Iowa State University Veterinarian Records, RS 22/6/0/9, University Archives, Special Collections Department, Iowa State University, http://www.add.lib.iastate.edu/spcl/arch/rgrp/22-6-0-9.html
Ovine progressive pneumonia (OPP), a chronic debilitating disease of adult sheep, develops insidiously over a long incubation period. The infection is widespread and may produce a fatal multisystemic lymphoproliferative disease. There is evidence that OPP virus infection is common in western and midwestern sheep flocks. A survey of cull ewes from western states revealed serological prevalence rates of up to 67 %. In Idaho, infection rates within range flocks were 51-90 %. A recent serological survey of the Iowa State University sheep flock indicated that 49 % of the ewes have opp virus antibodies and carry the virus.
Anson, Mary A. and Eness, Paul G.
"Ovine Progressive Pneumonia: A Brief Overview,"
Iowa State University Veterinarian: Vol. 47
, Article 10.
Available at: http://lib.dr.iastate.edu/iowastate_veterinarian/vol47/iss2/10