The number of cases of tuberculosis (TB) in India may be up to two to three times higher than current estimates, suggests a new study.

The research team, comprising researchers from Imperial College London, the Government of India's Revised National TB Control Programme, the World Health Organisation, and other organisations, have published their findings in The Lancet Infectious Diseases.

The team analysed drug sales data to show that the actual number of tuberculosis cases in India may be vastly under-reported, primarily because many people opt for treatment from private healthcare providers, who usually fail to report tuberculosis cases to public health officials.

TB is a bacterial infection, spread through inhaling tiny droplets from the coughs or sneezes of an infected person. In 2014, 9.6 million people fell ill with TB and 1.5 million people died from the disease. India has the highest number of TB cases in the world, and accounts for at least a quarter of all cases worldwide.

To estimate the number of cases of TB in India's private healthcare system, the researchers calculated nationwide sales of TB drugs across the private sector, and then used this figure to calculate the number of cases. This suggested there were plausibly 2.2 million TB cases, possibly more, treated in the private sector in 2014 -- two to three times higher than current estimates.

Dr Nimalan Arinaminpathy, lead author of the research, from the School of Public Health at Imperial College London said: "TB is the top infectious disease killer worldwide, yet we have had little idea of the true scale of the problem in India -- the worst affected country. This is because many patients in India use the private medical system as opposed to the state system. However, this vast private system consists of a huge number of providers and is largely unregulated -- meaning that most cases of TB seen in the private healthcare system are not reported to public health officials."

The WHO's Global TB Report 2015, estimated that only 12 per cent of all case notifications came from the private sector. As a result, the true burden of the disease in India is difficult to estimate.

The scientists warned that under-reporting significantly hampers the Indian health system's ability to know the true extent of the problem and thus adversely affects its ability to plan for it. Previous estimates pegged India's total burden of TB at 2.2 million. However this study estimates that over 2 million cases of TB were treated by India's private sector alone, and an additional 1.4 million were managed in the public system, in 2014.

Dr. Sunil Khaparde, Manager of India's Revised National TB Control Program (RNTCP) and one of the authors said: "While India's RNTCP has achieved tremendous success over the years, the country continues to have a large number of TB cases. Although we introduced the mandatory notification policy in 2012, and the proportion of cases notified by the private sector has increased, many still do not report cases. In the past three years, the RNTCP has introduced private sector engagement initiatives to encourage active reporting of TB cases and standardised management protocols by private providers. However, this study reflects the urgent need for us to amplify our efforts with much greater commitment and vigour."

The WHO Representative to India, Dr Henk Bekadem, said: "India has come a long way and done an excellent job of meeting its Millennium Development Goal targets for TB control. But the findings of this ground-breaking study with the estimates of people treated in the private sector for tuberculosis reveal that there could be well over 1 million TB patients not being accounted for in the national TB programme surveillance. The study reaffirms the need for innovative ideas to engage the private sector more comprehensively and for all stakeholders to unite if we are to End TB by 2035."

The researchers argue that their findings suggest the need to re-double efforts to address the TB burden in India, and urgently increase surveillance in the private sector, as well as ensure greater collaboration between the private and public sector in India.