Investigations were undertaken to isolate viruses
from sheep, and to assess their role in respiratory diseases
by means of epidemiological observations and experimental
studies of their pathogenesis.
Viruses were isolated from 0*7 per cent, of samples
taken at necropsies and from 16 per cent, of sheep in
3 of 4 flocks currently experiencing outbreaks of respiratory
disease, but not from 7 flocks with few or no signs of
An adenovirus (strain 7769) was isolated from rectal
swabs, but not nasal swabs, from 3 of 15 lambs during an
outbreak of pneumonia in a group of 4 to 10-week-old lambs.
This adenovirus differed antigenically from other ovine,
bovine and human adenoviruses, and the species of erythro¬
cytes that were agglutinated by strain 7769 differed from
those agglutinated by porcine, canine, equine and murine
adenoviruses. It was concluded that strain 7769 was a
previously unreported type of adenovirus and was designated
ovine adenovirus type 4 (0A4).
Although antibodies to the adenovirus group specific
antigen were detected in only 6 per cent, of 661 sheep sera,
neutralizing antibodies to 4 serotypes of ovine adenovirus
were more common. Neutralizing antibodies to ovine adeno¬
virus types 1-4 were more prevalent in animals over
12 months of age.
Following exposure of specific pathogen-free (SPF)
lambs to an aerosol of 0A4 virus, replication of this virus
occurred in the respiratory and alimentary tracts and liver,
and neutralizing antibodies could be detected in the serum
and nasal secretions as early as 8 days after inoculation.
Infection was associated with a mild clinical illness,
detectable by auscultation only, and accompanied by lesions
in the lungs and liver. The lesions found in the lungs
of infected lambs were pulmonary oedema and peribronchiolar
accumulations of mononuclear cells and in the livers were
focal necrosis, lymphangitis and occlusive cholangitis.
The clinical disease and pneumonic lesions observed
in SPF lambs infected with both ovine adenovirus type 4
and Pasteurella haemolytica were no more severe than those
in lambs infected with P.haemolytica alone.
Enzootic pneumonia was induced consistently in SPF
lambs inoculated with parainfluenza virus type 3 (PI3)
followed by P.haemolytica 4 or 7 days later. Seventyeight per cent, of lambs developed severe respiratory
disease by this method, 54 per cent, died and 95 per cent,
had macroscopic lung lesions. The illness and lesions were
more marked in lambs inoculated with both PI3 virus and
P.haemolytica than in lambs inoculated with either agent
alone and were associated with rapid multiplication of
P.haemolytica within the lung.