Bug out: Killing common bacteria may curb stomach cancer, studies show
This bug loves our guts — living silently in most folks, causing ulcers in others — but a new meta-analysis suggests that eliminating it entirely may help prevent stomach cancers.
The bacteria on the hit list is called Helicobacter pylori, or H. pylori — a strain that’s coexisted with humans for tens of thousands of years, residing today in the bellies of roughly two-thirds of all people. It’s also the primary cause of gastric cancers, the world’s fifth most common malignancy, experts say.
Permanently wiping out H. pylori with a brief blitz of two common weapons — antibiotics plus an acid-suppressing medication like Prilosec or Prevacid — may cut the risk of stomach cancers, asserts a paper published Tuesday in the Cochrane Library. The authors analyzed six clinical trials spanning nearly 6,500 patients who were treated for H. pylori.
“The findings of this systematic review and meta-analysis add to the increasing evidence that eradicating H. pylori in the general population has the potential to prevent gastric cancer,” said co-author Dr. Paul Moayyedi, who researches H. pylori testing and treatment at McMaster University in Hamilton, Ontario.
Among trial participants who were given one to two weeks of antibiotics and gastric-acid suppressing medicine, 1.6 percent later developed gastric cancer, the meta-analysis found. Among the participants who were given no treatment or a placebo, 2.4 percent later developed gastric cancer.
“International guidelines for the management of H. pylori infection may change as a result,” Moayyedi said.
Four of the trials reviewed by Moayyedi and colleagues were conducted in China and one was done in Japan. Those nations have two of the highest stomach cancer rates in the world.
Stomach cancer rates dipping
In the U.S., prevalence of stomach cancer has declined by an average of 1.5 percent per year for the past decade, making it the 16th most common cancer among Americans, reports the National Cancer Institute. In 2014, nearly 11,000 Americans were expected to die from the disease, while 22,000 people were expected to be diagnosed.
Risk factors for stomach cancer, the NCI says, include smoking, a family history, a diet high in salted or smoked foods, and a diet low in fruits and vegetables. But at the top of the NCI’s risk list: H. pylori infection.
“This study supports previous analyses that have found that antibiotic treatment to remove this risk factor can reduce a person’s gastric cancer risk, especially if treated early,” said Sarah Talarico, a researcher at Fred Hutchinson Cancer Research Center.
In fact, noninvasive tests for detecting and genotyping H. pylori will make it easier for scientists and physicians to screen populations at high risk of gastric cancer, determining antibiotic resistance and checking whether treatment was successful, Talarico said.
Current exams used to hunt for H. pylori in people include breath tests that require patients to swallow the human waste product urea, biopsies of the stomach lining, and blood draws that only show if someone has H. pylori antibodies, not a current infection. A stool test can detect H. pylori in feces.
Talarico has a patent pending for a noninvasive stool test that will provide genetic information about any H. pylori strain it locates, revealing, for example, if the bug has certain genes that boost the risk of gastric cancer, or if the strain is resistant to an antibiotic.
A spiral-shaped bacterium that’s long survived the gut’s harsh, acidic habitat, H. pylori is believed to spread mouth to mouth or through contaminated food or water. Infection is more common among people who live in highly congested areas with poor sanitation.
“I think eradicating Helicobacter is an excellent preventative strategy for gastric cancer,” said Dr. Nina Salama, a Fred Hutch microbiologist who studies H. pylori.
“The tricky part is doing effective eradication because H. pylori is difficult to treat — there’s increasing antibiotic resistance, and there’s no vaccine,” Salama said. “That’s the real challenge.”
And, there may be consequences to annihilating an organism with which humans have co-evolved. For example, there seems to be an association between the belly bug and a lower rate of esophageal cancer.
Does bug thwart another cancer?
People who tested positive for H. pylori had a significantly decreased incidence of aneuploidy — a measure of genomic instability that predicts someone’s progression to esophageal cancer —according to a study co-authored by Salama and fellow Fred Hutch researchers in June and published online in PLOS ONE.
Questions remain, the Hutch authors acknowledged, about whether the esophagus and the stomach are host to separate communities of microbes like H. pylori or if microbial communities in both locations are extensions of each other.
“So, without really knowing the mechanism, it’s hard to know whether it’s good or bad to eradicate H. pylori or not,” Salama said. “But the other thing to consider is that while esophageal cancer (specifically, adenocarcinoma), is a rapidly increasing cancer, it’s still a very rare cancer.
“Even in the U.S., a lot more people die of stomach cancer than esophageal adenoma. So, in places with high-risk populations — East Asia, Costa Rica, or Columbia — places that have really high incidence of gastric cancer, eradicating H. pylori is kind of a no-brainer.”