Safety in numbers: children who go to day care early in life are less likely to develop leukaemia. Credit: N. FEANNY/CORBIS SABA

It is finally possible to identify the environmental factors responsible for childhood leukaemia, researchers told a meeting in London last week. Now it's time to think about how to prevent the disease, they said.

Exposure to radiation, chemicals and power lines are not a significant cause, the meeting heard. In fact, most cases are caused by common infections in toddlers.

Leukaemia causes the production of abnormal white blood cells, and accounts for a third of cancers in children. Some genetic predisposition is involved, but for decades scientists have been trying to identify what triggers the disease.

The biggest effort is the United Kingdom Childhood Cancer Study, set up in 1991. It compiled information from more than 10,000 children, including some 1,700 with leukaemia.

Researchers from the project met last week to discuss the results. They were agreed on the role of chemicals and radiation. “Perceived risk factors such as living near sources of electromagnetic fields or natural radiation are not principal causes, if at all, of leukaemia in children,” says Mel Greaves of the Institute of Cancer Research in London.

Infections, on the other hand, induce a proliferation of white blood cells in bone marrow as part of the normal immune response. In children genetically predisposed to leukaemia, the researchers think that infection might cause an uncontrolled proliferation of cells, leading to cancer.

Although several studies have hinted that infection could be a cause, it has taken the size and statistical power of the UK project to convince researchers that there really is a significant link. To do this, epidemiologist Eve Roman of the University of York and her team analysed data on children's day-care attendance.

Records from East Germany had hinted that infants sent to playgroups from the age of three months were less likely to contract leukaemia. So Roman's team set out to test whether exposure to infections very early in life could somehow train the immune system to protect against the cancer.

They focused on acute lymphoblastic leukaemia, a common form of the disease that usually strikes between the ages of two and five — the time most children start going to playgroups. The team found that children who attended day care during the first three months of life had half the normal risk of developing the disease (C. Gilham et al. Br. Med. J. doi:10.1136/bmj. 38428.521042.8F; 2005).

Attention is now turning to how to prevent leukaemia. Encouraging parents to send their children to playgroups early in life is one obvious option. Identifying the infections responsible also raises the possibility of developing protective vaccines. Greaves notes that US and Finnish studies have suggested that the Hib vaccine against meningitis also helps to protect against leukaemia.

Charles Stiller, a cancer epidemiologist at the University of Oxford, UK, is excited about the shift but cautions against getting too carried away. “It's difficult to know what proportion of cases is accounted for by infections,” he says. “And there is more to be done on defining the mechanisms by which this might work.”