Genetic signature may lead to better TB diagnosis

A patient who tested positive for extreme drug resistant tuberculosis (XDR-TB) awaits treatment at a rural hospital at Tugela Ferry in South Africa's impoverished KwaZulu Natal province, in this photo taken October 28, 2006.

LONDON (Reuters) - Scientists have found a "genetic signature" in the blood of patients with active tuberculosis (TB) and believe their discovery could help develop better diagnostic tests for the disease, as well as better treatments.

More than 2 billion people, or a third of the world's population, are estimated to be infected with the organism Mycobacterium tuberculosis (MTB) which causes TB, but the vast majority have the infection in latent form and have no symptoms.

The British scientists said they had now found a pattern of genes in the blood which is specific to up to 10 percent of those 2 billion people who develop active TB in their lungs.

The genetic signature shows the extent of the disease in the lungs and disappears after successful treatment, they said in a study in the journal Nature Wednesday.

"Although people have been studying TB for more than a century, there is still a desperate need for better prognostic and diagnostic tests and more information about the body's response to MTB infection, which may also help in the design of vaccines and treatments," said Anne O'Garra, of the Medical Research Council, who led the study.

TB is one of the oldest diseases known to mankind and afflicts mostly the poor in developing regions such as sub-Saharan Africa, India and China. It is among the world's top 10 leading causes of death and killed 1.8 million people worldwide in 2008, or one person every 20 seconds.

Current drugs for TB are at least four decades old and must be taken for several months. Patients often fail to take the full treatment course, which has spawned drug resistant TB and made the disease more dangerous and more difficult to treat.

Doctors say current diagnostic tests for TB have barely been improved in the last 125 years.

BLOOD TEST?

O'Garra's study was conducted in London -- which has 40 percent of all British TB cases and where 3,500 people were diagnosed last year -- and then checked on a separate group of patients in Cape Town, South Africa, where TB is often found in people whose immune systems are weakened by the AIDS virus HIV.

Results showed that around 10 percent of those with latent infection also had the genetic signature for the active disease.

The scientists said it was too early to say yet whether this 10 percent would be the same 10 percent who are estimated to go on to develop active TB, but further studies were underway.

At this stage the findings are a significant step toward developing a blood test using the genetic signature to predict which people with latent TB will get sick, they said.

Robert Wilkinson, the director of Cape Town University's clinical infectious diseases unit, who also worked on the study, said such a test would enable thousands of people at risk of active TB to be diagnosed and treated earlier, and help doctors to avoid treating large numbers of patients unnecessarily.

"It's known that treatment for latent TB is effective and can contribute to TB control -- but the doctor's dilemma...is that...you are prescribing unnecessary treatment probably to 9 out of 10 people," he said in a briefing about the work.

"If there was a way of finding out who was most at risk, then that would greatly rationalize the treatment of latent TB."

(Editing by Nina Chestney)