Clinical Signs

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 weights, 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 often appear after periods of stress, exertion, and inclement weather. The clinical disease progresses slowly, resulting in chronic degenerative disease. 

Initially, diseased animals appear listless and suffer progressivelemaciation,  and exercise may induce dyspnia. Sheep tend to maintain a normal appetite and body temperature provided there is an absence of secondary bacterial infection. Animals may exhibit tachypnea ( 80 -120 breaths per minute), may have nasal discharge, and coughing may be observed. Ataxia, stumbling, and unilateral proprioceptive deficits may be the first signs noticed. Posterior neurologic manifestations can slowly progress over weeks to months to rear limb 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 the carpal joints, which showing obvious swelling. The affected sheep become lame and emaciated (source 2).

As the disease progresses, open mouth breathing, flaring of nostrils, forced expiration, and increased frequency of coughing can be noted. Indurative mastitis may develop in ewes, which is characterized by an enlarged (symmetrical or asymmetrical) hard udder with no abnormal secretions. In rare cases, non-suppurative arthritis, vasculitis, mastitis, encephalitis, and  posterior paresis, may be observed. Clinical signs last on average for 3 to 6 years. The outcome of the disease after clinical signs appear is 100% mortality as a result of animals dying or being culled (source 2).

lame (source 9).