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Stanford Report, January 12, 2000

Clues to how stomach bacterium spreads found in unsavory places


Stanford researchers are searching in some unsavory places to solve the riddle of how a common bacterium that causes stomach ulcers and perhaps some kinds of stomach cancer jumps from one human to another. The most obvious places the bacteria may be hiding are the vomit, feces and saliva of infected people.

"One of the real dilemmas of this organism is how it gets into people; how they get infected," said Julie Parsonnet, MD, associate professor of infectious diseases and geographic medicine in the department of medicine and of epidemiology in the department of health research and policy. "We want to figure out how it's transmitted, and the first step is to see how it's getting out of people. Since it lives in the gut, we presume it's getting out through the mouth or in stools," Parsonnet said.

Sixteen healthy people infected with Helicobacter pylori, which is carried by 50 percent of the population, volunteered for Parsonnet's study. Like the majority of those carrying the bug, these people showed no signs of infection, but evidence of the bacteria in their breath and antibodies in their blood confirmed H. pylori's presence. Ten people who tested negative for the bacteria were also studied as controls.

The volunteers were admitted to Stanford's General Clinical Research Center for 24 hours, and Parsonnet and her team collected all manner of biological samples for analysis.

The researchers first screened normal feces from each of the 26 volunteers. Helicobacter thrives in the stomach's acidic surroundings, but its survival outside this environment is limited. Normal stools can take days to traverse the colon and, not surprisingly, the researchers found no live bacteria in these samples. But, when given a substance that induced diarrhea, 11 of the 16 infected people passed stools from which the bug could be grown.

Parsonnet believes that diarrheal stools are expelled so quickly from the small bowel and stomach that live bacteria can be transported into the external environment. According to Parsonnet, H. pylori can survive outside the body for a few hours, which may be sufficient time to infect anyone who contacts the sample.

The researchers also gave the infected people, and one uninfected volunteer, a substance to make them vomit. The results showed that vomiting is a potent way of discharging millions of infectious H. pylori into the environment. The researchers found live bacteria in samples from all of the infected individuals. The concentration of Helicobacter in vomit was also much greater than in feces.

Transmission of H. pylori by this route supports what scientists already know about Helicobacter infection. Close living quarters, the presence of many siblings and poor household hygiene are factors known to increase the risk of infection. Children, who tend to vomit and "spit up" more frequently than adults, are particularly likely to transmit the bug to other children, said Parsonnet. "Imagine children in day care together; one throws up and another child is right next to him or her. Or contaminated toys are not cleaned up properly," she said.

Infection during childhood leads to more opportunities to pass the bug on to others and also increases the likelihood of serious disease, said Parsonnet. Although many people carry the bug without symptoms, a combination of factors -- such as the particular strain of H. pylori and the person's diet and genetics -- can lead to ulcers and/or stomach cancer. Infection at an early age gives the bacteria more time to wreak havoc.

Analysis of air and saliva samples completed Parsonnet's battery of tests. Although the researchers could detect traces of bacteria in the saliva of approximately half of the people carrying H. pylori, mouth-to-mouth transmission is not considered a primary route of infection. Sampling the air surrounding infected volunteers confirmed the pivotal role of vomiting in the potential spread of the bacteria. The researchers found no evidence of the bug until the onset of vomiting, when air sampled from approximately a foot in front of the volunteers yielded Helicobacter that had been exhaled in tiny aerosol droplets.

Scientists who study H. pylori around the world believe that the rate of infection is declining, and the results of Parsonnet's study suggest that a reduction in gastrointestinal diseases that cause vomiting and diarrhea may be an important cause of that decline. "H. pylori is very common in developing countries where diarrheal diseases are common," said Parsonnet. "Less child-to-child contact and better hygiene means that opportunities for getting diarrheal disease and transmission of H. pylori are decreasing."

Parsonnet is continuing her research with a follow-up study to determine if H. pylori is shed more readily from people with low levels of acid in their stomach. She also plans to find out if some strains of H. pylori are more transmissible than others.

The results of the study were published in the December 15 issue of the Journal of the American Medical Association. Thomas Haggerty, a research technician, and Haim Shmuely, MD, a visiting scholar in Parsonnet's lab, were co-authors of the study. Volunteers were paid $250 for participating. Funding for the study was provided by the National Institutes of Health and the Centers for Disease Control and Prevention. SR