African swine fever is a highly contagious pig disease caused by the African swine fever virus, posing a huge threat to the national pig industry. Genotype II ASF entered the country in 2018, and subsequently there were reports of genotype I ASF being detected in the country.
Genomic Analysis of African Swine Fever Virus Type 1
Genomic Analysis
In June 2021, fattening pigs on two pig farms in Shandong and Henan provinces developed chronic infection symptoms, including weight loss, intermittent fever, skin ulcers, and arthritis. Sporadic deaths were also observed. Samples, including lymph nodes and spleens, were collected from four dead pigs and sent to CNASFPL for ASFV testing.
qPCR testing targeting the viral p72 gene confirmed that all samples were ASFV-positive. Further sequence analysis of the p72 gene revealed that the ASFV strains in these samples belonged to type 1. Subsequently, two ASFV genotypes were isolated from samples from pig farms in Shandong and Henan provinces, designated Pig/Shandong/DY-I/2021 (SD/DY-I/21) and Pig/Henan/ZZ-P1/2021 (HeN/ZZ-P1/21).
Figure 1 Phylogenetic analysis of SD/DY-I/21 and HeN/ZZ-P1/21 and comparison with early genotype I isolates from Europe and Africa
Whole-genotype sequencing of the Chinese genotype I ASFV strains SD/DY-I/21 and HeN/ZZ-P1/21 revealed that they belong to the same clade as the Portuguese genotype I strains NH/P68 and OURT88/3, but differ significantly from previous genotype II strains from China. Both strains harbor 10 ORF deletions (e.g., in the MGF_360/505 family genes) and 5 ORF truncations (e.g., EP402R in the CD2v protein gene), potentially associated with reduced virulence. Compared with NH/P68, SD/DY-I/21 and HeN/ZZ-P1/21 each harbor 18 single nucleotide mutations (12-13 amino acid changes) and large deletions (e.g., a 686-bp deletion in MGF_110-4/5L in the HeN strain). The CVR spectra of the B602L genes of the two strains were unique, and there were significant genomic differences between them (such as 23 ORF mutations and 700 bp deletions), suggesting that they were independent introduction events and that strengthened monitoring was needed for tracing the source and prevention and control.
Clinical symptoms of the SD/DY-I/21 strain type 1
Clinical symptoms
#01 Animal experiments
Each pig was monitored daily for 28 days for changes in body temperature and symptoms including anorexia, depression, fever, purplish skin, staggering gait, diarrhea, and coughing.
#02 Body temperature changes
The SD/DY-I/21 strain was inoculated intramuscularly, and body temperatures were measured daily. From days 3 to 18, all challenged pigs exhibited intermittent fevers of varying degrees. In the low-dose group, only one cohabiting pig developed fever by day 26, while in the high-dose group, both cohabiting pigs developed fever by day 23. One pig in group 10 died by day 16 after intramuscular injection.
Infection with the genotype I ASFV isolate SD/DY-I/21 resulted in intermittent fever in all inoculated groups (from days 3 to 18 post-infection). The high-dose group (10⁶ TCID₅₀) exhibited more pronounced symptoms (including generalized papular rashes, arthritis, and skin necrosis), but all pigs survived the 28-day observation period. One pig in the low-dose group (10³ TCID₅₀) died from severe visceral lesions, while the rest survived. Viral DNA was persistently detected in oral swabs (as early as day 5), rectal swabs (day 7), and blood (day 7) for over 28 days, indicating prolonged viral replication and shedding.
#03 Symptoms
Papules (A, B), skin necrosis (B, C), and hind leg joints (B, D) in surviving pigs. One pig (no. 3) in group 10 became ill and died on day 16 post-infection. Necropsy revealed nasal bleeding and congestion and swelling of the spleen, liver, and lymph nodes.
Genotype I ASFV isolate SD/DY-I/21 is transmissible through contact, with infected pigs displaying chronic symptoms (fever, arthritis, and skin lesions). Viral DNA is detected early and persistently in oral swabs (5-9 days after infection), with later shedding in rectal swabs and blood. High viral loads are observed in tissues of infected pigs (spleen, lung, and bone marrow). Following contact transmission, the virus is present at low levels in tissues such as the lungs and tonsils. However, all exposed pigs survived, suggesting that transmission efficiency is limited but that persistent infection is possible.
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Replication and Detoxification of SD/DY-I/21 in Pigs
Replication and Detoxification
Oral and rectal swabs and blood samples were collected from pigs every other day and tested for viral DNA by qPCR.
Figure 3: Toxins in the mouth, feces, and blood
The mouth is still the first to be detoxified, but the CT values detected are all below 30.
Figure 4 Virus Carrier Status and Antibody Conversion
A, from the horizontal axis, represents heart, liver, spleen, lung, kidney, tonsil, thorax, adrenal gland, bone marrow, joint fluid, LN1 intestinal lymph nodes, LN2 inguinal lymph nodes, LN3 subcollar lymph nodes, LN4 bronchial lymph nodes, LN5 gastrohepatic lymph nodes, and LN6 mediastinal lymph nodes.
After infection with genotype I ASFV strain SD/DY-I/21, both vaccinated and contact pigs produced IgG antibodies against the p72 protein (detected on day 7 in the high-dose group and day 9 in the low-dose group). Antibody levels increased over time. Contact pigs seroconverted 21-27 days after infection, confirming that the virus can be transmitted through contact. The virus replicates and excretes in the pig's body for a long time. Viral DNA is continuously detected in oral and rectal swabs and blood for more than 28 days, and the viral load in infected pig tissues (spleen, lungs, lymph nodes, etc.) is high, indicating that the virus continues to replicate and shed during chronic infection, increasing the risk of transmission.
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Summary
Summarize
African swine fever genotype 1 strains have a high mortality rate, a slow course, diverse and insidious clinical manifestations, can cause chronic infection, and are highly transmissible.
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