Wednesday, February 2, 2011

Disease trackers take aim at dengue fever

Click on the animated map above to watch how a dengue epidemic spread to 390 confirmed cases over 25 weeks. New cases are shown in red, and previous cases in blue.

In late January of 2003, a woman returned from an extended holiday in Papua, New Guinea, to her home in Cairns, Australia, a tropical town in northeastern Queensland. The woman was sick with a fever and went to a clinic, which gave her pills to treat malaria.

It was a misdiagnosis. She actually had dengue fever, another vector-borne disease spread by mosquitoes. Global trends in population growth and travel have expanded the range of dengue transmission risk, and yet little is known about dengue epidemics in an urban environment.

In this case, the woman lived in an older neighborhood of Queenslander-style homes that have lots of unscreened windows with wooden shutters, kept open to let in the breezes. It was the steamy wet season. Within a few weeks, mosquito bites had spread the dengue virus within the neighborhood and beyond.

“It was a perfect storm for an outbreak,” says Gonzalo Vazquez-Prokopec, an Emory disease ecologist.

The dengue virus was soon confirmed as the culprit, and Queensland health authorities began aggressive intervention measures. By the time the outbreak ended several months later, there were 390 confirmed cases among Cairns residents, and one death.

Vazquez-Prokopec and Uriel Kitron, chair of environmental studies at Emory, conducted a statistical analysis of the Cairns outbreak, recently published in PLoS Neglected Tropical Diseases. They combined detailed data gathered by Queensland health authorities with geographic information system (GIS) technology and space-time analysis to plot the interventions alongside the spread of the virus. Click on the animated map above to watch the progression from the first to last case, during the course of the outbreak.

When a disease breaks out in an urban area, health officials must act fast, but they often have limited resources. The analysis honed in on key transmission patterns. The results showed that spraying 60 percent of the homes in close proximity to a home with a confirmed case of dengue was effective in reducing transmission.

"A big problem with mosquito control is there is not enough personnel to spray everywhere, so you need to strategically choose your targets during an outbreak. It's like fighting a forest fire," Vazquez-Prokopec says. "Our goal is to use spatial analysis as another tool to improve disease control and prevention where resources are limited."

New methods of dengue control are desperately needed, Vazquez-Prokopec says. An editorial he wrote on the rising threat of dengue epidemics will appear in the March issue of Future Microbiology.

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