SHIC Monitors Recombinant ASFV Genotype I/II Strain Emergence Globally

The US swine industry is vulnerable to the introduction of emerging pathogens and routine swine disease monitoring provides early warnings of global challenges that could negatively impact US swine producers. As part of its mission to identify emerging disease threats, SHIC funds Global Swine Disease Monitoring Reports led by Dr. Maria Sol Perez Aguirreburualde at the University of Minnesota. Recent reporting on African swine fever virus strain circulation has highlighted a concerning expansion of the ASFV Recombinant Genotype I/II strains being detected in Asia and now the Russian Far East. Evolving changes in globally circulating ASFV genotypes continue to pose risks for US introduction and inform ongoing prevention and preparedness activities to protect the health of US swine.

Background

Since first being reported in August 2018 on a pig farm near Shenyang, China, 19 additional countries have reported ASF outbreaks in the Asia and Pacific region as of October 2024. The first ASFV genotype occurring in Asia was genotype II, characterized by highly pathogenic strains causing high mortality in domestic pigs. Genotype II is the predominant virus in Europe, Russia, Asia, and the Americas. Ongoing changes in ASFV circulating genotypes are being monitored by the Swine Health Information Center and reported via its Global Swine Disease Monitoring Reports.

Key events altering the ASF epidemiological landscape in Asia

Between 2019 and 2020, there were reports of low pathogenic strains of ASFV genotype II in China and other Asian countries. These low pathogenic genotype II strains were detected during various surveillance activities. Later, in 2021, ASFV genotype I was reported in China which further complicated the epidemiology of the disease in the country and Asia-Pacific region. Prior to this report, ASFV genotype I was only known to be present in Sardinia, Italy, and on the African continent. In 2021, China also reported the discovery of ASF viruses that were a mix between the two genotypes (recombinant viruses).

The recombinant viruses in China were a mix of genetic types, genotypes I and II. These new strains were found in pigs from Jiangsu, Henan, and Inner Mongolia. The viruses were identified as genotype I based on one gene (B646L). However, they showed an unexpected characteristic—they were “HAD-positive” and caused red blood cells to clump together, something not seen in previous genotype I strains in China. This trait is linked to a specific protein, CD2v, encoded by another gene (EP402R). When the researchers sequenced the EP402R gene, they found that it matched genotype II viruses, suggesting a mix between the two genotypes.

In 2023, researchers in Vietnam reported the detection of the recombinant ASFV genotype I and II viruses in domestic pig farms in Northern Vietnam. The recombinant viruses found in Vietnam matched the corresponding sequences of recombinant ASFV I/II strains from China, except for one genetic region, the Central Variable Region. Further molecular analysis of the recombinant strains from Vietnam indicates three possible independent introductions into the country.

ASFV recombinant virus strains reported in Russia

Also in 2023, an ASFV isolate obtained from a domestic pig in the Primorsky Region, a Russian region bordering China, was found to be a recombinant strain. The recombinant genotype I/II virus found in Primosky shares similarities with the recombinant strains from China and has a 99.9% sequence identity, although it does not share identical viral gene sequences. This means the Primosky isolate may be part of a wider transmission network and has undergone minor evolutionary changes as it spread from China. The Primosky recombinant strain presented acute disease in domestic pigs when infected experimentally, which suggests similar characteristics to the Recombinant strains from China.

Timeline of ASFV strain discovery in Asia and the Russian Far East

August 2018 – ASFV emergence in Asia (China) – ASFV genotype II (highly pathogenic strains)
January 2019 – ASFV continues in China and other Asian countries – ASFV genotype II (low pathogenic strains)
December 2020 – ASFV continues in Asia – ASFV genotype II (low pathogenic gene deleted strains)
March 2021 – New outbreak in China – ASFV genotype I (low pathogenic strains)
December 2021 – New outbreak in China – ASFV recombinant genotype I/II
2023 – New outbreaks in Vietnam and Russia – ASFV recombinant genotype I/II

Characteristics of ASFV recombinant strain

Genotype I/II strains from China: