Serologic prevalence of ovine progressive pneumonia in a western range flock of sheep.

: Of 254 sheep examined for antibodies to the virus of ovine progressive pneumonia, 162 (63.8%) were seropositive. The mean age of the seropositive ewes was 4.9 +/- 1.8 years, which was significantly greater than that for the seronegative ewes (4.1 +/- 2.3 years). The age-specific prevalence increased with ewe age, from 25% among yearling ewes to 85% among 7-year-old ewes. Seropositive ewes produced more pounds of live lamb at birth than did seronegative ewes.