Clinical disease due to OPPV
is rare, despite OPPV's high
prevalence in certain areas of Canada (source 2).Subclinical infection of breeding
ewes in some flocks has
been associated with reduction of conception rates, reduced birth
and reduced growth rate in lambs. The reduction in growth rates is
associated with changes in the
udder of the ewe that result in decreased milk production.
Subclinical infection has no effect on the mature ewe body
weight or on the greasy fleece weight (source 2).
As previously mentioned, clinical
signs appear in older ewes
since the incubation period of the virus is between 2-4 years. Signs
appear after periods of stress, exertion, and inclement weather. The
disease progresses slowly, resulting in chronic degenerative
Initially, diseased animals appear
listless and suffer progressivelemaciation, and exercise may
Sheep tend to maintain a
and body temperature provided there is an absence of secondary
infection. Animals may exhibit tachypnea (
80 -120 breaths per minute),
have nasal discharge, and coughing may be observed. Ataxia, stumbling,
unilateral proprioceptive deficits may be
the first signs noticed.
neurologic manifestations can slowly progress over weeks to months to
paralysis or quadriplegia.
Arthritis is occasionally seen in
naturally infected sheep,
but this manifestation appears restricted to sheep in the US.
It occurs in sheep ranging from 1-6 years of age, and is localized to
joints, which showing obvious swelling. The affected sheep become lame
As the disease progresses, open mouth
breathing, flaring of
nostrils, forced expiration, and increased frequency of coughing can be
mastitis may develop in ewes, which is characterized by an enlarged
or asymmetrical) hard udder with no abnormal secretions. In rare cases,
non-suppurative arthritis, vasculitis, mastitis, encephalitis,
paresis, may be observed. Clinical signs last on average for 3 to 6
outcome of the disease after clinical signs appear is 100% mortality as
a result of
animals dying or being culled