As many as 32,000 children worldwide fall ill each year with a drug-resistant "superbug" strain of tuberculosis, suggests a new study which, for the first time, has quantified rates of this difficult-to-treat form of TB.
The study by United States researchers also shows that as many as one million children become ill with TB each year - about twice the number previously thought; of these, only a third of cases are ever diagnosed.
About 40 per cent of TB cases were in Southeast Asia and 28per cent in Africa, the study shows.
"A huge proportion [of children] are suffering and dying from TB unnecessarily," says Helen Jenkins of Brigham and Women's Hospital's division of global health equity, in Boston, the lead statistician on the study published this week in medical journal The Lancet.
The disease is caused by bacteria that typically attacks the lungs and is often spread through the air when people who have an active infection cough.
The study in a special issue of Lancet marking yesterday's World TB Day, offers the clearest picture yet of the global burden of tuberculosis facing children. It also estimates for the first time the problem of multidrug-resistant tuberculosis or MDR-TB.
"Despite children comprising approximately one quarter of the world's population, there have been no previous estimates of how many suffer from MDR-TB disease," says Dr Ted Cohen, also of Brigham and Women's Hospital, and co-author of the study.
Children are at a higher risk of disease and death from MDR-TB, but react well to medication. It is harder to diagnose in small children because they cannot cough up sputum samples for testing.
For decades, researchers had largely ignored tuberculosis in young children, partly because juveniles are less likely to transmit the disease than adults. TB is very hard to diagnose in children because the infection looks different in children than in adults.
"In kids, you are much more likely to have TB disease in other parts of the body, not necessarily in the lungs," Jenkins says.
Even when children do have TB in their lungs, there are fewer TB pathogens present "making kids with TB invisible" to current diagnostic methods, she says.
The World Health Organisation estimates 8.6 million people developed TB in 2012 and 1.3 million died from the disease. It says half a million people became ill with dangerous superbug strains of tuberculosis in 2012; it estimates that up to twomillion people worldwide may be infected with drug-resistant TB by 2015.
Jenkins says keeping track of TB rates in children is useful because those with drug-sensitive forms of TB generally respond very well to treatment, and their cases can offer clues about TB transmission in a community.
Additional reporting by Agence France-Presse