Why bus drivers face higher TB risk than passengers

Drivers of public vehicles such as buses in Dar es Salaam are eight times more likely to catch Tuberculosis (TB) in a period of one year compared to the  passengers they carry; a new study has shown in part.

However, compared to all city dwellers, it does not mean that the drivers face the highest risk of catching the bacterial disease. Their risk only stands at 20.3 per cent, the study reveals.

Prison inmates are the most at risk group, carrying 41.6 per cent, according to findings in the study titled: “Tuberculosis transmission in public locations in Tanzania” published in the Journal of Infection.

Scientists now urge the authorities to target public health interventions against TB transmission on those locations that the study has focussed on.

Through the study, the researchers from Ifakara Health Institute (IHI) adapted a method for identifying tuberculosis (TB) transmission hotpots in Dar es Salaam by using a new technique.

They collected data in form of carbon dioxide, one of the gases in the air that people breathe. This data was used to estimate the proportion of re-breathed air from other people in confined spaces, such as cars and rooms.

The Principal Investigator of the study, Jerry Hella, believes this approach can guide the identification of TB transmission hotspots and targeted interventions to reach World Health Organization’s ambitious End TB targets.

Being an airborne disease, TB is transmitted from one person to another through small air droplets (droplet nuclei) which can remain suspended in air from few minutes to two hours.

These droplets, according to Hella, are produced in massive amounts when a potentially infective patient (not yet on TB treatment) coughs and sneezes.

“Locations with poor ventilation increase the time for the TB particles suspended in the air – resulting in increased probability of being re-breathed in by the next person in the room,’’ he says.

In public transport, TB transmission is highly likely but not many people ever thought that drivers of public vehicles, such as those popular in Dar es Salaam as daladala, are at a higher risk compared to their passengers.

Your Health posed a question to travellers at various bus stops: Between you and the driver, who do you think is safe from catching TB? None of those who responded believed the driver was more at risk.

“Look at that bus, look at how congested it is. There are too many people in there. I think they can’t even breathe well and freely!’’ said Mr Sadik Mussa, “…but see where the driver is seated,’’ he went on as he described a minibus plying Mbagala-Mwenge route which had just made a stop at Tabata Relini to pick more passengers.

“I think the driver is enjoying his seat and he is safe, right? I don’t think he is at risk,’’ says Mussa. That’s what most people like him thought as Your Health interviewed them.

To most of them, congestion of the passengers is the main reason. However, during the study, the scientists from IHI found a different reality in Dar es Salaam

“From our point of view – congestion alone is not a problem as long as there is an adequate system to support good ventilation i.e, air exchange between the indoor location and the “air outside”…’’ says Hella.

Hella and his team went around the city, collecting the carbon dioxide data from public transport facilities as well as markets, prisons, night clubs, schools, religious and social halls.

When asked why drivers faced a higher risk than their passengers, he explains “…passengers [in public transport] are less at risk because they spend less amount of time in the bus on a day compared to drivers. So, this puts the drivers at higher risk.

“If you model this relationship during one year, the risk of TB transmission is higher among drivers,’’ he tells Your Health. “For example,’’ he further explains, “The probability of a passenger to be in a bus with one TB patient during one route is the same for the passenger and the driver.”

“However the driver will have many trips, increasing his/her probability [of getting exposed to more patients with TB] during the day and the year,’’ says the researcher.

What about prison inmates?

“…Most people who end up in jail,’’ he says, “…are from a low socio-economic status, succumb to poor nutrition while in jail and prisons are mostly overcrowded making TB transmission rates higher than in the general population.”

The researchers believe that policies which address both congestion in public locations, with clean air quality systems will result into better control of TB by reducing significantly the transmission from such locations.

Why focus on Dar es Salaam?

“We decided to carry out the study in Dar es Salaam as it’s the city with the highest population in the country and it’s the fastest growing city in Africa as per World Bank Reports.

“But also, it is the leading in TB notification each year, accounting for about of 22 per cent of all TB patients notified…but also…because our research activities are mainly based in the urban.”

“We are currently conducting a similar study in the rural setting at Ifakara, Morogoro, so that we can have a rural to urban comparison of our findings,’’ he hinted.

Low risk areas for TB in Dar es Salaam

The researchers found that other areas in the city have lower risk of transmission. They include central markets, where the risk was 4.8 per cent and 0.5 per cent for traders and their customers, respectively.

For passengers on public transport, the risk was 2.4 per cent while in public schools it was at 4.0 per cent. In night clubs, it was 1.7 per cent and religious buildings (0.13 per cent) and social halls (0.12 per cent).

Tanzania is the 15th among 22 countries with the highest TB burden, says the WHO.

Global efforts to combat TB have saved an estimated 53 million lives since 2000 and reduced the TB mortality rate by 37 per cent according to the Global TB Report 2017.

“While the world has committed to ending the TB epidemic by 2030, actions and investments don’t match the political rhetoric. We need a dynamic, global, multispectral approach.” said Dr Tedros Ghebreyesus, Director-General of WHO when commenting on the report.