I don't have a citation for you because I heard this from someone I met in university, but large farms take "bio-security" incredibly seriously. Trying to treat

I don't have a citation for you because I heard this from someone I met in university, but large farms take "bio-security" incredibly seriously. Trying to treat the livestock is far more expensive than simply culling them all and sterilizing the enclosure. Additionally, even if they tried to treat the flock instead of culling it, the disease would linger in the population and be a continual concern.