Untangling drivers of dengue in Thailand

A female Aedes aegypti mosquito acquiring a blood meal from a human.
A female Aedes aegypti mosquito acquiring a blood meal from a human. (Photo by James Gathnay, courtesy of CDC PHIL)

Dengue virus has circulated in Thailand for decades, and researchers are starting to understand some of the factors that move its four different variations, or serotypes, around the country. Dengue, like malaria, is spread by mosquitoes that acquire the virus when they bite an infected person or wild primate. They then transfer it into a new host when they bite again.

But the secret to how dengue moves around human populations may not be as closely tied to its winged vector as one may think, says UF biologist Derek Cummings. 

“We tend to think of mosquitoes as moving the virus, and that because they bite multiple people, their actions will dictate how the virus moves,” says Cummings, a biology professor in UF’s College of Liberal Arts and Sciences Department of Biology. “But that may be the wrong way to think of it. People have an order of magnitude more movement than the mosquitoes, it’s really the people who are moving it around.”

Hired under the university’s preeminence plan, Cummings investigates the transmission dynamics of various infectious diseases. His work pays close attention to factors that contribute to, or hinder, viral spread or sustained circulation in a population. Cummings is also a faculty member of UF’s Emerging Pathogens Institute.

In new work, Cummings and Henrik Salje—an affiliate faculty member of UF’s biology department at the time of the study—along with a team of researchers, inferred how people’s movements in Thailand affect dengue disease dynamics. The study authors used genetic sequencing and cell phone mobility data from Thailand to model how people’s movements contribute to spreading dengue. The study was published recently in Nature Communications.

“You can think of mosquitoes as these little spatial samplers,” Cummings says.

When someone gets bitten by a mosquito carrying the dengue virus, whether they are near their home or in a neighboring town, they may either acquire a virus from the biting mosquito or pass one on. But fewer than 1% of all dengue infections are genetically sequenced, which can pose challenges to understanding how certain genetic sequences are related to each other.

Genetically sequencing dengue viruses from infected individuals can help determine which cases are related, says Salje, who is now an assistant professor at Cambridge University. But the many, variable combinations of transmission chains across both time and space create a tangled web of pathways by which the virus moves through populations.

For example, imagine that dengue viruses sampled from two people living in the same town are found to be closely related genetically. The two people could have:

  • Traveled to the same neighboring town, been bitten by mosquitoes, and then traveled home, or
  • One person may have traveled, been bitten, and then brought the virus home where it was passed to a neighbor via a mosquito, or
  • One person may have acquired the virus from a mosquito near to home and passed it to a neighbor via a mosquito while both stayed close to home.

In the new work, Cummings and his coauthors combined genetic sequence data with mobility data, based on cell phones and other sources, to build a model that infers how people’s movements affect individual dengue transmission events. These individual events form the basis for modeling mechanisms of spread.

The main finding was that younger kids who stay near to home are likelier to become infected from localized transmission. Adults, in contrast, were more likely than kids to acquire dengue infections that had a signal associated with travel.

“I think in most societies, kids are more likely to spend more time near their home, compared to adults who are commuting or moving around a city more frequently,” Cummings says. “It was the adults who tended to move new sequences into a local area where it was then transmitted between the kids.”

The findings could help inform mitigation strategies, Cummings says, and support the idea that most dengue transmission is local. “People spend a lot of time at home, and they get bitten at home a lot,” Cummings says. But when new viral sequences move between communities, it’s most likely adults who facilitate those new introductions. 

The new methods to interpret sequence data to examine the transmission process could also be used to study the transmission of other pathogens, Salje says.

Using these methods with strong surveillance networks could help public health authorities respond to reported infections and target interventions to best reduce cases and transmission, Cummings says.

Written by: DeLene Beeland