SCWDS has been tracking EHD since back in the late 1950s. It seems to run on a 9-yr cycle, with a 4- yr cycle superimposed on it.
As a veterinarian, I remember major epizootics in 1988 or 89 and in 2007 & 2012, when we saw sporadic cases of clinical EHD in the cattle population...in an 'on' year, nearly 100% of cattle get infected, but only a fraction of a percentage of infected cattle ever have any clinical disease.