ADENOVIRIDAE

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 Adenoviridae, so named because the first isolates were made from adenoidal tissues of humans, currently has 62 recognised species divided into five genera: Mastadenovirus (mast: from mastos, the Greek word for breast; these viruses only infect mammals), Aviadenovirus (viruses that infect only birds), Atadenovirus (viruses that infect a broad range of hosts including reptiles, birds and ruminants), Siadenovirus (birds, reptiles and amphibians) and Ichtadenovirus (fish)¹. Consequently the only genera that infect livestock as defined in this publication are Mastadenovirus and Atadenovirus.

The genome organisations of adenovirus genera is used as a basis for classification, i.e. genome arrangement is characteristic for each genus. Thus, natural host species and genome organisation as well as phylogenetic distance between viruses determines categorisation. Currently, each virus species is named according to its natural animal host, its genus and, where there is more than one, a serial number, e.g. Bovine mastadenovirus 3 or Bovine atadenovirus 4. However, the current system of classification, in particular the distribution of virus species between Mastadeonovirus and Atadenovirus genera and lettering accorded to individual viruses, is sometimes apparently illogical².

It is probable that all vertebrates have one or more adenoviruses with which they co-evolved so only a few cause disease in their natural hosts. Most adenoviruses are associated with the upper respiratory or digestive tracts of their hosts and, in some cases infection may be prolonged, apparently as part of a transmission strategy. Well-known exceptions in respect of pathogenicity in domestic animals are the viruses that cause infectious canine hepatitis and contribute to the kennel cough syndrome (i.e. subtaxa of Canine mastadenovirus A). These viruses were formerly referred to as Canine adenovirus 1 and 2.

Apart from Equine mastadenovirus A and B infection of Arabian foals suffering from severe combined immune deficiency, no clearly defined disease syndrome of livestock, other than poultry, has been shown unequivocally to be caused by an adenovirus. Adenoviruses are, however, common in children and people suffering from immune deficiencies, such as those resulting from chemotherapy following organ transplantation or due to human immunodeficiency virus infection.

Adenovirus virions are unenveloped icosahedrons (i.e. roughly spherical in shape with 20 surfaces) comprised of 252 capsomeres and are 70-90 nm in diameter. The capsomeres include 12 pentons at the vertices; these provide the base for the ‘fibres’ that have a terminal knob projecting above the nucleocapsid surface. A single molecule of double-stranded DNA that codes for about 40 proteins makes up the genome. Viral replication occurs in cell nuclei and cause intranuclear inclusions – sometimes in the form of crytaline arrays.

Adenoviruses are relatively resistant to environmental conditions but are readily inactivated by conventional disinfectants.

Adenoviruses have been used for many decades to research fundamental phenomena in the field of virology, i.e. for academic research. That, among other things, led to the discovery that adenoviruses are able to cause tumours in newborn rodents. However, this does not occur in natural hosts. More recently there has been research into use of adenoviruses to construct recombinant viruses for use as vaccines against other viral infection, e.g. foot and mouth disease.

Adeno-associated viruses (AAVs) are parvoviruses classified in the genus Dependovirus, dependent on co-infection of cells with an adenovirus in order to be able to replicate. These viruses have been identified humans, cattle, sheep, horses, dogs and birds. However, no AVV has so far been associated with disease.

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, 5^th edition, Academic Press.