For the second time this year, health departments in Florida and the Centers for Disease Control (CDC) are reporting cases of dengue fever. Dengue, which is spread by mosquitoes, was, until recently, all but eradicated in the United States. Back in March a slew of new cases in Florida worried local health officials, but subsided. Then, earlier this month, more new cases showed up, and the number is growing. The latest outbreak has caused county officials in southern Florida to suspend blood donations. That could create a knock-on effect for the area's entire health system.
Why is this happening? Last year a team from Texas Tech University tried to figure out why "breakbone fever" had started to flare up again.
The assumption the researchers started with, and most people start with, is that tropical diseases like dengue will extend further north as the planet warms, widening the band we traditionally think of as "tropical." That's pretty intuitive: More habitat for disease-carrying mosquitoes means more habitat for the disease.
When the team looked closer, however, they found little evidence of that. They found lots of mosquitoes ranging throughout much of North America. But the disease usually wasn't present. The assumption that spreading mosquitoes would necessarily spread the disease was false.
Next, they fed what they knew about existing mosquito populations into several different models of climate conditions over the next century. The models differed in the degree they assumed greater and lesser changes in greenhouse gas emissions. The Texas Tech team focused on mosquito populations in Atlanta, Chicago, and Lubbock, Texas. Those cities mark the very edge of the habitat of one of the two mosquito strains that carry dengue, a bug called Aedes Albopictus.
Comparing the models revealed something new. The important climate impact wasn't a change in the bugs' territory or population—but their behavior. They found that hotter summers caused less bug activity, and also bug deaths. That led the mosquitoes to become more active, and breed and bite more frantically, in cooler seasons.
The new spikes-and-valleys pattern of mosquito activity led to matching spikes in dengue infections. Compressed mosquito activity meant more biting, in a shorter period of time. Thus, epidemics, rather than isolated cases here and there.
Is that what's happening in Florida this year? It's only one theory, based on a very limited sample. But 2013 Florida, with its spring and fall dengue outbreaks, sure looks like the Texas Tech team's graph. One hopes not. A "dengue season," like the flu and allergy seasons, is a nightmarish thought for hospital networks and blood supplies.