Clinical signs are more severe in dairy cattle and swine than in sheep and goats. However, the disease is present in many countries in Europe, Asia, Africa and South America. The virus is easily killed in the environment. However, it will survive for a considerable time in animal proteins such as meat and milk. Animals pick up the disease through direct contact with the virus.
Infection usually begins and builds up in the respiratory tract.
Infected animals breathe out large volumes of the virus in aerosol form, and milk, feces, semen, saliva and urine contain the virus. In aerosol form the disease can spread a considerable distance through air and wind, especially when humidity is above 60 percent. Haskell says milk tankers can spread the disease from farm to farm, and feeding infected meat meal is a common source of contamination. Pigs eating infected feed can easily spread the disease to cattle. However, mechanical spread of the disease by birds is considered unlikely.