Dengue fever spreads in a neighborly way

Dengue is a bit of a homebody. By mapping the spread of the virus across Bangkok, scientists found that infections were most likely to occur within a few minutes’ walk of the home of the first person infected.

Pinpointing where dengue is likely to be transmitted can better focus efforts to stop the spread of the disease, the researchers report in the March 24 Science.

“We often think of transmission and infection as occurring in this ubiquitous, pervasive and amorphous way,” says study coauthor Derek Cummings. But there is a pattern to how dengue spreads. This study, he adds, shows that scientists are “starting to have the tools and methods to really track how infectious diseases move across a population.”
Dengue is a viral disease transmitted by Aedes aegypti mosquitoes and can cause fever and muscle pain so excruciating that it’s also known as “breakbone fever.” In some cases, it can be deadly, resulting in more than 20,000 deaths per year.
Cummings and colleagues looked at both the genetics and locations of about 18,000 cases of dengue from 1994 to 2010 in Thailand, most from the capital Bangkok. If two cases of dengue evolved from the same parent strain of the virus within a season, or about six months, researchers considered the pair to belong to the same transmission chain, which connects dengue infections that spread from one person to the next. About 160 chains occur in Bangkok in a season.

The researchers found that 60 percent of dengue cases within a 200-meter radius in Bangkok were closely related. These infections with a particular dengue strain belonged to the same transmission chain, says Cummings, an epidemiologist at the University of Florida in Gainesville. In contrast, only three percent of cases separated by a greater distance, between one to five kilometers, were from the same transmission chain.
The new study’s combination of genetic and location information provides more details on the ecology of dengue than previous research, says Caroline Buckee, an infectious disease epidemiologist at the Harvard School of Public Health. “It would be great to see this kind of approach become a standard for studies of dengue transmission and epidemiology.”

When the researchers mapped the locations of cases within the same transmission chain, they found that the home of the person originally infected by a mosquito bite, the first link in the chain, is a good indicator of where new cases of dengue will occur.

Thailand’s Ministry of Public Health responds to dengue infections by spraying to kill mosquitoes. “Now, we have some quantitative details to start targeting control technologies,” Cummings says, to better focus spraying in high-priority areas.

The data may also be helpful for a vaccine. Though there is a dengue vaccine licensed for use in Thailand, Cummings says, researchers don’t know yet whether the vaccine will need to be updated with more strains of the virus over time, like the flu shot. Understanding the diversity of dengue strains and how they spread across Bangkok in a season may help researchers address this vaccine concern, he adds.

“Once we can understand these detailed patterns of how things spread, then we might be able to refine how we respond to the pathogen,” Cummings says.