A study of Northern California children finds that diagnoses of the digestive disease ulcerative colitis increased nearly three-fold between 1996 and 2006 - for reasons that are not yet clear.

Ulcerative colitis is one of the two major forms of inflammatory bowel disease (IBD), a group of conditions marked by chronic inflammation in the intestines, leading to symptoms like abdominal pain and diarrhea. The other major form is Crohn’s disease.

It’s estimated that about 1 million Americans have IBD. However, when it comes to children, relatively little has been known about the epidemiology of IBD - including its incidence rate and how that rate may be changing over time.

In the new study, researchers looked at IBD diagnoses among children age 17 and younger who were members of the large Kaiser Permanente health plan in Northern California.

They found that the rate of colitis increased from 1.8 cases per 100,000 children in 1996, to 4.9 per 100,000 in 2006. There was also an increase in Crohn’s disease diagnoses - from 2.2 to 4.3 cases for every 100,000 children - but the difference was not significant in statistical terms.

The study, published in the Journal of Pediatrics, cannot pinpoint the reasons for the colitis increase. But one factor may be the changing demographics of Northern California, according to lead researcher Dr. Oren Abramson, a pediatric gastroenterologist at Kaiser Permanente in Santa Clara.

Over the last 20 years, the region has seen a significant increase in the proportion of Asian and Hispanic residents. In this study, Asian and Hispanic children accounted for a growing share of ulcerative colitis cases over time; and in contrast to white children - who were affected by colitis and Crohn’s in roughly equal proportion - Asian and Hispanic children were more likely to have colitis than Crohn’s.

In an interview, Abramson said that more studies are needed to see whether the “Westernization” of Asian and Hispanic immigrants is playing a role in children’s risk of colitis.

It’s not clear how many Asian and Hispanic children in this study were from immigrant families, but immigration in general has accounted for a large share of California’s population growth in recent years, Abramson and his colleagues note.

Abramson also pointed out that a few studies have found evidence that certain immigrant groups’ risk of IBD goes up after they leave their home countries - including South Asian immigrants to the UK and Mexican immigrants to the U.S.

More studies, Abramson said, are needed to see whether changes in diet, lifestyle or environmental exposures are involved in that trend.

Such research, he added, should help in identifying the environmental factors important in IBD in general.

The precise cause of IBD is not known, but it is thought to involve an immune system overreaction that injures the body’s own intestinal tissue. In general, experts suspect that genetic susceptibility, coupled with some type of environmental trigger, cause IBD.

While the current findings point to an increase in the number of children being diagnosed with ulcerative colitis, it is not clear how widely relevant the figures might be. But, Abramson said, “I think they would be relevant to (U.S.) areas with similar demographics.”

He added that the findings suggest that pediatricians should give more consideration to the possibility of IBD in Asian and Hispanic children with symptoms like abdominal pain and diarrhea. In general, white Americans have higher rates of colitis and Crohn’s than minorities, and minority groups have been traditionally seen as “low-risk.”

Factors other than demographics are also likely involved in the increasing colitis rate seen in this study, Abramson said.

One interesting possibility, he noted, is the decline in smoking since the 1990s. Studies have linked smoking to a lower risk of colitis, with some research suggesting a protective effect of secondhand smoke as well.

It’s also possible that greater awareness of IBD among pediatricians is boosting diagnoses, Abramson said - though that, he added, would not explain why colitis diagnoses are rising, while Crohn’s cases have remained relatively steady.

SOURCE: The Journal of Pediatrics, online April 19, 2010.