ATLANTA — An unexpected big drop in new U.S. tuberculosis cases is probably because of stepped up screening and treatment of immigrants before they leave their native countries, health officials say.

America's TB rate dropped more than 11 per cent last year, the largest single-year decrease in more than 50 years of federal record keeping, the Centers for Disease Control and Prevention reported this week. TB rates have been falling, but generally about 4 per cent a year.

The drop was so steep that some experts are cautious about celebrating. While testing overseas may explain the decline, underdiagnosis and undertreatment could also be factors, the CDC said. A final report is due in the fall.

"This is provisional data and still needs to be scrutinized," said Dr. Michael Leonard, an Emory University infectious diseases specialist.

Screening changes made in 2007 call for more complete testing of prospective immigrants suspected of having TB. They also require screening of children younger than 15, who previously were not tested.

If TB is diagnosed, the changes ensure that immigrants receive at least six months of treatment and are no longer infectious before coming to the United States.

The CDC is trying to analyze immigrant infections before and after the 2007 changes.

Tuberculosis is caused by bacteria that usually attack the lungs, and is spread through the air when an infectious person coughs or sneezes. If not treated properly, TB can be fatal.

It once was a major cause of death and illness, and in the late 1800s killed one out of every seven people living in the United States and Europe. But the development of antibiotics and public health efforts succeeded in treating infections and tracking down those they infected.

Last year, 11,540 U.S. cases were reported, down from 12,904 the year before. About 60 per cent of the cases were immigrants, legal and illegal. About half were from just four countries -- Mexico, the Philippines, India and Vietnam. Cases from those countries have been falling.

The TB rate fell to 3.8 cases per 100,000 people, from 4.2 cases reported for 2008.

Health officials looked at other possible explanations for the overall decline, and discounted several of them.

For example, budget cuts at local health departments could mean that fewer cases are being detected and treated. But in the past, cuts in public health spending have caused TB rates to go up -- not down.

More aggressive measures to identify and stop the spread of TB may be paying off, said Dr. Carla Winston, a CDC epidemiologist who was lead author of this week's report.