Call your vet. he or she may likely advice you to increase the label dose substantially, my vet often does. That is certainly a disease that should be taken very seriously. This disease, also known locally as circling disease, often causes severe brain damage if not treated quickly, not to mention a very high mortality.
According to the latest edition of N.Bruce Haynes, DVM's book, keeping livestock healthy, any good first line, broad spectrum antibiotic may be tried for results.
One thing though, are you feeding any silage or other fermented feeds? As this is often the triggering factor in this disease.
