There is no evidence to show that dogs can cause breast cancer in their owners. The idea that there might be a link between breast cancer and dogs was widely reported in the media after a German study in 2006 showed women who had a dog at the time or in the previous 10 years were at 3.5 times higher risk of breast cancer. The researchers also found nearly 80% of the women with breast cancer in the group they studied had had ‘intensive contact’ with dogs for up to 30 years before their diagnosis. The study was relatively small and has been criticised for not considering factors other than pet ownership. The fact that there has only been one study into this theory suggests that few other research groups found worth in further investigation. However, scientists trying to understand the association – coupled with knowledge that breast (mammary gland) cancer is common in female dogs – have suggested it may be because of a virus that has been detected in both dogs and humans. The mouse mammary tumour virus (MMTV) has been known for about 30 years and has been shown to trigger breast cancer in mice. The virus also has been detected in dogs and cats with breast tumours. Some scientists hypothesise that dogs could transmit MMTV or MMTV-like viruses to humans, and that those viruses may lead to breast cancer in humans, but there is no credible evidence to support the theory. There is scientific debate as to whether a virus or viruses could be the cause of breast cancer and more research is necessary to understand how the virus works and whether it can be passed from mice to dogs to humans. It’s also important to note that people who own dogs tend to participate in more exercise, and being physically active is a proven way to reduce the risk of developing breast cancer.