RETROVIRIDAE

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.

Following the epoch-making discoveries by Ellerman and Bang in 1908 and Peyton Rous in 1911, which eventually led to the realization that viruses are involved in the aetiology of tumours in chickens, retroviruses have been associated mainly with the ability to transform normal cells to a cancerous state.1 Since then, a large number of retroviruses have been isolated and shown to cause leukaemias, lymphomas and sarcomas in cattle, sheep, chickens, mice, rats, cats and a variety of other animals. Only much later was it found that there are also non-oncogenic retroviruses, some apathogenic and others associated with so-called slow virus diseases.¹

In order to accommodate the different groups, the family Retroviridae is subdivided into two subfamilies, specifically Orthoretrovirinae and Spumaretrovirinae. The subfamily Orthoretrovirinae, which includes the major pathogens of animals, is further subdivide into seven genera as shown in (Table 1).²

All retroviruses have certain characteristics in common. The most important of these, from which the name retrovirus is derived, is the possession of a ‘reverse transcriptase’ or RNA-dependent DNA polymerase as part of the viral capsid. This enzyme, which is unique to the Retroviridae, enables the virus to make a DNA copy of its RNA genome. This DNA ‘provirus’ can integrate into the host-cell genome, giving rise to transcriptionally inactive (latent) infections that allow the virus to escape the host’s immune response. In the case of the oncogenic retroviruses the provirus can also act as a natural vector transmitting cellular oncogenes from one cell to another, which is one of the mechanisms of oncogenesis. The lentiviruses, although structurally very similar, have a completely different pathogenic effect. In all diseases associated with this group, including the human immunodeficiency virus (HIV), which causes the acquired immunodeficiency syndrome (AIDS), immunopathological changes are the primary lesions.

Table 1 Family Retroviridae, subfamily Orthoretrovirinae

Table 2 Diseases of livestock caused by members of the family Retroviridae, subfamily Orthoretrovirinae

* Livestock diseases caused by members of the subfamily Orthoretrovirinae are shown in Table 2.

References

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