ASFARVIRIDAE

The family Asfarviridae contains only one genus, Asfivirus, with African swine fever virus (ASFV) currently being the only species within the genus. ‘Asfarviridae’ is an acronym for ‘African swine fever and related viruses’. African swine fever virus has features in common with other large ‘nucleocytoplasmic’ DNA viruses that have been proposed as a ‘superfamily’, the other members being Poxviridae, Iridoviridae, Phycodnaviridae and Mimiviridae.¹  However, these families are currently unassigned to a viral order by the International Committee on the Taxonomy of Viruses although the Order Megavirales has been proposed.⁵ It is interesting that genomic sequences related to ASFV have been identified in sewage and human serum in Asia, ponds in the Mississippi Delta and in oceans during metagenomic sequencing projects.^{2, 4, 6, 8}

African swine fever virions are large (175 - 215 nm in diameter) and enveloped. The genome consists of a single molecule of linear double-stranded DNA that codes for between 150 and 170 proteins.³ Replication occurs primarily in the cytoplasm of the host cell, preceded by an earlier phase of nuclear replication necessary for DNA synthesis.⁷

It is probable that ASFV evolved on the African continent in association with wild suids and argasid ticks of the genus Ornithodoros in a true arboviral relationship; ASFV is the only DNA virus so far shown to be capable of behaving in this way. Like a number of virus infections of ungulates native to Africa, ASFV is essentially non-pathogenic for its natural hosts but highly pathogenic for closely related mammalian species, in this case domestic pigs and Eurasian wild boars. The pathogenicity of this virus for pigs was revealed when European settlers in eastern and southern Africa attempted to farm with domestic pigs, which develop severe disease with high mortality, and unlike the African wild suids are able to transmit the infection efficiently, including through direct and indirect contact, and thereby facilitate rapid dissemination. In terms of intercontinental spread over the last 60 years, few other animal pathogens have spread as widely and with greater effect than ASFV. This has been largely due to the ability of ASFV to survive in uncooked meat products derived from infected pigs, while the lack of an effective vaccine has facilitated its spread and endemic establishment after introduction to new destinations.

References

1. ALONSO, C., BORCA, M., DIXON, L., REVILLA, Y., RODRÍGUEZ, F., ESCRIBANO, J.M. & ICTV REPORT CONSORTIUM, 2018. ICTV Virus Taxonomy Profile. Asfarviridae. Journal of General Virology, 99, 613-614.
2. DIXON, L.K., ALONSO, C., ESCRIBANO, J.M., MARTINS, C., REVILLA, Y., SALAS, M.L. & TAKAMATSU, H., 2011. Asfarviridae. In: KING, A.M.Q., ADAMS, M.J., CARSTENS, E.B. & LEFKOWITZ, E.J. (eds). Virus Taxonomy. Classification and Nomenclature of Viruses. Ninth Report of the International Committee on Taxonomy of Viruses, pp. 154-163. Amsterdam: Elsevier/Academic Press.
3. GALINDO, I. & ALONSO, C., 2017. African swine fever virus: A review. Viruses, 9: 103.
4. LOH, J., ZHAO, G., PRESTI, R.M., HOLTZ, L.R., FINKBEINER, S.R., DROIT, L., VILLASANA, Z., TODD, C., PIPAS, J.M., CALGUA, B., GIRONES, R., WANG, D. & VIRGIN, H.W., 2009. Detection of novel sequences related to African swine fever virus in human serum and sewage. Journal of Virology, 83(24), 13019-13025.  
5. MACLACHLAN N.J. & DUBOVI, E.J. (eds.), 2016. Fenner’s Veterinary Virology (5th edn), Cambridge, Massachusetts: Academic Press.
6. OGATA, H., TOYODA, K., TOMARU, Y., NAKAYAMA, N., SHIRAI, Y., CLAVERIE, J.-M. & NAGASAKI, K., 2009. Remarkable sequence similarity between the dinoflagellate-infecting marine girus and the terrestrial pathogen African swine fever virus. Virology Journal, 6: 178.
7. ROJO, G., GARCIA-BEATO, R., VIÑUELA, E., SALAS, M.L. & SALAS, J., 1999. Replication of African swine fever virus DNA in infected cells. Virology, 257, 524-536.
8. WAN, X.F., BARNETT, J.L., CUNNINGHAM, F., CHEN, S., YANG, G., NASH, S., LONG, L.P., FORD, L., BLACKMON, S., ZHANG, Y., HANSON, L. & HE, Q., 2013. Detection of African swine fever virus-like sequences in ponds in the Mississippi Delta through metagenomics sequencing. Virus Genes, 46(3), 441-446.