The cows are killed because the disease is extremely contagious. The virus survives for a long time in the air, food, garbage, hides, hair and wool. The area has to be quarantined, and all infected animals slaughtered and burned. Any contaminated objects are disinfected, and the farm is left empty for months. In reality it isn't fatal for the cows and it isn't a health risk to humans.
Horses are resistant.
In surviving cows the flow of milk is greatly reduced, and abortions are common. Aside from the death to cows, farmers lose money. As you know when farms go broke people suffer both psychologically and financially. Child abuse goes up, as does alcoholism, divorce, suicide, etc., etc. While it may not be contagious to humans, it does indirectly affect their health.