The spread of resistance to rodenticides has health implications. Rats spread diseases, such as Weil's disease and salmonella, they spoil crops and stores of foods, and they trigger general contamination.
Rodenticides used by farmers in the UK since the early 1980s include chemicals such as difenacoum and bromadialone which are anticoagulants and cause rodents to bleed to death. They were introduced to replace previous poisons, such as warfarin, after rodents developed tolerance to them in many parts of the country, beginning in 1957. By 2000, resistance was developing to the standard second-generation rodenticides in areas in East Anglia, Yorkshire, Kent and elsewhere. This has put pressure on farmers to use even more powerful rodenticides, including toxins like brodifcoum and flocoumafen, which are only supposed to be laid down indoors by licensed operators.