Pest control experts have theorized that rats and mice in various parts of Sweden had developed some sort of immunity to commonly deployed rat poisons. Now their suspicions have been confirmed.
The results of 80 random tests performed across the country by Swedish extermination company Anticemex revealed poison-proof rats and mice in four locations: Kristianstad in the south; Linköping and Växjö in south central Sweden; and Uppsala in eastern Sweden.
Pest control expert Håkan Kjellberg with Anticemex said chemicals are likely to blame for the rodents having developed immunity to rat poison.
"It may have been rat poison, but also chemicals in their immediate environment that have caused the genetic makeup in their body to change," he told Sveriges Radio (SR).
According to SR, rats that are resistant to poisons have been found in many other countries, including Denmark, but this Anticemex study is the first to confirm the phenomena in Sweden.
The company said it may now be forced to resort to more potent poisons in more cases in order to keep Sweden's rodent population in check.