By Nina Agrawal

Most cases of stomach cancer are caused by bacteria. A majority of cervical cancers, as well as some genital and oral cancers, are caused by a virus. And certain chronic viral infections can lead to liver cancer.

Infections like these account for an estimated 13% of all cancer cases globally, according to a new report published Wednesday (18) by the American Association for Cancer Research. But knowing which infections can lead to cancer means scientists also have a good idea of how to prevent them from ever getting that far: There are effective vaccines and medications to prevent and treat these infections, and they can be detected early through screening.

Dr. Michael Pignone, a professor of medicine at the Duke School of Medicine and member of the steering committee that oversaw the report, said the progress made in preventing and treating these four infections, among others that can cause cancer, was one reason for highlighting them. We are now close to “turning what would have previously been some common cancers into rare diseases,” he said.

Human papillomavirus

There are more than 200 types of Human papillomavirus (HPV), including a dozen that significantly increase the risk of cervical, genital and oral cancers.

Most people infected with HPV will clear it on their own. But about 10% of women with HPV infection in the cervix will develop a persistent infection with a high-risk type. This can cause cells to replicate rapidly and inactivate proteins that suppress tumours, said Denise Galloway, scientific director of the Pathogen-Associated Malignancies Integrated Research Centre at Fred Hutch Cancer Centre in Washington.

Most sexually active people will be infected with human papillomavirus at least once in their lives. Using condoms can protect against infection with HPV, though not fully. Vaccination offers the strongest protection.

“If you vaccinate someone who’s young, the risk goes down to zero,” Galloway said.

The Centres for Disease Control and Prevention recommend two or three doses of the HPV vaccine starting around age 11 or 12 and through age 26. Some older adults may also choose to get the vaccine.

But research has shown that many young people who are eligible for the shots haven’t received them.

“Increasing vaccination rates is the most important long-term strategy,” Pignone said. Early detection is also essential to treating cellular abnormalities caused by HPV before they turn into cancer. Doctors can look for an HPV infection with a vaginal or cervical swab. Many people are tested for HPV at the same time as a Pap smear.

Hepatitis B and C 

These viruses primarily lead to cancer by causing inflammation in liver cells, said Dr. Sunyoung Lee, a gastrointestinal medical oncologist at MD Anderson Cancer Centre in Texas. Chronic inflammation leads to a build-up of scar tissue in the liver, called cirrhosis, which is a strong risk factor for cancer. In certain cases, hepatitis B can also directly cause cancer by altering healthy liver cells, Lee said.

Hepatitis B and C can both be transmitted through contact with blood, semen or other bodily fluids. In the United States, hepatitis C most commonly occurs among intravenous drug users who share contaminated needles.

Hepatitis B can commonly spread from a mother to her baby. The virus is more common in East Asia — China, Japan, South Korea and Vietnam — and among Asian patients in the United States who became infected via their mothers at birth, Lee said.

Doctors can detect both infections with blood tests.

There is a highly effective vaccine against hepatitis B, and it has been recommended to vaccinate infants against the virus since 1991. Adults up to age 60 and those of any age who have certain risk factors should be screened and vaccinated if they haven’t already.

There is no vaccine for hepatitis C, but not sharing needles is the best way to help prevent the risk of infection.

Antiviral medications can cure hepatitis C, Lee said. But patients often go untreated for years — either because they don’t realize their infection is serious and requires treatment, or because they lose contact with the health system.

Lee always asks patients when their hepatitis was diagnosed, he said. Some tell him it was 20 years ago. That prolonged exposure can lead to liver damage and put patients at a much higher risk of liver cancer.

Hepatitis B can range from an acute, mild infection to a chronic infection. These persistent infections require treatment, including with antiviral medications and, in some cases, interferon, a protein that helps the immune system fight off infections.

Because hepatitis B is commonly transmitted from mother to child, pregnant women should be tested, Lee said.

H. Pylori

H. pylori infections are very common: About half the world’s population carries the bacteria. But only 1% to 3% of them will develop cancer. Scientists aren’t completely sure why that is, or how the bacteria actually causes cancer, said Nina Salama, senior vice president of education at the Fred Hutch Cancer Centre who has studied H. pylori.

The bacteria are found in saliva, the plaque on teeth and faeces. Infections commonly occur in childhood through close family contact or crowded living quarters, Salama said, and most people are asymptomatic.

The infection produces chronic inflammation in the stomach lining, which promotes cancer, Salama said; the bacteria also bring toxic proteins into cells that can cause mutations. The strain of the bacteria and a person’s genetics can also play a role.

The best way to prevent spread within families is to avoid sharing food utensils, drinking glasses and toothbrushes when possible, Salama said. Washing hands well with soap and water for at least 20 seconds before preparing food or eating, as well as after using the bathroom, will kill the virus.

The United States does not routinely screen for gastric cancer, Salama said. But people with stomach ulcers, stomach pain or bloody stools should be tested for the bacteria.

Doctors treat H. pylori infections with antibiotics and also often prescribe drugs that reduce acid and protect the stomach lining, she said.

-New York Times