Rats!

More reports of bait-poison-resistant rats, drawing parallels to the development of antibiotic-resistant bacteria in hospitals (https://www.bbc.co.uk/news/uk-england-19974261). Some authorities have linked the spread of resistant 'super rats' to people administering the bait inappropriately so that a full lethal dose is not taken (the whole procedure is pretty barbaric as the lethally-dosed rat essentially bleeds to death via small lesions in the lumen of its intestine), whereas others have simply noted the arising of resistance genes that can be inherited from one or both parents. Rats actually have a very impressive (for a mammal) reproductive rate and any organism with such characteristics is well-placed to develop resistance to any of the treatments devised (or hijacked) by we humans.