Does city living increase TB?

Gary Finnegan

Gary Finnegan

December 1st, 2011

Gary Finnegan

‘New evidence on the spread of tuberculosis (TB) challenges the conventional wisdom that urbanisation is the main driver of increased risk of infection. They reality, says the author of the study, is more complex.’

TbTek-Ang Lim, an infectious diseases expert at the European Centre for Disease Prevention & Control (ECDC), says that while living in crowded slums is typically associated with the spread of disease, economically developed countries tend to have lower incidence of TB.

In some cases, regions with very low population density have high rates of TB but several factors – many of them linked to income inequality – are at play.

This can be attributed, at least in part, to better access to health services in urban areas and generally higher hygiene and sanitation facilities in cities compared to rural areas.

So while living in a slum carries higher risk of infection, living in poor, sparsely populated parts of the countryside brings its own set of health risks.

Read: TB – return of a preventable disease

In an interview with Vaccines Today following his presentation at the European Public Health conference, Mr Lim explained the findings of a 30-country study of TB in Europe.

“We think that the reason that could explain our findings is that countries with high TB incidence in Europe are also countries where the economic development is weaker compared to countries with low TB incidence,” he said.

“This might indicate that in such countries access to health care as well as hygiene and sanitary facilities are of better quality in urban settings than in rural areas.”

In addition, the ECDC researchers found a strong correlation between TB incidence and income distribution. In countries where TB incidence is medium to high, the results indicate that there are more cases of the disease when income distribution rises.

In general, population density can make it easier for communicable diseases to spread within a population, according to Mr Lim, and other environmental factors – such as poor air quality – are associated with urbanisation and can exacerbate respiratory diseases.

Health authorities in Europe have grown increasingly alarmed by the resurgence of TB, particularly given the rise in drug-resistance strains of the disease. The WHO has published a TB Action Plan to deal with the problem.

With more people living in urban areas than ever before, officials are concerned about the spread of infectious diseases. Addressing social equality – while difficult – could be part of a multifaceted solution.

Read: TB ‘back with a vengeance’ in Europe

The following is a Q&A written interview with Mr Tek-Ang Lim of the ECDC

You have looked at the impact of urbanisation on the spread of TB. What are the main findings of your study?

In this study, we have tried to study whether or not there is a correlation between population density and tuberculosis incidence in 30 countries in Europe (the 27 European countries + Norway, Iceland and Liechtenstein).

We conducted an ecological study using a very innovative statistical modelling for epidemiology (quantile regressions) and our analysis have shown that for countries with a medium to high incidence of tuberculosis (above the 50th percentile), there is a very strong correlation between population living in areas where the population density is below one in 100km2 and TB incidence.

This is a very interesting because in the scientific literature TB incidence has been found to be correlated with urban areas where population density is very high.

We think that the reason that could explain our findings is that countries with high TB incidence in Europe are also countries where the economic development is weaker compared to countries with low TB incidence. This might indicate that in such countries the access to health care as well as hygiene and sanitary facilities are of better quality in urban settings than in rural areas. In addition, we also found a very strong correlation between TB incidence and the distribution of income. In countries where TB incidence is medium to high (above the 50th percentile), our results seem to indicate that if the inequity in the distribution of income increases then the TB incidence will as well increase. With a very strong increase for countries with a high TB incidence (above the 75th percentile).

Why is population density so important?

Population density is indeed a very important dimension in the control of communicable diseases. Firstly, the population contact rates increase with population density: the more people you have in a limited geographic area the more contacts they will have. For disease that are very infectious (flu, measles, etc.), population density is very important.

Three other important dimensions are also associated with population density and as a consequence to communicable disease controls:

  • Access to health care seems to be better in areas where population density is high which we can explain with a higher potential demand for health care in urban areas than in rural areas.
  • Higher population density also favours more socio-economic disparities. The very rich live in the same urban areas as the very poor, there may be clusters of very rich and very poor but they share the same urban settings.
  • Last, the environmental quality is also affected by population density. We can think of higher particulate matters pollution in urban areas compared to rural areas.

But specifically for our study on tuberculosis, it is well-known that TB spreads readily in congregated and densely populated areas with poor sanitary conditions. In cramped households the airborne infection transmits easily.

Large European cities like Paris, London or Amsterdam have high-density regions. Is TB as common here as in cities in South-East Asia?

From other studies we have seen that we have two different epidemic settings in the EU. In high-burden countries the TB epidemic is more generalised in the population and distributed throughout the country and we do not see a big difference in notification rates for the main cities compared to the countries as whole. In low-incidence cities of Western Europe we see the opposite, in these countries the incidence in urban settings i.e. major cities are many times higher than the national average.

Apart from density, what are the other factors that influence the spread of TB?

The spread of TB is generally influenced by the possibility and how rapid infectious cases of TB are detected, diagnosed and provision of adequate treatment. There are certain risk factors for specific populations to develop infectious active TB and within these populations the infection might spread more easily.

With the population density come several effects which are linked to it. For example the issue of access to health care which in rural areas of high-incidence countries might be a problem. In urban settings (of low-incidence countries, but not only) we see a congregation of people with specific risk factors for TB, such as homeless people, migrants, alcohol abusers and illicit drug users. These vulnerable and hard to reach groups often happen to encounter problems in accessing health care.  

With a rising global population, and more people living in cities than ever before, what can be done to curb the spread of TB?

The ECDC has developed a Framework Action Plan to Fight Tuberculosis in the EU. This Action Plan, together with the global targets to eliminate TB and the Action Plan from WHO on how to control multi-drug resistant TB, is addressing the challenge of TB in a comprehensive way. In a follow-up to the ECDC Action Plan, we are monitoring the progress towards TB elimination in EU. There is a declining trend in the TB notification in the EU by about 3-4% per year.

To decrease the TB incidence from mid-high burden to low burden can be done efficiently by strengthening the general health care system which is accomplished with a stronger economic environment. To achieve elimination of TB in those countries who already have a low-incidence is posing a different set of challenges, requiring a different strategy for the prevention and control of the epidemic.

It is essential to have the necessary resources which can maintain the capacity to rapidly detect, diagnose and treat all TB patients to stop the chain of transmission in the society. The basal principles to prevent and control TB do exist, but they need to be fully implemented in the most appropriate way. To keep the health providers aware of TB when the incidence decreases and fewer cases are seen by the doctors is key for a rapid diagnosis. Also, partnership outside of the TB services and health system might be necessary to achieve the goals of TB elimination.

Have any large cities managed to control TB despite having high population density? Is there a good example of what should be done?

To address issues related to urban TB control is one of ECDC’s priorities in dealing with the prevention and control of TB. Ongoing work in several cities have the aim to find good practices used in urban settings on how to best target the vulnerable and hard to reach populations.

There is also a network called ‘Metropolitan TB Control in Europe’ established that is driving projects on this topic. More info about them can be found here