Understanding African Swine Fever Virus: How Its Structure and Replication Shape Disease Progression

African swine fever virus (ASFV) is unlike many other viral pathogens encountered in veterinary practice. Its large genome, complex multilayered structure, and efficient replication strategy contribute to its ability to establish severe infections in domestic pigs and wild boars. These characteristics also explain why the virus is remarkably resilient and why controlling outbreaks remains a significant challenge. 

For practicing veterinarians, understanding how ASFV enters host cells, replicates, and produces new viral particles provides valuable insight into disease progression and reinforces the importance of rapid diagnosis and strict biosecurity during outbreaks. 

A Structurally Complex Virus 

ASFV is a large, enveloped, double-stranded DNA virus and the only member of the Asfarviridae family. It is also the only known DNA virus transmitted by arthropod vectors^1,2. The virus possesses a multilayered icosahedral structure measuring approximately 200 nm in diameter, although purified viral particles measuring 260–300 nm have also been identified through cryo-electron microscopy^2,3. 

Its genome ranges from 170 to 194 kb and contains more than 150 open reading frames, encoding numerous structural proteins and enzymes involved in DNA replication, transcription, and protein modification. Despite extensive characterization, the functions of nearly half of these viral genes remain unknown². 

The mature intracellular virus consists of four distinct layers⁴: 

• A central nucleoid containing the viral genome 

• A protein core shell surrounding the nucleoid 

• An inner lipid envelope 

• An outer icosahedral protein capsid 

As viral particles leave infected cells by budding through the plasma membrane, they acquire an additional outer envelope while remaining infectious^1,5. 

Viral Proteins That Support Infection 

ASFV contains approximately 50 different proteins, each contributing to viral survival, replication, or assembly². 

The nucleoid houses the viral genome along with proteins involved in transcription, including viral RNA polymerases and other enzymes required for early gene expression⁴. Surrounding this is the protein core shell, composed primarily of polyproteins such as pp220 and pp62, which play essential roles in genome packaging and viral core assembly². 

The inner lipid envelope contains membrane-associated proteins including p54, p17, H248R, and p12, several of which participate in capsid assembly and viral attachment². The outer capsid is dominated by the major capsid protein p72, accompanied by several minor structural proteins that stabilize the viral particle³. Proper folding of p72 depends on the viral chaperone B602L, highlighting the coordinated process required for successful virus formation¹. 

How ASFV Enters Host Cells 

ASFV primarily infects macrophages, although many early observations of viral replication were generated using Vero cells⁶. 

The virus gains entry through clathrin-mediated endocytosis and macropinocytosis, both of which are dependent on cellular processes that transport viral particles into host cells. During endocytosis, the acidic environment within late endosomes disrupts the viral outer membrane, allowing the viral core to move along the microtubule network toward sites of replication⁶. 

Several host cell molecules, including CD163, CD45, and MHC II, have been proposed to participate in viral entry, although no single receptor has been confirmed to be essential. Notably, pigs lacking CD163 remain susceptible to infection, indicating that viral entry likely involves multiple cellular factors⁷. 

Cell membrane cholesterol also plays an important role during infection, emphasizing the complexity of virus-host interactions during the earliest stages of disease². 

The Replication Cycle Inside the Cell 

Once inside the host cell, ASFV rapidly initiates its own replication machinery without depending on cellular enzymes. Viral DNA replication begins within the nucleus before larger replication intermediates are transferred to the cytoplasm, where viral maturation occurs². 

Replication takes place in specialized regions adjacent to the nucleus known as viral factories. These areas become centers for viral assembly, surrounded by endoplasmic reticulum membranes and largely devoid of normal cellular organelles². 

Within these viral factories, membrane precursors derived from the endoplasmic reticulum gradually develop into polyhedral viral particles. Structural proteins assemble around these membranes, followed by the formation of the viral core and nucleoid. Proteins such as pp220, pp62, p72, p49, and XP124L coordinate this highly organized assembly process². 

Following maturation, newly formed viral particles are transported along intracellular microtubules to the cell surface, where they exit by budding through the plasma membrane and acquire their external envelope^2,4. 

Conclusion 

African swine fever virus combines structural complexity with an efficient replication strategy that enables rapid multiplication within host cells. Its multilayered architecture, extensive collection of structural proteins, and specialized replication process contribute directly to its persistence and pathogenicity. For practicing veterinarians, understanding these biological characteristics provides valuable context for disease recognition and highlights why preventing transmission remains a cornerstone of African swine fever control. 

References 

1. Alonso C, Borca M, Dixon L, Revilla Y, Rodriguez F, Escribano JM, ICTV Report Consortium. ICTV virus taxonomy profile: Asfarviridae. Journal of General Virology. 2018 May;99(5):613-4. https://www.microbiologyresearch.org/content/journal/jgv/10.1099/jgv.0.001049?crawler=true&mimetype=application/pdf 

2. Li Z, Chen W, Qiu Z, Li Y, Fan J, Wu K, Li X, Zhao M, Ding H, Fan S, Chen J. African swine fever virus: a review. Life. 2022 Aug 17;12(8):1255. https://www.mdpi.com/2075-1729/12/8/1255 

3. Wang N, Zhao D, Wang J, Zhang Y, Wang M, Gao Y, Li F, Wang J, Bu Z, Rao Z, Wang X. Architecture of African swine fever virus and implications for viral assembly. Science. 2019 Nov 1;366(6465):640-4. https://www.science.org/doi/pdf/10.1126/science.aaz1439 

4. Salas ML, Andrés G. African swine fever virus morphogenesis. Virus research. 2013 Apr 1;173(1):29-41. https://www.asf-referencelab.info/asf/images/ficherosasf/MORFOGENESIS.pdf 

5. Galindo I, Alonso C. African swine fever virus: a review. Viruses. 2017 May 10;9(5):103. https://www.mdpi.com/1999-4915/9/5/103 

6. Alonso C, Galindo I, Cuesta-Geijo MA, Cabezas M, Hernaez B, Munoz-Moreno R. African swine fever virus-cell interactions: from virus entry to cell survival. Virus research. 2013 Apr 1;173(1):42-57. https://pmc.ncbi.nlm.nih.gov/articles/PMC7114420/pdf/main.pdf 

7. Lithgow P, Takamatsu H, Werling D, Dixon L, Chapman D. Correlation of cell surface marker expression with African swine fever virus infection. Veterinary microbiology. 2014 Jan 31;168(2-4):413-9. https://www.sciencedirect.com/science/article/pii/S0378113513005580