I have them in my back yard in Yuma, Arizona. The larval stage appears every year between May and September. My back yard has lots of vegetation so it is a breeding ground for them. My immediate neighbors yards are filled with gravel and little else so they do not have them.
How they got there, I do not know, probably came in on a plant I purchased (I ended up with a snail infestation that way).
My doc informed that the reaction and blisters are classic examples of chigger feeding.
I have learned how to protect myself against them but I forget every year to take precautions until the first itching and blisters show up.
After September, the adults hibernate until the next breeding season.