TUESDAY, July 22 (HealthDay News) -- In 2006, 57 percent of all tuberculosis cases in the United State were among foreign-born individuals and, as time passes, that population is under increased threat from the disease, a new study warns.

Between 1993 and 2006, the total number of cases of TB in the United States decreased by 45 percent -- from more than 25,000 to less than 14,000. However, that decline occurred primarily among people born in the United States, note researchers at the U.S. Centers for Disease Control and Prevention.

"The number of cases among U.S. born individuals has been reduced by 66 percent whereas, among the foreign-born population, during the same period, the number increased by 5 percent, thus widening the disparity between the two groups," said study author Dr. Kevin P. Cain. His report appears in the July 23/30 issue of the Journal of the American Medical Association.

Needless to say, experts are anxious to find ways to control and eliminate the disease. Testing the more than 37 million foreign-born individuals currently living in the United States isn't feasible, so officials are focusing their efforts on people before they immigrate.

In the study, Cain's team reviewed 46,970 cases of TB among foreign-born individuals in the United States. They discovered that an immigrant's risk for TB was highest during the early years after first arriving in the United States, varied with age (older people had a higher risk), and also varied by country of birth.

"We found that over 50 percent of all cases of TB among foreign-born persons occurred among 20 percent of the overall foreign-born populations, especially persons born mainly in Southeast Asia [particularly the Philippines and Vietnam] and sub-Saharan Africa," said Cain, a medical epidemiologist with the division of tuberculosis elimination at the CDC. "This is actually quite helpful because it means you can try to prevent as many case as possible by focusing your efforts on the highest-risk population," he said.

Also, adding a sputum culture test (actually growing TB in culture) to the standard sputum smear (looking at sputum under a microscope) would have prevented the importation of about 250 cases of TB a year (there are about 7,000 TB cases among foreign-born persons annually in the United States).

"This is a nice, balanced study that gives a good perspective about what we could do potentially to reduce the incidence of TB in the U.S.," said Jeffrey Cirillo, an associate professor of microbial and molecular pathogenesis at Texas A&M Health Science Center College of Medicine, in College Station. "This is focusing us in the right direction."

Still, Cain acknowledged, "That is a relatively small piece of the overall problem, so clearly more needs to be done."

According to Cirillo, one-third of the world's population is infected with TB and a person dies from the respiratory illness every 15 to 20 seconds. Nine million people become infected with the TB bacterium each year, and 2 million die.

"We recognize national borders but microbes, including TB, don't," Cain said. "As long as TB is out of control globally, as it is, this will be a problem in the U.S. If you breathe, you're at risk for TB. This can affect anyone."

A second paper in the same issue of JAMA explored the difficulty of tackling TB in vulnerable, HIV-positive populations in "resource-limited" areas of the world.

The authors, from the University of California, San Francisco, proposed several strategies, including intensified efforts to identify HIV-positive patients with TB; treating individuals with active TB; making the antibacterial drug isoniazid and antiretroviral therapy more available; implementing TB infection control; and making sure health-care workers record and report TB cases.

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