulating strains.
Table 1. Gross pathological lesions in pigs infected with genotype I and II recombinant African swine fever virus (rASFV I/II) (n=10)
Table 2. Comparison of histopathological lesions of spleen, gastrohepatic lymph nodes, lungs, liver, and kidneys in pigs experimentally infected with genotype II ASFV (n=3) and rASFV I/II strains (n=3) at the early stage of viral infection (3 dpi).
4. Viral excretion and transmission:
The virus could be detected in rope-based oral fluid samples 2 days after inoculation, earlier than the appearance of clinical symptoms, providing an effective means for early monitoring; environmental samples (feces, feed, drinking water) were all positive within 4 days, indicating an extremely high risk of transmission.
Figure 4. Mean viral copy number over time in pigs infected with genotype I/II recombinant African swine fever virus (rASFV)
(A) Individual samples: blood, oral swabs, nasal swabs, and rectal swabs.
(B) Population samples: cord-shaped oral fluid, mixed feces, feed, and water samples.
5. Tissue load:
Viral loads in the liver, spleen, lungs, and kidneys were all >10^7 copies/mg, significantly higher than historical data for genotype II (p<0.05).
Figure 5. Viral genome copy numbers collected from 11 organs and tissues (spleen, SLN (submandibular lymph nodes), GLN (gastric and hepatic lymph nodes), MLN (mesenteric lymph nodes), tonsils, lungs, liver, kidneys, heart, brain, and spinal cord) after necropsy of pigs infected with type I and II recombinant ASFV.
Summary
This study is the first to systematically reveal the high pathogenicity and rapid lethality of rASFV I/II recombinant virus in pigs, highlighting its seriousness as a new threat. In practice, it is necessary to strengthen early monitoring by relying on rapid testing of oral fluid rope sampling (virus can be detected 2 days after inoculation), closely monitoring specific lesions such as jaundice and severe pulmonary and renal hemorrhage, and simultaneously carrying out dynamic disinfection of the breeding environment and emergency isolation of infected individuals to block the transmission chain. The research results not only warn Asian countries to strengthen virus monitoring and mutation tracking, but also provide important basis for vaccine development and adjustment of prevention and control strategies.
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