How Population Density Drives the Spread of Dengue in Cities

Dengue, a mosquito-borne illness that’s particularly common in tropical cities, poses a persistent public health challenge. But while it’s known to spread quickly in densely populated areas, understanding the specific patterns of transmission—the pathways through which it moves—can reveal new ways to control it. A study by Henrik Salje, Justin Lessler, and colleagues, published in Science in March 2017, explores these patterns in Bangkok, Thailand, showing how factors like population density and urban layout influence dengue’s spread. The research was a collaborative effort involving institutions like Johns Hopkins University, the Institut Pasteur, the Walter Reed Army Institute of Research, and the Armed Forces Research Institute of Medical Sciences.

How Dengue Transmission Works

Dengue is spread by the Aedes mosquito, a species that often stays close to where people live, only traveling a few hundred meters in its lifetime. This limited range means that dengue infections tend to cluster in specific areas, where one infected mosquito can pass the virus to multiple people nearby. These clusters of infection are known as transmission chains. Essentially, a transmission chain is a series of infections linked by close geographic and temporal proximity, where each new case can be traced back to earlier ones in the chain.

By analysing more than 17,000 dengue cases in Bangkok and other Thai locations, researchers looked at how close cases were in space and time, and compared genetic data to see if cases were likely part of the same transmission chain.

How Distance and Density Affect Transmission

The study found that dengue transmission is heavily localised. People living within 200 meters (about two city blocks) of an infected person in Bangkok had a 60% chance of being part of the same transmission chain, meaning they were likely infected by the same virus strain or an offshoot from the same source. However, for cases farther apart—between 1 and 5 kilometres—this likelihood dropped to just 3%. These findings highlight how spatial distance impacts dengue spread: it moves most efficiently in tight clusters, limited by the Aedes mosquito’s small range.

The study also found that population density affects the number of transmission chains in an area. Up to a certain point, as population density increases (i.e., more people live in a smaller area), the number of independent transmission chains also increases. As mosquitoes have easy access to more potential hosts without needing to fly far, this allows more chains of infection to take root. However, the relationship between density and transmission isn’t limitless. The study found that when population density exceeds 7,000 people per square kilometre, adding more people doesn’t lead to more chains. This suggests that once an area becomes too crowded, dengue strains may start “competing” for hosts—meaning that beyond a certain density, it becomes harder for the virus to spread further.

This relationship is an example of density-dependent transmission, where the risk of infection increases with population density, but only up to a point. Beyond this threshold, increased density may even limit further spread, as virus strains face more competition for the same hosts.

Seasonal Patterns and Regional Containment

The study also looked at how dengue spreads across time and space within and between seasons. A dengue season is an interval where transmission is heightened and virus lineages actively circulate. Here, a season is defined by pairs of cases within six months of each other, where cases are likely linked due to proximity in time and transmission.

Within a single dengue season, the virus spreads widely across a city like Bangkok. By the next season, strains are evenly mixed citywide, suggesting that dengue in urban areas doesn’t stay confined to any one neighborhood but instead disperses widely within the city limits. Despite this citywide mixing, regional spread is limited. Researchers found that dengue viruses in Bangkok rarely spread to neighbouring countries, even within Southeast Asia. This suggests that while large urban centers enable rapid and widespread local transmission, they don’t necessarily drive regional outbreaks across borders. Phylogeography (the study of the geographic distribution of genetic lineages) showed that similar environmental conditions might exist in nearby areas, but the viruses themselves don’t often travel across borders.

Implications for Public Health and Dengue Control

These findings provide valuable insights into how dengue can be more effectively controlled in cities. Since transmission is concentrated in highly populated areas, vector control (methods to reduce mosquito populations) might be most effective if targeted in dense neighborhoods. Interventions such as targeted spraying or removing standing water where mosquitoes breed could help curb the virus more effectively than broad, citywide efforts.

Moreover, since dengue spreads widely within cities each season, continuous monitoring across urban centers may help public health officials detect and contain outbreaks quickly. By understanding how local factors like population density shape dengue’s spread, health agencies can focus on specific hotspots, making control efforts more precise and efficient.

This research underscores how important it is to understand the dynamics of transmission chains, density-dependent spread, and seasonal mixing. By focusing on these factors, public health strategies can be better tailored to the unique challenges of dengue in dense, interconnected urban settings, ultimately reducing the impact of this disease.