THERE is no association between exposure to electromagnetic fields and the risk of childhood leukaemia. But infants who attend day care are much less likely to develop leukaemia than those who do not mix with other babies, a large study in the UK shows.
The results suggest that early exposure to a range of infections has a protective effect, somehow teaching the immune system to respond to infections in an appropriate way. Without this training, it appears that infections later in life are more likely to trigger leukaemia. This cancer is caused by the uncontrolled growth of white blood cells, which are part of the immune system.
The findings are part of the United Kingdom Childhood Cancer Study (UKCCS), which has been tracking more than 11,000 children for 15 years. In particular it has looked at leukaemia, the most common childhood cancer in the industrialised world.
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There is growing evidence that leukaemia is caused by a “double whammy”, says Mel Greaves of the Institute of Cancer Research in London. In the first “hit”, a DNA break produces a mutant gene. As many as 1 in 20 newborns have this gene, a small study by Greaves suggests. Of these only 1 per cent develop leukaemia, which suggests a second hit, perhaps an infection later in life, is what triggers the disease.
To test the idea that early exposure to infections can protect children, Greaves’s team compared 6300 children without cancer with 3100 children with cancer, including acute lymphoblastic leukaemia (ALL). The team found that increased social activity outside the home reduced the risk of ALL, and that the reduction in risk was greatest in children who attended formal day care before they were three months old (British Medical Journal, DOI: 10.1136/bmj.38428.521042.8F).
“One explanation for this link is that infections early in life establish the appropriate immune response,” Greaves says. An inappropriate response to infections, by contrast, might encourage white blood cells to proliferate, triggering leukaemia.
If this theory is right, it might be possible to develop a vaccine to train babies’ immune systems. Studies in the US and Finland suggest that one existing vaccine, the Hib vaccine, has some protective effect.
As part of the UKCCS, researchers also investigated whether the risk of leukaemia was increased by exposure to electromagnetic fields, such as those from power cables, or vitamin K, which is injected into newborns to prevent haemorrhagic disease. An expert panel found no link, a press conference in London was told last week. The team hopes the results will reassure parents. “Once the spectre of risk is raised, getting rid of it is very difficult,” says Eve Roman of the University of York, a member of the panel.
The study did not look at proximity to nuclear power stations. Clusters of leukaemia around two stations in the UK have been blamed on radiation, but an alternative theory is that the cause is an unidentified infectious agent brought into the areas by migrant workers.
Roman says the latest work neither supports nor disproves this theory. “All our research is showing is that it’s good to start priming of the immune system as soon as possible after birth.”
