It has not been transmitted from infected rams via the semen. The exact mode of escape of the scrapie agent from the body is not clear, although nasal secretions and placenta have been shown to contain the agent in some studies. Therefore, there is less risk of a sheep contracting scrapie as it gets older. It is unlikely that spread occurs from the sire to offspring. It appears that scrapie is not a genetically trans mitted disease but rather a disease caused by an infectious agent with genetically influenced suceptibility.
Studies indicate that certain genes influ ence the incubation period.
Sheep with short incu bation genes develop scrapie in about two to four years. Sheep with long incubation genes are either resistant to infection or die from other causes before the clinical disease develops. Preliminary results indicate that testing for the gene and selec tive breeding for freedom from clinical disease may be practical. Over 90 percent of confirmed scrapie cases in the U.S. have been in black-faced sheep, mainly Suffolks. It is not known whether this is due to genetics or management practices.