The incidence of rheumatoid arthritis among American women has unexpectedly jumped in recent years, according to new research from Mayo Clinic investigators.

This is a very troubling finding. Rheumatoid arthritis is a chronic, painful and often disabling autoimmune disease that causes the body's joints to become inflamed and misshapen. It can also affect the heart and other internal organs. Rheumatoid arthritis is difficult to treat and has no known cure. Research suggests that people with rheumatoid arthritis tend to die at a faster rate than those without the disease.

"We were surprised to see the trend going up when it had been going down for 40 years," says Dr. Hilal Maradit Kremers, a Mayo Clinic epidemiologist and one of the authors of the new study, which was presented recently at the American College of Rheumatology annual scientific meeting in San Francisco. "We need to find out why."

Using Olmsted County data
According to the U.S. Centers for Disease Control and Prevention, rheumatoid arthritis affects about 1.3 million adults in the United States, and three-quarters of them are women.

This new Mayo study suggests, however, that the actual number of Americans with the disease may be significantly higher.

For the study, Kremers and her colleagues analyzed patient data from Minnesota's Olmsted County from early 1995 to the start of 2005. (Olmsted County was used because it has a stable patient population and medical data going back 50 years.) They then compared that data with that of the previous decade. They found that the annual number of women who developed rheumatoid arthritis had risen from 36 to 54 out of every 100,000 — a jump of 50 percent. The incidence for men, however, remained steady at about 29 per 100,000.

"We think it's the same in other parts of the country," says Kremers.

A medical mystery
No one knows why rheumatoid arthritis is striking an increasing number of women. "That's the next stage — to figure out what is driving this," says Kremers.

Research suggests that environmental factors — particularly a viral or bacterial infection — can trigger the onset of rheumatoid arthritis in people who are already genetically susceptible to the disease. A Swedish study published in October found that babies who experience serious infections during their first year of life were more than twice as likely as other babies to develop rheumatoid arthritis as young adults. Such early infections may alter the body's immature immune system in some as-yet-unknown way.

So is rheumatoid arthritis on the rise among women because of an increased exposure to infections? No one knows.

Given that rheumatoid arthritis is so much more prevalent in women than in men, scientists strongly suspect that hormonal factors are also involved. Indeed, some (but not all) studies suggest that women who take oral contraceptives are less likely to develop the condition. And women with rheumatoid arthritis often see their symptoms temporarily improve when they become pregnant, a time when their hormonal levels recalibrate. In addition, breastfeeding for at least one year has been found to reduce the risk of rheumatoid arthritis by about 20 percent.

Could more women be developing rheumatoid arthritis because they're having fewer babies or breastfeeding less or otherwise changing their hormonal profiles? Again, no one knows.

Arthritis and heart attacks
In another study, which was also presented at the ACR meeting, the Mayo Clinic researchers found that people with rheumatoid arthritis who had a heart attack were more likely to die from heart-related complications than arthritis-free heart attack patients.

"We didn't find any differences in treatment," says Kremers, "but we need to look in more detail." It may be that people with rheumatoid arthritis receive treatment that is subtly less aggressive because of their already poor health status, she says.

If you've been diagnosed with rheumatoid arthritis, doctors recommend taking steps to lower your risk of heart disease. It's the same routine as for everybody else: Eat a heart-healthy diet, maintain a healthy weight, exercise regularly, and quit smoking. (Smoking is a major risk factor for rheumatoid arthritis.) And be sure to get regular heart check-ups.