RABIES has increased dramatically in the United States over the past eight years; and cases of rabies in horses are being reported more frequently. Rabies vaccination should be done yearly, and because this is a very inexpensive vaccine against a 100-percent fatal disease, there is no reason not to vaccinate.POTOMAC HORSE FEVER is a disease that was first diagnosed in the Potomac River Valley of Virginia and Maryland.abies vaccination should be done yearly, and because this is a very inexpensive vaccine against a 100-percent fatal disease, there is no reason not to vaccinate. POTOMAC HORSE FEVER is a disease that was first diagnosed in the Potomac River Valley of Virginia and Maryland.