Two million children are infected by a drug-resistant form of tuberculosis across the world, according to a new study that highlights the importance of the disease among teenagers. 

Tuberculosis (TB) in children is increasingly being recognized as a significant public health issue and now the new figures have estimated that at least 67 million children were infected by Mycobacterium tuberculosis with 850,000 developing active disease. 

Of these children, two million were estimated to be infected with multidrug-resistant (MDR)-tuberculosis strains, leading to 25,000 cases of MDR-TB disease requiring expensive and toxic treatment. 

The new figures were compiled as a result of "innovative modelling and statistical analysis" carried out by researchers from Sheffield University, Imperial College London and the World Health Organization. 

"Our report shows far more drug-resistant TB occurs in children than is diagnosed, and there is a large pool of drug-resistant infection," said Peter Dodd, an infectious disease epidemiologist from the university's School of Health and Related Research (ScHARR). 

"If they are not identified as having drug-resistant TB, children are unlikely to receive appropriate and effective treatment. After infection with Mycobacterium tuberculosis, young children are at particularly high risk of progressing to tuberculosis disease," Dodd said. 

TB in children is an important element of the total global burden of the disease, the researchers said, adding that Africa and Southeast Asia were found to have the highest numbers of children with TB. 

"They are also more likely to develop more severe forms of disease such as TB meningitis and disseminated TB," Dodd said. 

The report, published in The Lancet Infectious Diseases, warns that the identified cases of drug-resistant TB in children are the tip of the iceberg and there is a large unmet need for diagnosis, drug-susceptibility testing and appropriate treatment. 

Improved estimates of the rates of drug resistance in children are important because pediatric tuberculosis can be more difficult to diagnose, more challenging to test for drug sensitivity and more likely to cause extra-pulmonary infection, the study said. 

The findings could have implications for approaches to treatment and preventive therapy in some parts of the world.