Bovine Adenovirus

Introduction

Bovine Adenovirus belong to the family Adenoviridae.
Bovine adenoviruses have been isolated from healthy cattle, but more often, the isolations are associated with some form of clinical disease. Most adenovirus infections in cattle involve either the respiratory (e.g. rhinitis) or gastrointestinal tracts. In addition there have been reports of adenovirus associated with conjunctivitis, keratoconjunctivitis, and weak calf syndrome. It is now also thought to contribute to the disease complex Enzootic pneumonia of calves.
Currently the International Committee on Taxonomy of Viruses recognizes 10 types in cattle (Benkö et al., 2000). Bovine adenoviruses are found worldwide as indicated by either serology or virus isolation, and they are particularly widespread in Central America and Africa. BadV antibodies have been found in sera in virtually every cattle population tested.

Pathogenesis

The respiratory and enteric tracts are the primary targets for adenovirus infection. Infection with adenovirus usually results in cell lysis and virus shedding, but some cells accumulate virus particles in the nucleus without lysis establishing persistent infections. Respiratory and faecal shedding usually last for about 10 days and, where the kidney is involved, virus can be excreted for over 10 weeks in urine (Aldásy et al., 1965). With persistent infection, lysis of fragile infected cells produces virus-shedding resulting in infection of susceptible animals that come in contact with the virus.

Adenovirus infection is common in younger animals. Maternal antibodies provide protection from infection by homologous BAdV types. As specific maternal antibodies wane, calves can become infected if that particular adenovirus type is present in the calf’s environment. Depending on maternal antibody level, virus exposure can result in either a subclinical infection followed by an active antibody response or if the antibody level is high enough, virus replication can be prevented.
Adenoviruses are generally confined to one host species or closely related species and are classified on the basis of species of origin and antigenic reactivity. Because bovine adenoviruses or closely related antigenic viruses have been isolated from a variety of other ruminant species (Belák and Palfi, 1974; Davies and Humphreys, 1977; Baber and Candy, 1981; Boros et al., 1985), the potential for infection across species exists among adenovirus isolates from ruminants.

Signalment

Bovine Adenovirus can infect cattle and zebu of any breed, sex or age, however, younger animals are more at risk as their maternal antibody begins to wane, around the age of 2 weeks to 4 months old. Experimentally, the virus has also been shown to infect goats and sheep.

Clinical Signs

Signs involving the gastrointestinal tract include diarrhoea with mucous, melena or haemoatochezia, anorexia or reduced appetite, abdominal distension and possible dysphagia.
Respiratory signs include coughing +/- blood, serous nasal discharge, dyspnoea and tachypnoea. This may progress to bronchopneumonia if secondary bacterial infection is present.
Generalised clinical signs include pyrexia, weight loss, sudden death, lymphadenopathy and generalised weakness. The animal will also be dull, depressed and lethargic.

Diagnosis

Clinical signs and physical examination may add Bovine Adenovirus to the differential list, but definitive diagnosis cannot be achieved in this manner.
Because adenoviruses can be isolated from apparently healthy cattle, isolation of adenoviruses from clinically sick calves does not necessarily mean that the isolated adenovirus type plays an aetiological role in the clinical disease. For adenovirus to be considered as the aetiologic agent in a particular disease, the antibody titre should be low at the onset of the infection and result in at least a four-fold increase in neutralizing antibodies to the virus type in question.
Adenovirus infection can be diagnosed morphometrically, serologically, and by virus isolation. Rapid presumptive diagnosis can be made either by observation of characteristic virus morphology in intranuclear inclusions by transmission electron microscopy or by immunofluorescent or immunohistochemical labeling of adenovirus antigens in tissues with gross lesions. Serotype-specific diagnosis, while not important to the treatment of clinical disease, is important in the development of a database from which to evaluate the role of each viral serotype in disease production. For a serotype to be considered as the aetiologic agent in a clinical syndrome, it must be isolated from many cases with similar clinical syndromes and be capable of reproducing the disease experimentally.
Because of the number of types of adenoviruses infecting cattle, virus isolation is necessary to definitively identify the virus. Virus can be isolated from nasal secretions, tracheal fluids, intestinal contents and tissue homogenates.
On post mortem examination characteristic features may lead to the diagnosis of BAdV, such as atelectasis and consolidation of the lungs and erosions, ulcerations and haemorrhage in the intestinal tract. Bronchiolar, mediastinal, and mesenteric lymph nodes are usually enlarged. Microscopically the basic lesion is bronchiolitis with necrosis and sloughing early and hyperplasia later in the course of the infection. Amphophilic, intranuclear inclusions are seen in swollen cells in the respiratory epithelium and sloughed in the lumen. Where the gastrointestinal tract is involved, the basic lesions are fibrinonecrotic plaques overlying foci of haemorrhage and necrosis. Amphophilic intranuclear inclusions are seen in enterocytes as well as in vascular endothelial cells.

Treatment and Control

When economically feasible, cattle can be treated to provide relief from clinical signs associated with adenoviral infection. Because secondary bacterial infections such as Mannheimia haemolytica, Pasteurella multocida and Haemophilus somnus are common in cattle, antibiotics are often used as part of the treatment.

The main control measure is to ensure adequate colostrum at birth as passive transfer provides immunity to calves. Other control strategies include preventing mixing of calves of different age groups and ensuring good hygiene and ventilation in calf housing.
Both modified live and inactivated adenovirus vaccines have been developed and evaluated for use in cattle and they should be administered when maternal antibodies have waned, but 2 to 3 weeks before calves from different places are assembled under stressful conditions. Such vaccines are available in Europe and Japan, but there are no commercial adenovirus vaccines available in the USA. Most vaccines are formulated in combination with other agents. Two to 4 doses of vaccine administered subcutaneously or intramuscularly are recommended to provide proper protection. Vaccination has not eliminated infection entirely, but has resulted in the reduction in disease incidence and treatment costs.

References

Áldásy P, Rusvai M, Bartha A, 1965. Pneumo-enteritis in calves caused by adenoviruses. Acta Veterinaria Academiae Scientiarum Hungaricae, 15:167-175.
Baber DJ, Candy JB, 1981. Isolation and characterization of bovine adenoviruses types 3, 4 and 8 from free living African buffaloes (Syncerus caffer). Research in Veterinary Science, 31(1):69-75.
Belák S, Palfi V, 1974. An adenovirus isolated from sheep and its relationship to type 2 bovine adenovirus. Archiv für die gesamte Virusforschung, 46(3-4):366-369.
Benkö M, Bartha A, Möstl K, Bürki F, 1989. A heteroploid permanent cell line originating from embryonic calf thyroid supporting the replication of all known bovine adenovirus serotypes. Veterinary Microbiology, 19(4):317-324; 10 ref.
Benko M, Harrach B, Russell WC, 2000. Family Adenoviridae. In: Van Regenmortel MHV, Fauquet CM, Bishop DHL, Carstens EB, Estes MK, Lemon SM, Maniloff J, Mayo MA, McGeoch DJ, Pringle CR, Wickner RB, eds. Virus Taxonomy. Seventh Report of the International Committee on Taxonomy of Viruses. New York, San Diego, USA: Academic Press, 227-238.
Boros G, Graf Z, Benkö M, Bartha A, 1985. Isolation of a bovine adenovirus from fallow deer (Dama dama). Acta Veterinaria Hungracia, 33(1-2):119-123.
Davies DH, Humphreys S, 1977. Characterization of two strains of adenovirus isolated from New Zealand Sheep. Veterinary Microbiology, 2:97-107.