The US is actually winning the war on mosquito-borne disease

After months of watching Zika infect thousands of people throughout Brazil and then much of Latin America, the virus is now being spread by mosquitoes on the US mainland.

So far, local transmission of Zika has been limited to part of the Wynwood neighbourhood in Miami and a 1.5 square mile swath of Miami Beach. And while there will certainly be more cases in the continental US, there’s at least one reason to believe that Zika won’t become endemic here the way it has elsewhere: We’ve actually been through this before.

While Zika is new to North Americans, viruses spread by Aedes aegypti mosquitoes are not. In fact, dengue virus — which usually causes mild symptoms like headaches and joint pain but can also (rarely) lead to internal bleeding, difficulty breathing, and death — has popped up on the US mainland for hundreds of years.

Since it spreads just like Zika does (not only via the A. aegypti mosquito but by A. albopictus too — a possibility for Zika as well), it’s instructive to look at how the more established virus has spread on the US mainland versus other countries, particularly in Latin America.

The good news, as Dr. Marc Siegel also noted in Slate, is that this history should actually help quell panic on the US mainland (while we focus our efforts on countries where the risk is greater). “We’ve fought the battle against [Zika’s] host, the Aedes aegypti mosquito, before. Many times, in fact,” Siegel wrote. “We’ve always come out on top.”

A model for the spread of Zika?

Dengue, “the most common and fastest-spreading mosquito-borne virus in the world,” infected 3.2 million people around the world last year, including many throughout the Americas.

While dengue is most severe in developing countries, its history on the US mainland is long, as researchers detailed in a paper in the journal Public Health Reports.

The first report of a “dengue-like outbreak” here was in Philadelphia, four years after the signing of the Declaration of Independence. In the nineteenth and early twentieth centuries, dengue outbreaks with thousands of cases were reported in southern states every few years. But after World War II, efforts to eradicate yellow fever, which is also spread by Aedes aegypti mosquitoes, changed that.

Between 1945 and the 1980s, there were only sporadic dengue outbreaks in the US, generally near the Mexican border. Then, as now, the vast majority of cases diagnosed in the US were imported from other countries, not acquired here, suggesting that dengue has not set up permanent shop in our mosquito population.

Today, however, dengue is on the rise around the world.

Dengue mapBouri et. al., Public Health Rep. 2012 May-Jun; 127(3): 259 — 266.The range of Aedes aegypti and Aedes albopictus mosquitoes (in black, as of 2009) and locally acquired dengue cases in the US between 1980 and 2010 (grey dots).

In 2009, the CDC announced that a doctor had discovered a case of locally acquired dengue in Key West, Florida. It was the state’s first locally acquired case since 1934. Ultimately, 65 cases of dengue were discovered nearby, and the CDC estimated that the infection rate in the Key West area was 5% — no small thing. But that outbreak was contained.

In the past 20 years, there have also been dengue outbreaks in Hawaii and Texas. Last year’s dengue outbreak on the Big Island in Hawaii, in which there were 264 confirmed cases, was the largest in the US in decades. Yet that was also contained within months. By April, the Hawaii Department of Public Health said that no one on the island was infectious.

So far, in 2016, 302 cases of dengue have been reported in North America, compared to 1.7 million throughout the rest of the Americas. The virus has not become widespread on the US mainland the way it is in Puerto Rico and in many countries in Central America, South America, and the Caribbean.

That’s in large part because our highly developed public health system has learned, over many decades, exactly how to contain the fearsome little beasts that spread these viruses: by ridding communities of standing water (where mosquitoes like to breed) and spraying insecticides widely. People who live in the US are also lucky to be in a country where both window screens and air conditioning are very common, helping to keep mosquitoes away.

We can’t be certain, but given that dengue and Zika are spread in the same way, we can expect that Zika’s impact on the US mainland will be similar to dengue’s. That means we will most likely see pockets of locally-acquired Zika cases in the Gulf states and occasional outbreaks, but not the same kind of widespread epidemic that’s happening in places like Brazil.

Mosquitoes are among humanity’s greatest foes, leaving a trail of death and destruction in their wake. That problem is far from solved. But most Americans should worry more about mosquito-borne illnesses around the world, not here at home, where we’re prepared to win this battle — as we have before.

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