A common childhood virus may significantly boost the risk of breast cancer, according to researchers who think infection with it late in life may be a trigger for some of the cancers.
Dr Ann Richardson, an epidemiologist at Otago University's Christchurch School of Medicine and Health Sciences, uncovered the intriguing new links between cytomegalovirus (CMV) and breast cancer.
CMV is a common virus that infects most people at some time during their lives but rarely causes obvious illness. It is a member of the herpes virus family.
Richardson published a study in the British Journal of Cancer, showing women who developed breast cancer when aged under 40 years were more likely to have recently been exposed to CMV, than were women without breast cancer.
If late infection with CMV is proven to increase the risk of breast cancer, some countries may consider immunising children against it, Dr Richardson said.
The research appeared six years, to the day, after she and Otago colleague Associate Professor Brian Cox first published research in the journal investigating CMV and another common virus, Epstein-Barr virus (EBV).
EBV is a common human virus, both it and CMV can cause glandular fever.
The team made its latest breakthrough by comparing stored blood samples in Norway with a cancer register and showed that elevated antibody levels in the blood for CMV preceded the development of breast cancer in some women.
They did not find a similar link between EBV and breast cancer.
Richardson said the pattern of infection with CMV differed between countries, and in countries with the lowest incidence of breast cancer, nearly everyone was exposed to CMV in early childhood.
But in countries such as New Zealand, where the incidence of breast cancer is high, many women are not exposed to CMV until adulthood.
CMV is spread in breast milk as well as saliva, urine, cervical secretions and semen.
Initial work with Cox and Professor Margaret McCredie at the Dunedin School of Medicine, and researchers at the University of Melbourne, found the risk of breast cancer in 377 Australian women aged under 40 rose with the level of CMV antibodies in their blood.
The team concluded higher CMV antibodies measured in young women with breast cancer could be the result of more recent viral infection.
Cox said the latest study, of blood samples taken up to 17 years before Norwegian women were diagnosed with breast cancer, found women infected with CMV as adults had four times the breast cancer risk in women not infected as adults.
And this increased risk was nine-fold in women who had had children.
The increased risk was seen in only a small number of the breast cancer patients - other factors likely contributed to cancer in most women.
The link between adult CMV infection and breast cancer fitted neatly with a lower risk of cancer seen in women who started giving birth while relatively young and went on to have a number of children.
New young mothers not previously infected with CMV might pick up the virus when their children caught it from playmates, and would be able to develop immunity while young.
And the more children in the family, the more likely it was that a mother would be exposed to CMV while young.
Several other cancers such as cervical cancer, liver cancer and a type of leukaemia are known to be caused by viruses, and a vaccine has been produced to protect women against the virus involved in cervical cancer.
About 2,000 women develop breast cancer in New Zealand every year, making it the most common cancer in women, but most are over the age of 40.