CIRCOVIRIDAE AND ANELLOVIRIDAE

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 Circoviridae (circo = circular) comprises a group of small DNA virus pathogens of plants, birds and pigs which are unique in that they are amongst the smallest known autonomously replicating viruses.^1,2 Virions are non-enveloped with icosahedral symmetry and are 17 to 22 nm in diameter. The form of their viral DNA, as a single-stranded, circularized DNA molecule, is also distinct. The family contains two genera: The Gyrovirus genus is represented by chick anaemia virus which is the type virus for this group, and the Circovirus genus which contains both porcine circoviruses (PCV-1 and PCV-2) and psittacine beak and feather disease virus and other avian circoviruses. The avian members of the Circoviridae are well known viral pathogens. Similarities between animal circoviruses and plant geminiviruses and nanoviruses have been identified and these similarities have led to the suggestion that PCV-1 may represent the most ancient mammalian DNA replicon. Recent work has re-enforced this belief and indicates that PCV-1, and presumably also PCV-2, may have originated by recombination of a plant nanovirus with a vertebrate virus. In plants, circoviruses have long been associated with disease and the plant nanoviruses are an important group of viral pathogens in their respective host plant species.

Originally viruses in the family Anelloviridae were often grouped with circoviruses because of their morphological and genomic similarities with that family, however their genome replication strategies are distinct. These viruses infect a wide variety animals including humans, non-human primates, bats, tree shrews, pigs, wild boar, dogs, cats, horses and marine mammals. However, none have so far been unequivocally associated with specific disease; they are also difficult to grow in cell culture and therefore challenging to investigate. In pigs, torque teno sus viruses (there are currently four species listed in two genera) have been shown to have a worldwide distribution but, as with most other torque teno viruses of other species, their pathogenicity is a matter of speculation. There is nevertheless a growing body of evidence that torque teno sus viruses have an effect on the pathogenicity of PCV-2 infections.

References

1. International Committee for Taxonomy of Viruses https://talk.ictvonline.org/taxonomy/ (accession date: 26/05/2017)
2. MACLACHLAN N.J. & DUBOVI, E.J. (eds.), 2016. Veterinary Virology, 5th edition, Academic Press.