Timing of exposure to bacteria and viruses suspected to play significant role.
A decade-long investigation of childhood leukaemia has come to the conclusion that the disease is probably often triggered by common infections in toddlers, scientists announced today in London.
By identifying the exact types of infections involved, one might be able to prevent some cases of leukaemia, the researchers say. They add that exposure to infections very early in life may help to develop children's immune systems and protect them against developing tumours in later years.
The cause of leukaemia has been the subject of much debate. Some have suggested that it is influenced by exposure to radiation and chemicals, and others have controversially proposed that electromagnetic waves from power lines might also contribute to cases.
But the United Kingdom Childhood Cancer Study, which compiled information from studies of more than 10,000 children, including 1,737 with leukaemia, concludes that infection is a far more important factor. The group says their work agrees with previous studies that have shown no connection between power lines and leukaemia.
Leukaemia accounts for a third of all cancers in children under the age of 15, mostly affecting industrialized countries. The disease commonly strikes infants in the first two to four years of life, and is treated with aggressive chemotherapy. About 80% of those affected survive the first 5 years following diagnosis.
Researchers first proposed a link between childhood leukaemia and viruses such as measles in the 1920s. Five decades later, researchers discovered that viruses were to blame for the onset of leukaemia in cats and cattle. And one particular virus has been found to be a trigger for a rare form of leukemia that occurs primarily amongst adults in Japan.
But it took the UK assessment to convince researchers of a credible link between infection and most types of leukaemia.
Infections induce a proliferation of white blood cells in bone marrow as part of the normal immune response. In those genetically predisposed to leukaemia, the infection could cause an uncontrolled proliferation of cells, leading to cancer, says Mel Greaves of the Institute of Cancer Research, London.
Safety in numbers
Exposure to pathogens in the first year of life may help to train a child's immune system to somehow prevent this, says Greaves. He points out that the incidence of childhood leukaemia in East Germany before reunification was a third less than its Western counterpart. This may be in part because every three-month-old infant in East Germany was sent to playgroups, he says, exposing them to a healthy dose of disease.
A paper published online in the BMJ this week1 reaches a similar conclusion. From a survey of more than 9,000 UK children, it concludes that infants who attend group day care twice a week in the first few months of life are half as likely to develop childhood leukaemia.
Greaves notes that US and Finnish studies have suggested that the Hib vaccine against acute respitory infection helps to protect against childhood leukaemia. But he cautions that many details about the biological mechanisms remain unknown.
GilhamD., et al. BMJ, Online publication: doi:10.1136/bmj.38428.521042.8F (2005).
Institute of Cancer Research, London
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Khamsi, R. Common infections blamed for childhood leukaemia. Nature (2005). https://doi.org/10.1038/news050418-16