“One in six cancers caused by preventable infections,” the Daily Mail reported today. The story comes from a study estimating that, of 12.7 million new cases of cancer that occurred worldwide in 2008, about 2 million were caused by infectious diseases.
The infections that cause cancer include Helicobacter pylori (the bacteria that cause stomach ulcers), hepatitis B and C (viruses that cause inflammation of the liver) and human papilloma viruses (sexually transmitted viruses that cause several cancers, notably cervical cancer in women). In women, cancers of the cervix were estimated to account for about half of infection-related cancers and in men, liver and gastric cancers accounted for more than 80%.
This important study suggests that certain treatable infections are a significant cause of cancer worldwide. It implies that tackling these infections (particularly in developing countries) may be a more effective way to reduce the number of global cancer deaths than focusing on treatment for the cancers.
It is of note that the proportion of cancers attributable to infection varied widely according to region, for example, in Europe 7% of cancers were attributed to infection while in sub-Saharan Africa this figure was 32.7%. It should also be noted that the calculations used by the researchers to identify the scale of cancer attributable to infections might be imprecise, partly due to the scarcity of cancer incidence data in some countries.
In the UK, infections such as H. pylori can be treated with antibiotics, vaccination against human papilloma virus (HPV) is offered by the NHS for girls aged 12 and 13, and a vaccine for people considered to be at high risk of hepatitis B is available.
While infections play a role in the development of several cancers, it is important to remember that there are many risk factors that affect how likely you are to develop cancer. These include smoking, diet and family history.
Where did the story come from?
The study was carried out by researchers from the International Agency for Research on Cancer, France. It was funded by Fondation Innovations en Infectiologie (FINOVI) and the Bill & Melinda Gates Foundation.
The study was published in the peer-reviewed medical journal The Lancet Oncology.
Generally, the media reported the story accurately, although headlines focused on the more alarming global figure of one in six cancers caused by infection than the estimated UK figure of 3.1% (just over one in 30).
What kind of research was this?
This was a narrative review in which the researchers estimated the proportion of cancers that could be attributed to infection, both worldwide and within eight geographical regions.
The authors point out that infection is recognised as a major cause of cancer worldwide and that prevention and treatment of infectious agents has already had a substantial effect on cancer prevention. Their review is an update of a previous review carried out in 2002.
What did the research involve?
The researchers reviewed the infectious agents that have been classified by the International Agency for Research on Cancer as causing cancer in humans and the cancer sites with which they are associated. There are thought to be 10 infections that can cause cancer, including:
- H. pylori (stomach)
- hepatitis B and C (liver)
- HPV (cervix, penis and other sites)
- Epstein-Barr virus (lymphomas and nose/throat)
- human T-cell lymphotropic virus type I (T-cell leukaemia and lymphoma)
- human herpes virus type 8 (Kaposi’s sarcoma)
- Chinese and South Asian liver flukes (gall bladder and bile duct)
- Schistosoma trematode worms (bladder)
The researchers obtained estimates of the number of new cancer cases in 2008 using statistics from an established source, the Globocan 2008 report, which provides age-specific and sex-specific incidence for 27 cancers in 184 countries.
For each of these cancers, they calculated the “population attributable fraction (PAF)”. PAF is an estimate of the proportion of cases of a disease that could theoretically be avoided, either by protection against or treatment of a specific risk factor. For example, H. pylori can be treated with antibiotics before it leads to stomach cancer. PAF uses a formula that combines the size of the effect of a risk factor with the distribution of that risk within a population. The researchers used various sources to calculate the PAF, including studies on risk factors associated with these cancers and the prevalence of infection.
Using the PAF they calculated the number of new cancer cases attributable to infection in 2008 worldwide and in eight geographical regions:
- sub-Saharan Africa
- North Africa and West Asia
- Central Asia
- East Asia
- South America
- North America
What were the basic results?
- Researchers found that, of the 12.7 million new cancer cases that occurred in 2008, the population attributable fraction (PAF) for infectious agents was 16.1%, meaning that around 2 million new cancer cases were attributable to infections. This is the one in six figure quoted in the media.
- This fraction was higher in less developed countries (22.9%) than in more developed countries (7.4%), and varied from 4% in North America to 32.7% in sub-Saharan Africa.
- Helicobacter pylori, hepatitis B and C viruses, and human papilloma viruses (HPV) were responsible for 1.9 million cases of cancer, mainly gastric, liver and cervical cancers.
- In women, cervical cancer accounted for about half of the infection-related burden of cancer. In men, liver and gastric cancers accounted for more than 80%.
- Around 30% of infection-attributable cases occurred in people younger than 50 years.
How did the researchers interpret the results?
The researchers say that since infection-related cancers have high mortality rates, the proportion of cancer deaths attributed to infections is probably higher than 16.1%. They estimate that of the 7.5 million deaths from cancer in 2008, 1.5 million were caused by infections – roughly one in five cancer deaths worldwide.
Public health measures to prevent infections, such as vaccination, safer injection practice or antimicrobial treatments, could substantially reduce future burden of cancer worldwide, the researchers argue.
This important study highlights the potential role played by certain infections in causing cancer. It uses the highest quality available evidence to calculate the proportion of cancer caused by infectious agents, worldwide and by region.
However, as the authors point out, their calculations may be imprecise. For example, many countries have very sparse data on cancer incidence and the prevalence of risk factors for specific cancers. To obtain global estimates researchers had to extrapolate data from other areas. They also say they had to make certain assumptions, for example, that the risk of infection was constant across populations and sexes. They also point out that there was a lack of high quality data from some of the research sites in the studies.
In the UK, a vaccine against strains of HPV that cause cancer (as well as genital warts) is now offered to girls between the ages of 12 and 13. A vaccine for people considered at high risk of hepatitis B is also available. Helicobacter pylori is usually treated with antibiotics when diagnosed. These may all contribute to a further reduction in the proportion of cancers caused by infectious diseases in this country, which may be some way below that suggested in the headlines.
It’s important to bear in mind the other risk factors, such as smoking, diet and family history, that can contribute to your chances of developing cancer.