Wonder if its the same chemical RATT used for their hair in the 80s
http://news.yahoo.com/giant-rat-kills-predators-poisonous-hair-231002214.html
It is unknown why the rat doesn't die from chewing the poison, though it could be resistant somehow. "The rats should drop dead every time they chew this stuff but they are not," Kingdon said. "We don't have the slightest idea how that could be done."
These rats are the carriers and are immune to the poison, that's just how it is.