One in eight global cancer cases linked to infections, new study finds

One in eight new cancer cases diagnosed worldwide in 2024 were caused by infectious agents, according to new research published Monday.

The study, published in The Lancet Oncology, linked an estimated 2.3 million new cancer cases, 12 percent of all new cases, to viruses, bacteria and other microbes.

"There's been new infectious agents identified in this time as being cancer-causing, and even the infectious agents that we knew about in the past, the evidence has increased about the number of different cancers that they can cause," said study co-author Gary Clifford, a cancer epidemiologist based in Lyon, France.

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The research was conducted by a team at The International Agency for Research on Cancer, an arm of the World Health Organization.

The bacterium Helicobacter pylori, which increases the risk of stomach cancer, accounted for the largest number of cases globally at 760,000, predominantly in eastern Asia.

Human papillomavirus followed close behind, linked to nearly 750,000 new cases globally, with high rates in sub-Saharan Africa. HPV causes 100 percent of cervical cancer cases and also anal, vulvar, vaginal, penile and some head and neck cancers.

Hepatitis B virus and hepatitis C virus were also linked to a significant number of liver cancer cases.

For the first time, researchers linked even more cancers to infections, expanding their list to include 16 additional ones.

Some of those new infectious agents included HIV and its link to cervical cancer, and a very rare virus called Merkel cell polyomavirus that can lead to an aggressive form of skin cancer.

Another significant shift is the recognition of the role of Epstein-Barr virus, linked to about 260,000 cases globally in 2024.

"That's quite a big burden of cancer caused by Epstein-Barr virus that was not counted in the past, but is now recognized," Clifford said.

Epstein-Barr virus can lead to nasopharyngeal cancer, some gastric cancers and Hodgkin's lymphoma, and is also a key risk factor in developing multiple sclerosis.

"The fact that we're seeing Epstein-Barr has caused all these chronic diseases, it's increasingly becoming a target that people need to focus on for prevention," he said.

The global report highlights vast regional differences, mostly driven by historical disparities.

High-income countries that established cancer screening and vaccination programs over the last few decades are doing much better compared to lower- and medium-resourced countries where access to prevention and treatment is limited.

"We really still have a lot of work to do implementing things that we know work and we've known for a long time but haven't really made it into all populations in the world," Clifford said.

In North America, the burden of cancer-causing infections is much lower, but H. pylori and HPV are the most common sources.

In Canada, a 2019 study estimated the rate of cancers attributable to infections was four per cent.

"It tells us how important each of the 12 infections are," said Karena Volesky, an epidemiologist in Montreal who led the Canadian research team while at McGill University.

Her study estimated these infections lead to more than 7,000 cancer diagnoses each year in Canada.

"HPV is certainly an infection that we're keeping an eye on in Canada," said Elizabeth Holmes, the director of health policy at the Canadian Cancer Society.

Holmes said cervical cancer rates had been steadily declining in Canada due to robust childhood vaccination programs, but plateaued last year.

"That's certainly a concern, that we might not meet our goal of elimination of cervical cancer by 2040 unless those rates start to decline again," she said.

Human papillomavirus is transmitted through sexual contact, but the HPV vaccine protects against the most common forms of infection that can cause cancer.

Canada's cervical cancer elimination plan aims to increase vaccination rates to 90 percent, increase screening rates to 90 percent and ensure timely followup care after screening.

Holmes said only about five provinces are currently above 80 percent vaccination coverage, with at least one as low as 66 percent.

Helicobacter pylori a 'horrible disease'

The infection leading to the most cancer cases is Helicobacter pylori, a bacterium first discovered in the early 1980s that mainly leads to gastric cancers.

While the research found the highest burden in eastern Asia in countries like Japan, South Korea and China, it's a problem in Canada too.

"It's a horrible disease to die from," said Dr. Paul Moayyedi, a leading researcher and gastroenterologist at McMaster University.

About 2,000 Canadians die every year from stomach cancer.

Moayyedi wants to see a universal screening program for the bacterium, which can be detected through a blood or breath test, stool sample or endoscopy and treated with a combination of antibiotics over a two-week period.

Helicobacter pylori is associated with crowded living conditions and poor sanitation in childhood.

While most people with it can live a long life without any issues, if left untreated, it can cause ulcers and stomach cancer for some.

Moayyedi said stomach cancers may not be seen as high priority compared to other types of cancers, but believes screening programs for those at high risk, such as people with a family history of stomach cancers or First Nations communities, would save lives.

"In consultation with the elders of local communities it would be important to at least offer those groups screening if they would want it," he said.

With files from CBC News