There are genetic risk factors behind post-infectious irritable bowel syndrome, say researchers studying residents sickened by tainted water in Walkerton, Ont.

In 2000, seven people died and 2,300 suffered symptoms including bloody diarrhea after the drinking water in the southwestern Ontario town was contaminated with E. coli and Campylobacter jejuni, another water-borne bacteria that causes fever, headache, muscle pain, vomiting and diarrhea.

Of the 2,300 people who were sickened, 36 per cent developed post-infectious irritable bowel syndrome or PI-IBS.

Patients suffer from chronic abdominal pain and discomfort, bloating, and defecation problems that develop suddenly after an episode of acute gastroenteritis or inflammation of the stomach and intestines. Doctors diagnose PI-IBS after ruling out structural and biochemical abnormalities and other known disorders like Crohn's disease or ulcerative colitis.

The sheer size of the outbreak and follow-up health studies on residents gave researchers a unique opportunity to study genetic factors in the development of PI-IBS, said Alexandra-Chloé Villani, a research associate at McGill University in Montreal.

"We were wondering how come 64 per cent didn't develop anything and 36 per cent came to develop this syndrome," said Villani.

When the 228 cases and 581 controls were compared, three genes were linked to an increased risk of PI-IBS, the team reported in the March edition journal Gastroenterology.

The genetic risk markers all slightly increase or decrease the probability of developing IBS. But the markers can't be used to predict which individuals will get the condition or recover from it, said principal investigator Dr. John Marshall, a gastroenterologist at McMaster University in Hamilton, Ont.

The genes encode proteins involved in the barrier and immune system roles played by the intestines, the researchers said.

In 2008, researchers reported that Walkerton residents who fell seriously ill were at higher risk of permanent kidney disease.

The study was funded by the Crohn's & Colitis Foundation of Canada and the Ontario Ministry of Health and Long-Term Care.

With files from The Canadian Press