Poorer kids have much lower risk
of leukemia
Children living in Canada's poorest neighbourhoods have a sharply lower risk of developing leukemia compared to their wealthier peers, according to an intriguing new study.
While the research does not reveal why poverty provides a shield against the most common childhood cancer, researchers suggest the most likely explanation is the so-called hygiene hypothesis — the notion that being exposed to infectious agents early in life bolsters the immune system.
“In a poorer neighbourhood with crowded conditions, there may be more exposure,” Dr. Marilyn Borugian, a senior scientist at the B.C. Cancer Agency, said in an interview.
She stressed, however, that this is just a hypothesis, and there may be other explanations.
“We know that income is not a cause of disease, but there are a lot of lifestyle and socio-economic factors that can influence risk,” Dr. Borugian said.
The research, published in today's edition of the medical journal Epidemiology, shows that children in Canada's poorest neighbourhoods have a 13-per-cent lower risk of developing leukemia than children in the country's richest areas.
Dr. Borugian noted that low income is usually associated with worse health outcomes. “Low income usually has a negative connotation, so this is a bit counterintuitive,” she said.
To conduct the study, the research team collected data on childhood leukemia cases from all provincial cancer registries between 1985 and 2001, and then, using the postal codes of patients, determined the income level of the neighbourhoods where they lived.
There were a total of 5,411 cases of leukemia — more than half of them in children under the age of 4.
Leukemia is a cancer of the early-forming cells, usually the white blood cells that are a key component of the immune system. Most childhood leukemia starts in the bone marrow and spreads to the lymph nodes.
The new research focused specifically on lymphoid leukemia.
Childhood leukemia is highly treatable, and more than 80 per cent of children are cancer-free five years after diagnosis and treatment.
The new study is not the first to suggest that children who get infections, particularly viral infections, at a young age, have a lower risk of cancer.
Previous research has shown that babies who attend daycare have lower rates of cancer.
There is also strong evidence that early exposure to germs reduces the risk of multiple sclerosis, asthma and allergies.
According to the hygiene hypothesis, the modern obsession with cleanliness and a sterile Western diet deprives children of exposure to common germs and that leaves their immune system weak and vulnerable as they grow.
What is not at all clear, however, is what sort or level of exposure provides protection, and at what age children need to be exposed to germs to benefit.
On average, 1,285 children are diagnosed with cancer in Canada annually, and there are 227 deaths. Leukemia accounts for one in four new cancer cases.
The study is the largest ever to look at the link between socio-economic status and a childhood cancer, but the B.C. Cancer Agency is currently doing a similar analysis involving other cancer sites, including brain cancer, bone cancer and Hodgkin's lymphoma.
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