In addition to local factors and genetics, my hunch is that there is a climatological connection.

The Parkinson's distribution maps hint at a relationship between prevailing wind direction and PD. In itself this is not causal. However, one possible causal mechanism follows from the wind affecting surface ozone levels [1]. Especially in summer, these seem to be distributed in a similar way to PD. In turn, ozone levels affect nitric oxide levels in the body [2]. NO may be implicated in Parkinson's both directly [3] and indirectly, by increasing the permeability of the BBB [4].

Proper statistical analysis is required.

[1] http://acdb-ext.gsfc.nasa.gov/Data_s...ce/gif/cl2.gif [Shows tropospheric ozone levels, not ground levels.]
[2] http://www.ncbi.nlm.nih.gov/pubmed/17662977
[3] http://www.ncbi.nlm.nih.gov/pubmed/16005074
[4] http://www.liebertonline.com/doi/abs...08601300185223

John