CHILDREN who are regularly looked after in day-care centres in the first year of life run half the risk of developing childhood leukaemia as other children, research suggests.

The largest study of cancers in the young indicates that children not exposed to common infections in infancy — in group settings such as crèches — are more likely to develop the disease because their immune systems are less robust.

The United Kingdom Childhood Cancer Study pinpoints infection as by far the most likely trigger of childhood leukaemia. It also offers the best evidence yet that exposure to radiation such as pylons, neonatal use of vitamin supplements and other common theories are not causes of the illness.

Findings from the 15-year study indicate that most childhood leukaemias have their origins before birth, when babies often develop cell mutations. A later infection in infancy or childhood is then thought to trigger an abnormal immune response that causes the disease.

Rates of leukaemia in children have been rising, with about 500 new cases in under-15s diagnosed each year. Scientists plan to identify the timing and types of infection that prompt the onset of the disease, with the hope of developing vaccines that can block the process.

Other research from the study, which is published on the website of the British Medical Journal, supports the infection theory, showing that babies who attend day-care centres are 52 per cent less likely to develop the disease.

The child cancer study, which was supported by the Leukaemia Research Fund, gathered information on 3,838 children in whom cancer is diagnosed, including 1,737 with leukaemia, and compared them with 7,629 youngsters without cancer. The researchers looked at exposures including household levels of background radiation, parental smoking, breast-feeding, and vitamin K, which is given to babies shortly after birth.

Parents of the children, aged between 2 and 14, were also questioned about day care and social activity with children outside the family during the first year of life.

The researchers found that increasing levels of social activity outside the home were linked to consistent reductions in the risk of leukaemia, with the risk halved for those children attending formal day care during the first three months of life. Children exposed to informal day care, such as that provided by friends and family, had a 38 per cent drop in risk, while those who had some social activity, but not day care, had a 27 per cent lower risk.

Mel Greaves, from the Institute of Cancer Research in London, said that the project had been the most exhaustive and detailed study carried out into identifying the possible causes of leukaemia. Experts from around the world this week signed a consensus statement supporting the infection theory as the most likely explanation for the disease.

Professor Greaves said that although analysis of the data continued, “it is clear that perceived factors such as living near sources of electromagnetic fields of natural radiation such as radon are not principal causes, if at all, of leukaemia in children”. He said the evidence pointed to an abnormal response to infection in children with a particular defect in their chromosomes that occured while in the womb.

He said that about one child in twenty could be affected by the chromosomal differences they had outlined, but only one youngster in 2,000 would develop leukaemia. Those children who encountered more infections early in life appeared to be more protected from the illness, he said.

Other cancer trends support this notion, including a reduced rate of leukaemia in East Germany before unification in 1990 compared with the rest of Western Europe. The rate increased dramatically after unification in 1990, when compulsory state day care for infants was abolished.

THE CONDITION

• Cancer is diagnosed in about 1,500 children under the age of 15 each year, a third of which are leukaemia cases
  
• Rates are rising, but medication is improving; 75 per cent of cases are treated successfully
  
• The UKCCS project lends further weight to the “hygiene hypothesis”, which links the rise in some illnesses to the inability of children’s immune systems to cope with infection because modern environments are too sterile

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