The frequency in which horses are vaccinated depends on their risk of encountering a large number of the disease-causing organisms. With regards to many respiratory tract diseases, risk is increased for most horses in areas where they congregate (shows, trail rides, etc.) For other diseases, such as encephalomyelitis (‚Äúsleeping sickness‚Äù) or tetanus, all horses in a given area are basically at equal risk. It is important to note that vaccines are ideally administered when the need for immunity against a disease is at its highest. For example, diseases carried by insects like West Nile, Potomac Horse Fever, and Eastern and Western Encephalomyelitis should be vaccinated against in the springtime each year.