As is the case with Covid-19, breathing spreads TB and not coughing: study

For centuries it's been believed that tuberculosis is transmitted mainly through coughing, but new research by the University of Cape Town has cast doubt on this theory and now suggests that, just like with Covid-19, breathing may be a bigger contributor to the spread of TB.

Researchers believe the latest finding helps explain why overcrowded spaces, like prisons and informal settlements, often are breeding grounds for TB.

Presenting the latest research at The Union World Conference on Lung Health, which is being held virtually this week, the researchers, who conducted the study on TB patients in Cape Town, using a specially constructed respiratory aerosol sampling chamber (RASC), found that as much as 90% of TB bacteria released from TB patients may be carried in tiny droplets, known as aerosols, which are expelled when a person exhales deeply.

While coughing produced proportionately fewer particles compared to heavy breathing, the TB germ was detected consistently in all forms of exhaling after a five-minute sampling, with 66% found in normal breathing and 70% during heavy breathing, while 61% was during coughing.

If the number of organisms detected reflects infectivity, researchers argue this would suggest that normal breathing might contribute to asymptomatic transmission in high-burden endemic settings.

Covid-19 and TB are today the world's two leading infectious disease killers. The pandemic has refocused attention on how infectious diseases transmit from person to person and has catalysed innovations in sampling and diagnostics.

Lead author of the study and PhD candidate, Ryan Dinkele from UCT's molecular mycobacteriology research unit, said if tidal or normal breathing is a primary driver, or even as important as cough in TB transmission, then symptomatic screening for TB-transmitters may not be useful in slowing the spread of the disease.

Because TB transmission is poorly understood and difficult to study, control efforts so far have been based on the treatment of passively identified TB patients upon first presentation at a clinic. Researchers believe this is dangerous, because it means that transmission that happens before treatment if initiated can continue without interruption.

“The current approach, which relies on the testing and treatment of passively identified individuals may not be a reliable response to preventing transmission, as it depends on people feeling sick enough to seek treatment. It may also shed light on why constructing transmission chains is so challenging in high TB burden settings.”

Guy Marks, president of The Union and the conference convener, said the latest study will hopefully generate more focus on the topic of airborne spread of respiratory diseases.

“This study is an important step forward in our understanding of aerosol transmission of disease and its findings are as relevant for Covid-19 as they are for TB,” he said

The disruption to TB services during the pandemic has highlighted just how important it will be going forward that testing for and treating TB are made simpler and easier to access.

Another TB innovation announced at the conference on Tuesday, is a finger-stick blood test that can detect TB in less than an hour. The development of a fast and accurate, non-sputum-based point-of-care triage test, was tested in SA, Uganda, Gambia and Vietnam, and is set to have a major impact on combating the TB burden worldwide.

Lead researcher Jayne Sutherland of the MRC Unit in Gambia, reported interim results that were analysed using the Xpert-MTB-HR prototype technology in these four countries. The device is the first to meet the World Health Organisation target product profile for a triage test for TB regardless of HIV status.

“It takes under one hour and uses finger-stick blood, rather than sputum, which reduces biohazard risk and increases likelihood of diagnosis in individuals who cannot readily produce sputum, such as children and people living with HIV,” Sutherland said.

“Our interim data indicates that the Cepheid MTB-HR cartridge reaches the minimal target product profile for a point of care triage test for TB using finger-stick blood, regardless of geographic area or HIV infection status.”

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