Adenoviridae

Adenoviridae

Adenoviruses (from the Greek word adenos meaning gland) were first isolated from explant cultures of human adenoids. They are non-enveloped, icosahedral viruses, 70–90 nm in diameter, containing a single linear molecule of double-stranded DNA (Fig. 48.1). One to two protein fibres protrude from each of the 12 vertices of the capsid. The family Adenoviridae contains five genera designated Mastadenovirus, Aviadenovirus, Atadenovirus, Siadenovirus and a genus of fish adenoviruses, Ichtadenovirus. Mammalian adenoviruses, assigned to the genus Mastadenovirus, share a common antigen and are serologically distinct from those that infect birds, genus Aviadenovirus. Many adenoviruses agglutinate rat or monkey erythrocytes. The fibre protein is responsible for this property and contains type-specific determinants. The haemagglutination inhibition test is used to confirm serospecificity. Adenoviruses are moderately resistant, surviving in the environment for days or weeks under suitable conditions. Adenoviruses can withstand freezing, mild acids and lipid solvents. Infectivity is lost following heating at 56°C for more than 10 minutes. The adenoviruses of veterinary importance are shown in Figure 48.2 and Table 48.1.

Table 48.1

Adenoviruses of animals

Virus	Host species	Disease
Canine adenovirus serotype 1	Dog, fox	Cause of infectious canine hepatitis, a generalized infection affecting liver and blood vascular system
Canine adenovirus serotype 2	Dog	One of the causes of infectious tracheobronchitis (kennel cough), a highly contagious respiratory disease complex
Equine adenovirus A	Horse	Generally subclinical or mild respiratory infection but also associated with fatal pneumonia in Arabian foals affected by combined immunodeficiency disease
Bovine adenoviruses	Cattle	Occasional outbreaks of enteric and respiratory disease
Ovine adenoviruses	Sheep	Occasional outbreaks of enteric and respiratory disease
Porcine adenoviruses	Pig	Occasional cases of diarrhoea, usually subclinical
Fowl adenoviruses	Fowl, quail	Numerous serotypes. Frequently isolated from healthy birds or following mild respiratory disease. Associated with inclusion body hepatitis and quail bronchitis
Duck adenovirus A	Fowl, duck	Cause of egg drop syndrome in laying hens
Turkey adenovirus A	Turkey, pheasant	Cause of turkey haemorrhagic enteritis which affects four- to 12-week-old turkey poults and of marble spleen disease which affects two- to eight-month-old pheasants

Figure 48.1 Electron micrograph of adenoviruses.

Figure 48.2 Classification of adenoviruses of veterinary significance.

Adenoviruses replicate in the nucleus of the cell. Newly assembled virions form crystalline aggregates which appear as intra-nuclear basophilic inclusions when viewed using light microscopy. Infection with adenoviruses is common in many animal species and man. They are generally host species-specific having a natural host range confined to a single species or closely related animal species. More than 50 human serotypes, grouped into seven virus species, have been identified to date. However, the vast majority of infections appear to be subclinical or mild with more severe disease signs confined to immunodeficient individuals. A similar situation pertains in the case of many animal adenovriuses. In contrast, canine adenovirus type 1 causes a generalized disease affecting the liver and vascular system of dogs.

Avian adenoviruses

Adenoviruses have been isolated from fowl, turkeys, pheasants, pigeons, ducks, quail, geese, guinea fowl and budgerigars. Some avian adenoviruses exhibit a marked species specificity while others have a broad host species range. The distribution of infection is worldwide and extremely common in poultry flocks. Faeces are an important source of infection. Egg transmission also occurs. Maternal antibodies control infections in young birds. As these antibodies are lost, the birds may undergo infection. The majority of fowl adenovirus infections are subclinical or only associated with mild disease. A number of fowl adenoviruses are associated with a specific clinical syndrome such as inclusion body hepatitis.

Inclusion Body Hepatitis

Inclusion body hepatitis (IBH) occurs chiefly in broilers but can also be seen in rearing pullets. Several fowl adenoviruses have been associated with IBH but the exact aetiology and pathogenesis requires further clarification. There is a sudden increase in mortality in affected flocks. Mortality averages 10% but may be as high as 30% where there is immunosuppression due to infectious bursal disease or chicken anaemia virus infection. Lesions include an enlarged, friable liver with haemorrhages on the surface, intramuscular haemorrhages and anaemia. Histologically there is hepatic necrosis and intranuclear inclusions in hepatocytes. Diagnosis is usually based on clinical signs and post mortem examination because apparently healthy birds can also excrete fowl adenoviruses and possess antibodies. Control measures are general in nature because vaccines are not routinely available and the aetiology and pathogenesis of IBH is not fully understood.

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Tags: Clinical Veterinary Microbiology

Jul 18, 2016 | Posted by admin in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off

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