Could Dogs Pose a Risk Factor for Breast Cancer?

Zosia Chustecka


June 28, 2006 — Breast cancer patients were twice as likely to have kept a dog as a pet in the past 10 years than age-matched controls in a small study conducted by researchers at the University of Munich, Germany. They suggest that dogs may offer a route of transmission for the mouse mammary tumor virus (MMTV), as well as other microbes, which could participate in the first steps of carcinogenesis in humans. Writing in the June issue of Medical Hypotheses, they speculate that pet dogs harboring such risk factors offers a possible explanation for the increase in breast cancer in Western countries and its correlation with a higher standard of living.

Asked to comment on the paper, Jennifer Wheler, MD, special fellow at the Memorial Sloan-Kettering Cancer Center, New York, said, "This is a controversial area of research." The hypothesis is not new, as there have been scientific papers in the past suggesting that MMTV or MMTV-like organisms may contribute to the development of breast cancer. However, she told Medscape: "This is a hypothesis that requires much more study."

In the United Kingdom, where several newspapers picked up the story about a potential link between dogs and breast cancer, cancer charity spokespersons were dismissive. Kat Arney, MD, information officer for Cancer Research in London, commented: "There is currently no scientific evidence to support the suggestion that women can get breast cancer from contact with dogs. This study was very small and does not take other factors into account." Sarah Rawlings, from Breakthrough Breast Cancer in London, pointed out that the researchers have not shown that the dogs "carried the virus, let alone that the virus moves from dogs to people, so we find it difficult to support this study and advise all dog lovers not to worry."

Patients Consulted for Immunotherapy

The researchers were based at the Institute of Immunology, at a clinic within Munich University Medical School, where they regularly ask consulting patients whether they keep household pets to identify zoonotic factors. From a group of 69 women with breast cancer who consulted on immunotherapy, they found that 26 patients (37%) had a dog at the time of consultation and throughout the previous 10 years. Using data collected for a public opinion poll held locally, the researchers found 1320 age-matched controls, of whom 195 (14.8%) had dogs. They report a relative risk of 3.5 (P = .0000003).

The researchers note that there were further associations with dogs that were not included in this statistical analysis. An additional 11 patients with breast cancer had kept a dog at some stage in the past 10 years but did not have one at the time of consultation, and 16 patients had intensive contact with dogs. Two further patients had received dog bites. "Taken together, 79.9% of all patients had intensive contact with dogs before the diagnosis of breast cancer, encompassing a time period up to 30 years," the researchers note.

No significance differences were found with regard to keeping cats as pets (20.6% of breast cancer patients vs 21% of controls).

From Mice to Dogs to Humans?

MMTV, an oncogenic B retrovirus that can induce breast cancer in mice, has been known for more than 30 years, but whether it is a risk factor for human breast cancer remains a "tantalizing" question, the researchers comment. Many groups have found MMTV-like sequences highly expressed in human breast cancer, but 2 previous studies did not, they point out.

The researchers of the current study note that in Western countries, a high incidence of cancer has been correlated with the dominance of a particular mouse strain, Mus domesticus. Dogs may offer a route of transmission for MMTV as well as other viruses and bacteria, the researchers suggest. With dogs' noses often close to the ground, they may inhale infectious excretions from small animals such as mice and could pass along viruses to their human owners without succumbing to cancer themselves, although the group points out that breast cancer occurs frequently in dogs.

Med Hypotheses. 2006;67:21-26

Thank you. I needed to be sure the hypothesis was offal. If all data were interpreted that way, then we'd be living in hermetically sealed rooms. At least they suggest that cats (pets, in general, I presume) are equally "risky." Why hasn't this vector also been suggested for the Hanta virus? Was there a positive or negative correlation to existing studies that demonstrate "companion animals" help most owners live longer, happier lives?