Looks like it'd map nicely to population density.
Not by the numbers. There was at one time a hypothesis that rural residents had a higher prevalence, but that turned out not to be true... and the reverse was true but not by much. The map is showing ten times the prevalence between some areas.
Highly urban counties 1,706 per 100,000
Highly rural counties 1,371 per 100,000
I got this in my PPMI newsletter, and I also learned the difference between:
Incidence - new diagnoses in a given time period.
Prevalence - total cases in a given demographic.