Adenovirus infections

Adenovirus infections

G R THOMSON

Adenovirus infections of cattle

Introduction

Adenovirus infections of cattle probably occur universally, although most are subclinical. Bovine adenovirus infections have been associated with a variety of respiratory and alimentary tract diseases but their role in the causation of these diseases remains uncertain.

Aetiology

Adenovirus virions are 70 to 90 nm in diameter (i.e. a medium-sized virus), non-enveloped and have icosahedral symmetry. They have a buoyant density of 1,32 to 1,35 g/ml in CsCl. Each virion is made up of 252 capsomeres which are 8 to 9 nm in diameter. The capsomeres at the 12 vertices of the virion (also known as vertex capsomeres or penton bases) differ from the other 240 (known as hexons) in that each penton base carries one (Mastadenovirus) or two (Aviadenovirus) glycoprotein projections with terminal knobs, known as fibers, each of which is 10 to 50 nm in length (the length being characteristic for each serotype).40, 82, 99

The genome comprises a linear molecule of doublestranded DNA, to the 5’-end of which a viral-coded protein with a molecular weight (MW) of about 55 kDa is covalently linked. The genomes of viruses in the genus Mastadenovirus are smaller (MW = 20 − 25 × 106 ) than in Aviadenovirus (c. 30 × 106 ) and there are also differences in G+C content — 48 to 61 per cent for Mastadenovirus and 54 to 55 per cent for Aviadenovirus. Inverted terminal repetitions (50 to 200 base pairs) of the type abc–c’b’a’ 48 have been found in all adenoviruses so far sequenced.82, 99 The DNA is tightly bound to protein VII and is also associated with four other proteins. This complex (DNA-proteins) makes up the viral core.67

In addition to the 55 kDa protein linked to the 5’-end of the DNA, virions contain at least 10 polypeptides with MWs varying between 5 and 120 kDa. Each fibre polypeptide, the only glycosylated structural protein, contains two molecules of N-acetylglucosamine.

Hexons have been shown by x-ray crystallography to have a pseudohexagonal base and a triangular top and to consist of three molecules of polypeptide II, while penton bases are pentamers or trimers of polypeptide III.67

Adenoviruses attach to specific cell receptors by the fibres and are probably internalized by endocytosis. The viral core enters the cell nucleus where transcription, DNA replication and virus assembly occur. Host-cell DNA synthesis is shut off early during replication and RNA and protein synthesis at a late stage. DNA replication occurs by strand displacement, using viral-coded DNA polymerase and terminal protein priming.82 As adenoviruses are being assembled in the nucleus they form crystalline arrays which appear as basophilic or amphiphilic intranuclear inclusions when examined by light microscopy.34

The 10 recognized serotypes of bovine adenovirus45 are divided into two genera, Mastadenovirus and Atadenovirus, the former consisting of bovine adenovirus 1, 2, 3, 9 and 10, and the latter of bovine adenovirus 4, 5, 6, 7 and 8.

Epidemiology

Although the limited number and extent of serological studies so far undertaken have failed to give an accurate indication of the prevalence rates of infections with adenovirus serotypes in cattle in different geographic locations and in various sex and age classes, it is probably safe to presume that infection with adenoviruses is widespread in cattle populations.15, 17, 62, 88 For example, 76 and 59 per cent of calf sera collected at two Austrian abattoirs had virus neutralizing antibodies against selected serotypes.15, 73 In a group of 29 heifers monitored serologically in Idaho, USA, from birth to 24 months of age, there were 28 bos 3 and 33 bos 7 infections (representatives of subgroups 1 and 2 respectively), some of which occurred in the face of maternally derived antibody.88

It is presumed that most infections occur by close and direct respiratory transmission or by the faecal–oral route. The virus has been detected for up to eight days in the respiratory secretions and rectal swabs of experimentally infected calves.74 Transplacental transmission has also been shown,5 but how common this is remains uncertain.

Apart from evidence of infection in sheep and African buffaloes (Syncerus caffer), antibodies to bovine Adenovirus 3 and 7 have been detected in the sera of pronghorn antelope (Antilocapra americana) in Idaho.89 However, it is not clear whether this was cross-reacting antibody or antibody specific to these two serotypes.

Pathogenesis, clinical signs and pathology

Although most adenovirus isolations have been made from cattle that are apparently healthy,32, 53, 54, 91 a variety of conditions, most of which involve the respiratory or alimentary tract, have been associated with adenovirus infections (Table 75.1). The two most comprehensive studies on the association between adenoviruses and disease in cattle, both conducted in Britain, reached conflicting conclusions.

One, based on the isolation of viruses from nearly 1 600 diseased and healthy calves over a period of 43 months, found that adenoviruses played no significant part in the causation of respiratory illness.91 The other, based on serological monitoring of young bulls over a period of 18 months, demonstrated a statistically significant association between serological responses to adenovirus infections, especially those caused by subgroup 1 viruses, and the occurrence of...

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