Aedes aegypti, a mosquito species known for spreading dengue fever, feeds on a human (Image: James Gatheny).

Dengue fever was eradicated from the US nearly 70 years ago, but the devastating tropical disease has made something of a comeback in recent years, following outbreaks in Florida, Hawaii, and Texas. Unlike dengue outbreaks in other parts of the world, its American resurgence has so far been limited in reach — though researchers are still struggling to explain why it hasn't spread further.

The mystery grew more complex today at the annual meeting of the American Society of Tropical Medicine and Hygiene, where scientists from the National Center for Atmospheric Research (NCAR) presented preliminary findings of a study comparing two cities: Key West, Florida, and Tucson, Arizona. Dengue reappeared in Key West four years ago, but has yet to surface in Tucson, despite the fact that both cities are home to Aedes aegypti — a species of mosquito known to carry the disease — and that dengue remains endemic in neighboring Mexico.

Commonly known as "breakbone fever" because of the severe aches and pains it entails, dengue is fatal in only rare instances, though it’s remarkably difficult to treat. Symptoms include high fever, severe joint pain, and rashes, and can sometimes lead to a deadly form of hemorrhagic fever.

"It is still a mystery as to why dengue infection has not shown up here."

Kacey Ernst, an infectious disease epidemiologist at the University of Arizona in Tucson and co-author of the study, says she chose to compare the cities because at first glance, they appear to share commonly-cited dengue risks. People surveyed in both cities said they spend about an hour outside every day, and nearly identical proportions reported using central air conditioning, which is believed to lower the risk of dengue infection. In fact, Tucson may be at greater risk of a dengue outbreak because mosquito control programs there have been less aggressive than in Key West, and because many Tucson homes use cooling systems that increase humidity, making them more habitable for mosquitoes.

For some reason, though, Tucson remains dengue-free, while the disease continues to spread from Key West into the Florida mainland. Ernst speculates that there may be human or biological factors that would limit the ability of mosquitoes to transmit the disease in Tucson, though she and co-author Mary Hayden, a scientist at NCAR, have yet to determine what those may be.

"It is still a mystery as to why dengue infection has not shown up here," Ernst said at today’s meeting in Washington, DC. "When researchers looked at why dengue is not more common along the Texas side of the Mexico border, they cited factors limiting contact with mosquitoes, like people spending a lot of time in sealed, air conditioned buildings. Those issues are extremely important considerations, but we don't think they fully explain why Key West has dengue and Tucson doesn't."

Global dengue cases increased thirtyfold last year

Dengue emerged as a global threat in the 1950s, but infection rates have accelerated dramatically in recent years. Earlier this year, the World Health Organization (WHO) announced that global cases of dengue fever had increased by a factor of 30 in 2012, making it the world's fastest-spreading vector-borne viral disease, outpacing both malaria and West Nile virus. Overall, there are between 50 million and 100 million dengue infections each year, the WHO estimates, and about 12,000 dengue-related deaths. It's not clear what drove its resurgence in the US, though many have speculated that climate change may be responsible.

Dengue has traditionally been most prevalent in tropical climates, and especially in poorer countries, where poor sanitation systems give rise to widespread mosquito breeding grounds. The Indian city of Delhi recently saw a particularly devastating outbreak, though it's certainly not alone. According to the WHO, an estimated 2.5 billion people — about 40 percent of the world’s population — are currently at risk of contracting the disease.

Strong mosquito control and sanitation systems have been credited with limiting dengue’s spread in the US, though its irregular patterns add an extra layer of mystery to an already puzzling disease. Unlike malaria, dengue can’t be cured or treated with drugs, and there is currently no dengue vaccine on the market. As a result, the only way to prevent infection and spread is through mosquito control. Yet even detecting it can be a challenge for American physicians who have little experience with dengue, leading Ernst to speculate that the disease may have already appeared in Tucson without being identified.

"One of the tricky things with dengue fever is that in many cases, when people are infected they may show minimal symptoms and not seek medical care," Ernst tells The Verge. "Given our lack of history with dengue in the United States, many physicians are unlikely to think about dengue as a differential diagnosis, unless there is travel history."

Satellites and genetics

Scientists are working toward a dengue vaccine, though attempts to combat all four (and possibly five) forms of the disease have so far fallen short. Others have begun developing new ways to track and neutralize Aedes aegypti. Researchers in Australia recently genetically engineered a dengue-resistant form of the mosquito to be deployed in Vietnam, while scientists at NCAR have begun work on a computer program capable of scanning aerial satellite images for cans, buckets, and other water-filled containers where Aedes aegypti lay their eggs.

"The commercially available imagery from these satellites are now capable of providing clear images of relatively small objects on the ground,” says Paul Bieringer, a scientist at NCAR who collaborated on the satellite project with researchers at the Colorado-based STAR Institute. The idea, he explains, is to create maps that can simulate the spread of of dengue from water containers in particular areas, which may provide a deeper understanding of the meteorologic and human-based factors that drive it. This could prove particularly valuable, he notes, "in areas where ground-based surveillance is limited."

Ernst and Hayden, meanwhile, plan to continue their research with a comparison of two cities in Mexico that have somehow remained dengue-free while surrounding areas have not. They hope further comparisons will help shed light on how the disease moves and behaves.