What’s really causing childhood leukaemia?

Few issues are as emotive as that of childhood cancer. It affects around 1,500 children under the age of 15 every year, and – thanks to research – most children are now cured. But there is understandable concern among parents as to possible causes. Could we be unwittingly be risking our children’s health by giving them plastic bottles, or failing to buy them organic veg?

The reassuring news for parents is that many of the scare stories are simply unsubstantiated by significant scientific evidence, or the research suggests extremely small (if not negligible) increases in risk. But if that’s the case, then what is actually causing childhood cancer?

A risky infection?
A theory that’s growing in weight is the idea that it’s not mysterious chemicals or electrical fields that are a major cause of acute lymphoblastic leukaemia (ALL), the most common childhood cancer in the West. In fact, some experts believe that the cause of some cases of ALL may simply be that most innocuous and ubiquitous of kiddie problems – picking up an infection.

So what’s the idea here? Researchers have found that the incidence of childhood leukaemia is higher in towns that undergo a rapid influx of people from elsewhere in the UK or further afield (known as population mixing). For example, this can be seen in rural New Towns, and in areas with a high population of Armed Forces, migrant workers, or even just commuters.

This kind of mixing is thought to bring in ‘fresh’ germs from outside, which the children in the indigenous population may not have as much immunity towards. There’s also evidence to suggest that the incomers may also lack immunity to the infections in their new community. As a result, in very rare cases, a child’s response to an (unknown) infection is believed to be a step on the road to leukaemia, probably due to stress on the immune system.

It’s worth emphasising that this is rare. Before people start panicking about every sniffle their child gets, it’s important to remember that although ALL is the most common childhood cancer, it is very rare. Around 370 children are diagnosed with ALL every year in the whole of Great Britain. That’s an annual incidence rate of about 36 children in every million.

Latest findings
New results from the Childhood Cancer Research Group at the University of Oxford, now add further weight to this idea. Led by Charles Stiller, the researchers looked at the incidence of ALL among children under the age of 15 in England and Wales. Then they looked at data from the 1991 national census, which reveal the extent of population mixing and therefore act as an indication of the likely influx of new infections. The team also took into account the levels of social deprivation in different areas.

After some intensive number-crunching, the team found that the incidence of ALL was higher in children aged 1-4 from rural areas that had recently welcomed incomers from further afield. They also found that young children in rural populations were at slightly greater risk, as well as those from more affluent families.

But what does it all mean? The analysis of all the results supports the idea that at least one form of ALL (a type called precursor B cell ALL) may be due to an unknown but common childhood infection, and those at greatest risk are children who have been in social or geographical isolation early in life, but are then exposed to new infections.

The researchers suggest that children in such populations have a lower than average ‘herd immunity’ to the infection, so are more likely to have an exceptional response to it. The results also explain the increase in risk seen in affluent rural populations, who tend to be isolated in geographical terms, but also move around the country.

In summary
Although infections can’t explain all cases of childhood cancer, there is certainly growing evidence that it may play a role in a small proportion of cases. For example, a group of Cancer Research UK scientists have found a link between childhood brain cancer and infection.

There’s also research to show that regularly mixing with other children (for example at nursery) may help to reduce the risk of childhood cancer (for example, in the case of lymphoma, although the evidence isn’t clear for ALL)- presumably by exposing them to a wide range of infections on a regular basis. And other researchers have suggested a role for allergies (caused by an overactive immune system) in causing ALL.

We need to bear in mind that this research actually tells us more about the likelihood of infection playing a role in childhood leukaemia, rather than highlighting specific risks for individuals. This is certainly not about the tabloid-esque notion of “people moving into our towns and causing cancer”, as the increase in risk could apply to children moving into a neighbourhood, as well as those with parents who are commuters or travelling workers.

Finally, we still don’t know which infections might be involved, or the background against which they occur. What are the bacteria or viruses that are causing cancer in this specific group of children? And if they are widespread (as they presumably are), why do they only cause cancer in such a relatively small number? Is it better to send children off to nursery for the immune boosting onslaught of a plethora of bugs, or to keep them at home? For the moment, the answers to these questions remain a mystery.

Kat