Tuberculosis (TB) is responsible for the deaths of millions across the globe each year.

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It’s been assumed the illness spreads through droplets released by the coughs and sneezes of people who test positive for TB.

But scientists warned that the deadly illness could be spreading silently, as common tests fail to catch cases.

People may still carry the infection despite testing negative for TB, thus unknowingly transmitting it to others, the researchers said.

There is a vaccine for TB called the BCG vaccine and many Brits will have already received it.

TB is often spotted by taking samples of someone’s mucus – also referred to as sputum or phlegm.

But a study by scientists from Amsterdam UMC and the University of Cape Town found that this method sometimes failed to identify when someone could spread the illness.

It comes as scientists recently cast doubt on the assumption that TB only spreads through droplets released by coughs and sneezes, suggesting that the simple act of breathing could be enough to spread the infection.

The team of Dutch and South African scientists used a new method of testing that was able to spot even low concentrations of mycobacterium tuberculosis (Mtb), the bacteria causing TB infections.

They analysed samples from over 100 patients visiting community clinics in Cape Town, as well as aerosol samples collected in a community-based dedicated TB aerobiology lab.

Researchers found that Mtb was present in the exhaled breath of 90 per cent of people suspected of having TB, including those whose sputum test had come back negative.

Frank Cobelens, professor of Global Health at Amsterdam UMC and senior fellow at the Amsterdam Institute for Global Health and Development (AIGHD), said: “This rather shatters the paradigm on the transmission of tuberculosis.

“Previously we understood that Mtb was only expelled by those who have the disease, but this study shows that also those with symptoms who test negative do this and probably spread the infection.”

Ben Patterson, external PhD candidate at Amsterdam UMC and AIGHD, added: “If someone carries Mtb in their respiratory tract, this may also mean they can spread it.

He said the study’s results – published in Proceedings of the National Academy of Sciences – suggest there’s “a much broader range of people transmitting TB than previously recognised”, which would have “significant implications” on public health systems designed to stop transmission that are currently in place.

Researchers collected aerosol samplings on three separate occassions over six months for all participants.

The presence of Mtb gradually decreased in those receiving treatment for TB, as well as for not on treatment.

But 20 per cent of all participants continued to test positively for Mtb with aerosol tests after six months.

This suggested that transmission can continue over a period longer than previously thought, according to researchers.

Indeed, a recent study from the University of Cape Town suggests that tuberculosis could be present in the lungs for up to four years prior to the onset of symptoms.  

Dr Cobelens said: “Together, our results indicate how complex tuberculosis is, and perhaps also why it is so difficult to eliminate tuberculosis in endemic areas.

“Even when public health agencies work, according to the current guidelines, effectively against symptomatic TB cases.

“In this sense, a revaluation of our practices is necessary.”

Health officials recently raised the alarm over a surge of TB cases in England.

Cases of the Victorian disease increased by over 10 per cent last year, according to the UK Health Security Agency – up from 4,380 in 2022 to 4,850 in 2023.

It prompted health officials to last month warn Brits to get checked for TB if they have a lingering cough, as the deadly disease can easily be mistaken for a cold or the flu.

Dr Esther Robinson, head of the TB unit at UKHSA, said: “A cough that usually has mucus and lasts longer than three weeks can be caused by a range of other issues, including TB.

“Please speak to your GP if you think you could be at risk.”