Along with smallpox and measles, it appears that the Spanish Conquistadors brought the bacterium that causes stomach ulcers to the New World, according to a genetic study.
Comparing pieces of DNA from Helicobacter pylori, scientists discovered that strains in Peru resemble those from Spain, rather than those in eastern Asia. Their finding suggests that ulcers may not always have been part of the human condition.
Douglas Berg, a member of an international team that analyzed more than 500 strains of the bacterium on five continents, said:
"My favorite interpretation of this is that the Spanish brought H. pylori to Peru when they conquered the Inca empire nearly 500 years ago, and that the bacterium was not present in the Inca's ancestors who crossed the Bering Strait from Asia more than 10,000 years ago."
Berg is a professor of genetics at the Washington University School of Medicine in St. Louis. His team's results are being published in the June issue of the Journal of Bacteriology.
The bacterium is carried in the stomachs of more than half the world's population. While some suffer no ill effects, others develop peptic ulcer disease.
Until now, scientists had assumed that humans began carrying H. pylori far back in evolutionary history. "People haven't paid any attention to the possibility that human H. pylori infection might have become widespread only in more recent history," Berg said.
Researchers focused on a few highly variable genetic regions in the bacterium and classified them into five DNA blueprints.
One motif was most common in strains from Spain, Peru, Guatemala and South Africa. Type 2 was most common in Chinese and Japanese strains, while a third type was predominant among Asian Indians. All three motifs were common in bacterial isolates from northern Europe. A rare fourth variation was found only in one English strain and in two strains from West Virginia. And the last DNA scheme turned up in just a few Asian Indian strains.
"So we know we can type H. pylori strains from different societies by differences in their DNA," Berg said. "One of the most striking differences was between strains from eastern Asia and strains from Amerindians in Peru."
Berg argues that humans only became widely infected with H. pylori when people shifted from hunting and gathering to agricultural societies that brought them into closer contact with animals.
"Many other diseases, like tuberculosis, whooping cough, mumps and chicken pox, are of animal origin and probably came into the human population when our ancestors started to practice agriculture and when their population densities increased," he said.
The genetic differences in H. pylori strains probably reflect different animal hosts that spread the disease to different populations of humans. For instance, European strains might have come from mice or sheep, while Asian strains could have originated in pigs, cats or Mongolian gerbils, Berg said.
For more information about H. pylori and ulcers on the Web: check the American Gastroenterological Association site: http:www.gastro.org.