New, Effective Way to Diagnose Latent TB
News May 15, 2014
The results of a research partnership between the University, the Royal Centre for Defence Medicine, based at the Queen Elizabeth Hospital in Birmingham, and the Liverpool School of Tropical Medicine will help to improve the UK’s ability to identify people entering the country who have been infected with the bacterium that causes TB.
Diagnosing latent TB is important because the bacterium can persist in the body like a “Trojan Horse”, only to emerge and reactivate months or even years later. Treating individuals with latent TB can prevent disease developing and so prevent them infecting others.
The research study centred on Nepalese soldiers recruited to the Brigade of Gurkhas of the British Army, who were tested for latent TB using all the tests currently available. One of these, called T-Spot, was found to be better than others at detecting latent TB when used on its own.
As a result, this test has been adopted by the Ministry of Defence (MoD) as a more efficient way to identify latent TB infections. The study’s authors say it offers a strategy for diagnosing this “silent infection”, which is notoriously difficult to detect, and could be used in civilian populations.
TB is caused by the bacterium Mycobacterium tuberculosis, and is spread through the inhalation of tiny droplets from coughs or sneezes of an infected person.
Most people acquire infections abroad and will not develop disease. However, about two billion people around the world carry the bacteria in their body and five to ten per cent go on to develop active TB.
While treatment can be effective, it lasts for six months and failure to complete it can result in resistance to drugs and the reactivation of infection, making cure more difficult. This makes diagnosis of latent infections a key strategy to control disease in countries like the UK.
More than 70 per cent of cases in the UK occur in people born overseas, often where TB is endemic, such as the Indian sub-continent and Africa. According to Public Health England, there are now about 9,000 new cases of TB a year in the UK – 6,000 of which affect people born elsewhere, in whom the incidence rate of TB is more than 20 times higher than among those born in the UK.
The study, published this month in the scientific journal PLOS ONE, was funded by the MoD and is co-authored by Adam Cunningham, Professor of Functional Immunity at the School of Immunity and Infection at the University of Birmingham and by Dr Matt O'Shea from Oxford and Birmingham Universities who is a Surgeon Lieutenant Commander in the Royal Navy, together with Dr Tom Fletcher from Liverpool University, who is a Major in the Royal Army Medical Corps, and Duncan Wilson, Colonel and Professor of Military Medicine at Birmingham.
“Every year, the UK recruits Gurkhas, a tradition that has been part of the British Army for more than 200 years,” said Prof Cunningham. “They are excellent soldiers, but they come from Nepal, a country where TB is endemic. Although they get screened for active TB before they arrive in the UK, this does not exclude the chance that they have latent TB.”
The study, carried out over several months in 2012, involved 166 Gurkha recruits – nearly the whole of that year’s cohort – and found that nearly 20 per cent had latent TB using the T-Spot test. There are more than 3,000 Gurkhas in the British Army at any one time.
“One of the problems with latent TB is that it is very tricky to diagnose,” said Prof Cunningham. “We have found this test is the best way for diagnosing latent TB in this group. This matters, not just for Gurkhas entering the British Army, but because about ten per cent of the UK’s soldiers come from TB-endemic countries. So it’s been policy-changing for the MoD.
“The other reason it’s important for the military is that if soldiers with latent TB become active, they can’t take part in operations. You also have to screen other soldiers who may have been exposed to an active case, which is costly, and may limit operations.
“It has a wider importance because the way the study was performed allowed a more accurate assessment than has been possible previously.”
At the moment, there are three tests used to detect TB: two called Interferon-Gamma Release Assays (IGRAs) are blood tests based around a molecule called IFNg.
“What we found was that one IGRA was more effective at identifying latent TB infected individuals than the other tests,” explained Prof Cunningham. “This commercially-produced test, called T-Spot, is a blood test that measures T cells specific to certain Mycobacterium tuberculosis antigens. It is cost-effective because individuals only require one visit to be tested.
“Because much of the TB burden comes from migrants, the test now offers a better way to assess whether people coming to the UK have latent TB.”