In Medellin, Colombia, low-income residents who lived in close proximity to new public transit stations had increased rates of mosquito-transmitted dengue fever, according to a new study.

The study, published May 18 in Nature Scientific Reports, found that poorer neighborhoods closest to public transit stations had the highest dengue infection rates while equivalent socioeconomic neighborhoods farther from these stations had lower incidence. Zones with greater percentages of people who reported using public transit also had higher numbers of dengue infections.

In wealthier neighborhoods, distance from transit had less effect on rates of dengue infections.

“Construction of public transportation is one of the most widely recognized methods to reliably improve people’s economic conditions, access to resources and social networks,” said Laura Harrington, a senior author of the paper and professor of entomology.  “Our study highlights the necessity of providing adequate and well-targeted dengue surveillance and control concurrently with infrastructure investments, as they are likely to alter disease dynamics and infection risk.”

Medellin, which has one of the best public transit systems in Latin America, expanded its metro system between 1994 and 2016, with many new stops opening between 2010 and 2016. This is believed to be the first study analyzing geographic data of dengue incidence before and after construction of public transit lines.

The researchers found that in the first two years after a station opened in low income neighborhoods, dengue incidence spiked significantly, said first author Talya Shragai, Ph.D. ‘20, a former member of Harrington’s lab who is currently an epidemic intelligence officer at the Centers for Disease Control Emergency Response and Recovery Branch in Atlanta, Georgia.

The reasons behind the correlations between public transit, distance to a metro stop, socioeconomic status and dengue fever rates are unclear and will require more research, but the results point to the need for more public health support in low income areas near public transit, the researchers said.

“Information like this can help target limited mosquito control resources,” Shragai said. “We have good data that shows the earlier that you start mosquito control efforts in a dengue outbreak, the more effective you are at reducing cases. So it’s important to be able to target where you’re applying mosquito control efforts.”

Public health services to mitigate outbreaks could include access to doctors and health facilities, insecticide spray and public education on how to limit mosquitoes that carry the dengue virus by dumping standing water and putting up screens. In 2017, the city released replacement Wolbachia mosquitoes, which are modified with an intracellular bacteria that makes them resistant to dengue.

For the study, Shragai, who worked in Medellin from 2018 to 2020, discovered the city had collected nine years of data (2008-16) showing dengue infections and addresses where patients lived, which allowed for detailed geographic analysis of dengue incidence. The researchers also had access to demographic data, results of mobility surveys in 2011 and 2016 revealing public transit usage in the previous 24 hours, and locations and years when new public transit stations were built.

In other studies, relationships between lower socioeconomic status and higher dengue rates have been very inconsistent, though these findings suggest other factors may result from socioeconomics. For example, people in wealthier neighborhoods may drive or ride taxis as opposed to taking public transit. More research is needed to understand whether higher mobility enabled by public transit combined with higher risk conditions in poorer neighborhoods (including greater density of people, lack of air conditioning and screens and more breeding places for mosquitoes) led to the dengue hotspots.

Guillermo Rúa-Uribe, a faculty member at the University of Antioquia in Medellin, Colombia, is a senior author of the study.

The study was funded by the Max Planck Foundation, the Administrative Department of Science, Technology and Innovation in Colombia, and the U.S. Department of Agriculture.