We hunt Jo Davies County, IL just across the river from Dubuque and one of the farmers we hunt with was bitten by a timber rattler about 20 years ago. They had to bring him to Des Moines for anti-venom and it turned out he was allergic to that too, needless to say he had a long 2 week hospital stay and nearly lost his life.
After that he had a snake hunter come and walk his land to see if it was a fluke or if there were more, 2 times a total of 22 rattlesnakes removed.
On another note this spring when we went to SD to turkey hunt my dad came 2 feet from getting taked by a Prairie Rattlesnake (it was laying right next to our gun cases sunning itself in a dirt pile), luckily it was early for them and he was a little slow and sluggish so my dad was able to avoid him