Delhi is once again in the grip of a dengue outbreak with municipal corporation data showing a sudden spike in the number of cases over the last two months. More than 1,000 cases were reported in October and more than 500 in November, taking the total number of cases so far this year to more than 2,100.

However, a report by the non-governmental organisation Praja, which works on civic matters in Delhi and Mumbai, suggests the number of cases of dengue and other vector-borne diseases has been greatly under-reported. According to the report, information procured under the Right to Information Act shows there were 7,153 dengue and 4,205 malaria cases in Delhi between April 2017 and March 2018. Praja’s own survey across 12 zones in Delhi threw up even higher numbers – 1,06,456 cases of dengue and 1,26,334 cases of malaria in the same period.

In India, collecting data on diseases is problematic because of under-reporting as well as patchy disease surveillance in many parts of the country. But the data that is there suggests dengue epidemics are becoming more frequent, said Olivier Telle, urban health researcher at the Centre National de la Recherche Française, head of territorial dynamics at the Centre de Sciences Humaines and senior visiting fellow at the Centre for Policy Research in Delhi. Moreover, unlike malaria and cholera that occur in the same geographical areas, dengue outbreaks keep jumping locations. Telle spoke to about his research on dengue in Delhi and how it points to the need to rethink urban planning and connect all services with health.

Excerpts from the interview:

Oliver Telle is urban health researcher at the Centre National de la Recherche Française.

What are the main findings of your research on dengue in Delhi?
Dengue does not follow what we know about disease diffusion within cities. For example, diseases like malaria and cholera are found in the most deprived areas of cities, which helps prioritise areas where disease control efforts are to be implemented. For dengue, this is not the case. You can find the dengue virus anywhere within a city, in developed as well as deprived areas, because all spaces that make up a city are connected through the daily movements of individuals, allowing viruses to travel along with humans. For example, in South Delhi, we found fewer mosquitos than elsewhere, but because this is a central place through which people travel, they bring the viruses with them.

What explains this difference between malaria and dengue transmission?
The mosquito transmitting malaria [Anopheles] bites at night. So people tend to be infected where they reside. Aedes [which spreads dengue], on the contrary, bites during the daytime and this means you can be infected at your workplace or at the market. The place of infection is not necessarily the place of residence. This makes mobility a very important factor at the individual and collective level in the transmission of dengue. That is why central areas of Delhi are more affected by dengue despite having lower numbers of mosquitos, and this is why the dengue virus spreads this fast within and between cities.

We also found that 80% of dengue infections in Delhi are asymptomatic. As a consequence, infected individuals will continue to move in the city and will subsequently disseminate the virus.

Unlike outbreaks of malaria and cholera that are quite stable and occur in the same places, dengue outbreaks occur in different places every year (as shown in the map) and it is difficult to predict an epidemic based on a past one. For example, South Delhi was not affected in the 2008 outbreak but was a major hub of infection in 2009. (Credit: Olivier Telle)

When we say mobility here, we are talking about the movement of people who are potential carriers of the virus. What about the spread of the vector, the mosquito, itself?
The Aedes mosquito does not move much more than about 100 metres because in a city like Delhi, human density is so high that it does not need a greater range than that. So yes, it is human movement that brings the virus to new places. That is why by protecting deprived areas from dengue, you can protect richer areas and the city. But this is something that city authorities do not really understand, not only in India but in Europe as well.

So richer communities that have good sanitation may think they are protected but are really not because of this kind of transmission?
Exactly. You know, Paris is Paris because of cholera. In the mid-19th century, more than 30,000 people died in two cholera epidemics, after which the rich decided that to avoid cholera, they had to have the city provide good water, electricity and a good living environment for the poor. It was not philanthropic perspective, but they understood that by protecting the poor, they could protect the most privileged citizens. The rise of vaccination and other individual measures completely disconnected sanitation and health, but the idea of proper sanitation is to put health at the centre of human development would protect everyone. Viruses do not really understand social boundaries.

What are the implications of this? You have argued that this underlines the importance of better urban planning and sanitation throughout the city.
While studying epidemics of dengue in India and in Brazil, Laos and Thailand, I found that dengue control and urban planning are completely disconnected, not only for the administration in charge of epidemic control but in research as well.

In Delhi, a study I coordinated with the Centre National de la Recherche Scientifique, Institut Pasteur and the Indian Council for Medical Research found, for example, that dengue epidemics always start in high-density areas with a lack of infrastructure. These dense areas are warmer than others. In winter, when Delhi’s temperature is 5 degrees Celsius at city scale at night, the temperature in these areas can be 15 degrees Celsius because of the high density of buildings, which keeps the heat in. It helps mosquitoes and viruses survive in these environmental niches, waiting for climatic factors to be more suitable to spread to other areas. If we can avoid mosquito breeding sites in such areas in January, February and March, it could help control dengue for the city as a whole. This requires inducing a scientific and political paradigm shift: controlling virus spread before it appears on the surveillance system.

We need to prioritise preventing the disease from emerging, thinking long-term, reconnecting sanitation with its original aim, which was to develop proper living conditions for all urban dwellers to prevent epidemics of infectious diseases. This is quite complicated but possible. This is the second paradigm shift: developing the city for all.

What are the more immediate measures that can be taken?
We are now implementing mosquito traps to observe the potential diffusion of the dengue virus at low levels and to control it, but the only way to control infectious diseases in the long term is to put health in general and health administration in municipalities at the centre of urban planning and development. For example, to control mosquito breeding sites during pre-epidemic seasons, there needs to be proper access to water for all. People who do not have access to tap water store water in their homes and these become mosquito breeding sites. It may not necessarily help during an epidemic but during an inter-epidemic period, it is a major factor that helps control mosquitos.

Researchers from the Centre for Policy Research, the National Institute of Malaria Research and officials of the Municipal Corporation of Delhi set up mosquito traps. (Credit: Olivier Telle)

What kind of place would mosquito traps be most effective in?
That depends. During an epidemic, we could locate a central place where infections would occur. To do so, we would need to analyse big data on work-related population mobilities; this will help us localise areas where disease control would be the most effective. In the pre-epidemic season, we are trying to put some traps in places where we think the disease emerges every year. By controlling mosquitoes there, we might be able to control the viral load 10 months later during an epidemic. We, however, need important field and laboratory work to confirm that this will indeed be effective. If it works, this strategy could be implemented in India and in other countries as well.

How effective is this responsibility of providing proper sanitation at the ward level?
It is good to have the responsibility at the lower levels but you need coordination across a city and among cities, and that is lacking. Delhi is doing quite a good job trying to control the disease but people are moving across the area from Gurgaon, from Faridabad, from everywhere. There are so many people moving that if you do not control the disease in Faridabad, for example, you will not be able to control it in the areas under the Municipal Corporation of Delhi. Right now, there is no coordination between municipalities in the area. We also see that in France where we look at the problems in one area without coordinating with other areas. So, this is a global problem.

Even in research, there is little coordination with the city. Researchers, and I include myself, come up with very costly solutions. For example, to trap dengue mosquitos, you need to put mosquito traps every 30 metres to 40 metres, which in Delhi is completely insane. You cannot cover Delhi with mosquito traps like this. So, what we are trying to do is locate areas where the traps will have the maximum effect. Science has to provide solutions that can be implemented by the authorities, and this has not been done for a long time. That is why, at the Centre for Policy Research, I am trying to transform my scientific knowledge into practical ways to deal with diseases control, thinking with municipal authorities in charge of disease control on the best ways to control the spread of diseases.

There is a dire need for public transportation in Indian cities, but since that is another source of transmission, does that then become a focus for vector control?
Yes, definitely. You have seven million people travelling everyday to Central Mumbai. If you do not think about developing transportation along with what it means for the transmission of diseases, it could be problematic. For example, Gurgaon [in the National Capital Region] was developed as a special economic zone without thinking of the health aspects and so, for a long time, there was no surveillance for dengue in Gurgaon. It is the same again with smart cities where no health planning is operational. Health budgets are dedicated to hospitals but health is much more than this.

Developing transportation is good but it must be accompanied by health protection, which is not happening in India or anywhere else. In France, we have, for example, had a resurgence of tuberculosis in the deprived areas of Paris, which says a lot about the gaps there.

What happens when we go beyond cities? Do we know how these mosquito-borne disease are currently spreading between cities or in rural areas?
Cities are not geographically isolated and we actually have systems of cities. Delhi is connected with many medium and small cities. So, while we have in Delhi a proper dengue surveillance system that helps sustain scientific research, we may not know what happens in a small city. Some of these small cities are not even recognised as cities but as rural areas, so they lack the proper management a municipality in a recognised city is able to provide. They might actually be the most suitable places for virus spread and play a major role in upcoming governance of diseases. Providing sustainable disease control for all would help at all scales – between countries, between and within cities.