Child leukaemia linked to reduced exposure to germs

Tuesday May 22 2018

Why is leukaemia in the news?

"Childhood leukaemia is preventable and likely caused by keeping babies too clean, according to a landmark study," the Daily Mirror reports. This is somewhat of an oversimplification of a review that presented the hypothesis that the most common form of child leukaemia (acute lymphoblastic leukaemia, or ALL) may be caused by a 3-stage process:

  • an initial genetic mutation that occurs when the child is still in the womb
  • reduced exposure to infection in the first few years of life resulting in the immune system not developing fully
  • in those children with both of these events, a subsequent infection triggering other genetic changes that lead to ALL

This hypothesis is written by Professor Mel Greaves, who has spent decades studying the causes of leukaemia.

What is acute lymphoblastic leukaemia?

Leukaemia is a cancer of the blood cells. Acute lymphoblastic leukaemia, or ALL for short, is a cancer affecting the white blood cells that fight infection, called lymphocytes. ALL is called "acute" because it can develop quickly. Underdeveloped lymphocytes that don't function properly grow and multiply very quickly, building up in blood, the lymph nodes and spleen (the channels and glands that help fight infection). When the body is producing high numbers of these abnormal cells, it can't produce enough normal white blood cells, red cells and platelets.

This can cause problems like:

  • susceptibility to infections
  • anaemia
  • a tendency to bleed more than normal

How is ALL treated?

ALL is the most common form of childhood leukaemia, but it is still rare in general terms. In the whole of the UK around 800 people are diagnosed with ALL each year. The condition would usually be fatal if untreated, but about 90% of those affected can be cured by chemotherapy.

Researchers hope that by better understanding the causes of ALL, they may be able to take steps to prevent it.

What does the article say causes ALL?

The article reports that up to now, the main thing known to increase risk of ALL is exposure to high amounts of radiation, such as the levels people were exposed to after the use of atomic bombs in Japan.

Many other everyday things in the environment have been suggested to potentially contribute to causing ALL. However, studies on these potential risk factors have not always found strong or consistent evidence of a link.

The author discusses the evidence relating to a number of potential risk factors for one type of ALL called B cell precursor ALL (BCP-ALL). In this case the abnormal cells are early-stage cells that would normally go on to produce a type of lymphocyte called B cells. These are the cells that make antibodies. He concludes that genetic factors and the lack of exposure to infections in early life are 2 of the things that affect a child's risk of BCP-ALL.

How do genetic factors affect risk?

The author discusses his and other's research that suggests some children develop genetic changes in their blood cells while still in the womb, which predispose them to developing BCP-ALL. Researchers don't know why some foetuses develop these changes but not others.

What about infections: how might they cause ALL?

Over time, researchers developed the idea that the immune system needs exposure to infections in early life to "learn" to function properly. If this doesn't happen, when the child is exposed to common infections later the immune system could react abnormally. In children with the genetic changes that predispose them to BCP-ALL, this abnormal reaction could lead to them developing the condition.

What evidence suggests lack of infections causes ALL?

A number of studies have looked at whether common infections in early life, or factors linked with exposure to infections – such as attending day care or living with an older sibling – are linked to risk of ALL.

Several (but not all) of these studies are reported to have found a link. For example, some studies have found that:

  • children who develop ALL are less likely to have gone to day care early in life than those who do not develop the condition
  • children who have older siblings are less likely to get ALL
  • children who are delivered by caesarean section (who are not exposed to microorganisms by passing through the birth canal) are more likely to get ALL
  • children who are breastfed for at least 6 months are less likely to get ALL

However, not all of the evidence agrees with the theory. One study, for example, found that children who went on to develop ALL had more infections recorded in their medical notes than those who did not have the condition. The author suggests that this might be because mild infections may not be recorded.

It is also worth bearing in mind that these types of observational studies can suggest a link, but cannot conclusively prove on their own that lack of infection causes ALL.

Can I reduce my child's risk of acute lymphoblastic leukaemia?

The evidence isn't well enough established to suggest definitive recommendations that will reduce risk of child leukaemia.

Media suggestions, such as The Times' claim that children should be made to "play in the dirt" are not guaranteed to be effective. Another suggestion is that very young children should be encouraged to mix with other children; possibly at nursery or day care.

Obviously, parents make their decisions about childcare based on a number of factors. They should not worry that they are endangering their child by not sending them to a nursery or day care.

And while most childhood infections are benign, some less common ones bring their own set of serious risks. So this review certainly shouldn't be taken as an encouragement to ditch important methods of infection prevention, such as frequent handwashing and making sure your child's vaccinations are up to date.

The author himself notes that the cause is partly genetic, and it's not appropriate for people to send children to nurseries early in life (before the age of 1) just to try and reduce leukaemia risk.

The author is quoted on the Cancer Research UK science blog as saying: "Parents are in no way to blame for this" and “If their child has an accidental mutation in the womb, that's nobody's fault.”

He suggests that once we know more about the link between infection and ALL, a better option may be for researchers to develop a vaccine that could mimic early exposure to relevant infections.

Analysis by Bazian
Edited by NHS Choices