Dengue-Free in the Desert

An aedes aegypti mosquito. (Wikimedia Commons)

An aedes aegypti mosquito. (Wikimedia Commons)

New findings—or rather, lack thereof—have raised an intriguing mystery for disease control experts: Why has dengue fever, one of the world’s most prolific mosquito-borne illnesses, skipped over Tucson, Arizona despite what would appear to be optimal conditions for infection?

On Thursday, researchers from the National Center for Atmospheric Research released the results of a comprehensive survey comparing Tucson to Key West, Florida, an area where dengue (nicknamed “breakbone fever” for the excruciating muscle and joint pain it causes) has firmly re-established itself in recent years.  Although the two towns share numerous risk factors, including a warm climate and an outdoor-minded population, Tucson has somehow remained dengue-free.  In fact, all of surrounding Pima County has zero confirmed cases of far in 2013 and just 3 total since 2010, a miniscule amount compared to 144 cases (and counting) in Miami-Dade County over the same time period.  More puzzling still: mosquito eradication campaigns are far less numerous in Arizona than they are in Florida.

Dengue is notoriously resilient, owing largely to the hardiness of its primary carrier, the aedes aegypti mosquito.  Aedes aegypti larvae can hatch in just trace amounts of water, such as that which collects in a windowsill or a tire well.  This makes it fairly easy for dengue to hitch rides to new locations and re-infect areas that had previously been cleared.  The U.S. was dengue-free for more than 50 years prior to 2009, but ever since then, cases have sprung up in 39 different states.  There is no effective cure or vaccine.

So far, a dengue vaccine has remained elusive.  The disease has four separate serotypes, or strains, and immunizing a patient against any one of them will leave him/her at greater risk for the other three.  Last year, a promising vaccine candidate fell short in clinical trials, leading some to conclude that mosquito suppression methods (such as infecting males with dengue-proof bacteria or using genetic manipulation to create “flightless females”) offer the only hope of combating the disease in the near term.

Aedes aegypti mosquitos are quite comfortable living near humans and thrive in and around stagnant water sources.  But unlike the malaria-carrying Anopheles mosquitos, which feed at night, dengue carriers feed during the day when more people are out and about.  The widespread use of central air conditioning systems reduces the risk somewhat when indoors, but the study found that Tucson and Key West were more or less equal in that regard.

All of which brings us back to Tucson, with its hot climate and its 500,000 residents, 20% of whom have outdoor swimming pools.  What to make of this strange contrast?  The researchers imply that some human behavioral factors must be in play in order to explain this strange dengue blind spot, which could very well be true.  Arizona residents might have some regional quirks that make them more adept at preventing mosquito bites.  But there’s another possible answer out there: desert climate.

Ever since my sister and I climbed Mount Sinai in the pre-dawn hours wearing only T-shirts and flip-flops a few years ago, I’ve been all too aware of just how bone-chillingly cold deserts can get at night.  The temperature swings are extreme: 45 degrees or more between sundown and sunrise.  To illustrate what I’m talking about, here’s Tucson’s temperature chart for the past 12 months:

Tucson

photo: weatherspark.com

The key threshold here is 50° F, which is widely considered to be the lower bound at which mosquitos can reasonably function.  Any colder and they have to retreat into hibernation.  Even at 60° F, they begin to get lethargic and sluggish.  Looking at the chart above, you can see that while Tucson has some wild spikes, it spent five consecutive months with lows that regularly dipped below 60° F (six, if you count November 2012).

Now look at Key West’s chart:

KWEST

photo: weatherspark.com

Quite a difference, right?  Notice the scale on the Y-axis: the daily temperature has a lower overall variance, but there are relatively few days that dip below 60° F and exactly zero below 50° F.  Hence, mosquitos in Key West have all 12 months to establish themselves without interruption, as opposed to Tucson where the deck is stacked heavily against them for half the year.  The similarly dengue-free zone of Nogales, Mexico, would seem to fit this same criteria.

I am not suggesting that temperature range is the simple solution to this difficult problem; it’s surely more complicated than that, and tracking infectious disease vectors always involves numerous interconnected factors, not the least of which is human mobility and the ease with which mosquitos can spread out within a population center.  However, I’d be curious to see climate differentials investigated in greater detail than they were in this particular study.  It could be that these two test cities are not, in fact, all that similar when this wrinkle is taken into account.

Dengue has been a seemingly intractable global problem for several decades now, but Tucson has defied the odds for longer than expected.  Let’s hope that the trend continues long enough for scientists to figure out the city’s secret.

Trent Knoss is the digital editor at Backpacker Magazine and lives in Boulder, Colorado.

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