Just like drug resistant germs (we’ve discussed earlier), the rats are evolving.
“They’ve also mutated genetically and are bred to be immune to standard poisons.
“We have had to start using different methods such as trapping and gassing, which can be less effective and more costly.”
–Graham Chappell, from Rapid Pest Control in Newbury in Rowley (2012): Home counties demand stronger poison to deal with mutant ‘super rats’ in The Telegraph.