GENERAL INTRODUCTION: ASFARVIRIDAE

ASFARVIRIDAE

A General Introduction has been added to each disease chapter in an attempt to give a brief updated overview of the taxonomic, biological and other characteristics of the virus family or group of bacteria /protozoa that cause disease in livestock and, where relevant, involve wildlife. As the text of the three-volume book Infectious Diseases of Livestock is currently under revision the Editors are aware that there are inconsistencies between the updated introductions to chapters and the content of the chapters themselves. Once the chapters have been updated – a process that is currently underway – these inconsistencies will be removed.

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, Ascoviridae, Phycodnaviridae and Mimiviridae (Dixon et al.,2011).  However, these 6 families are currently unassigned to a viral order by the International Committee on the Taxonomy of Viruses although the Order Megavirales has been proposed (MacLachlan & Dubovi, 2016). 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 (Dixon et al., 2011).

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 about 50 proteins. Replication occurs in the cytoplasm of the host cell, although the nucleus is needed for viral DNA synthesis.

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 contributed to by 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. DIXON, L.K., ALONSO, C., ESCRIBANO, J.M., MARTINS, C., REVILLA, Y., SALAS, M.L. & TAKAMATSU, H., 2011. Asfarviridae, in A.M.Q. King, M.J. Adams, E.B. Carstens & E.J. Lefkowitz (eds), Virus Taxonomy. Classification and Nomenclature of Viruses. Ninth Report of the International Committee on Taxonomy of Viruses, pp. 154-163. Elsevier/Academic Press, Amsterdam.
  2. MACLACHLAN N.J. & DUBOVI, E.J. (eds.), 2016. Veterinary Virology, 5th edition, Academic Press.