Analysis of breast cancer cases by researchers at the University of Munich showed that patients with cancer of the breast were significantly more likely to have kept a dog than a cat. In fact, 79.7 per cent of all breast-cancer patients had regular contact with dogs before they were diagnosed. Only 4.4 per cent of the patients did not have pets at any time, compared to 57.3 per cent of a healthy control group. This, according to the researchers, shows a 29-fold increased risk for pet owners.

The researchers also point to another study in Norway, which reported a very high level – 53.3 per cent – of breast cancers in 14,401 dogs. The team then tried to isolate a virus that could be common to both dogs and humans.

The one they homed in on is the mouse mammary tumour virus or MMTV, which triggers breast cancer in mice, and has been investigated for possible links to human breast cancer. The theory is that dogs, and possibly other pets, harbour and transmit MMTV or MMTV-like viruses that can induce human breast cancer.

According to the researchers, the theory may help to explain why women from the Far East are at greater risk of breast cancer when they move to Western nations, because Asian or Oriental women seldom keep dogs as pets.