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An Aedes albopictus mosquito, which health authorities worry may begin to spread Zika. Photograph by James Gathany, CDC.

An Epidemic 14 Years Ago Shows How Zika Could Unfold in the US

If the Zika virus comes to the United States, we could face the threat of the same sort of virgin soil epidemic—an infection arriving in a population that has never been exposed to it before—that has caused more than 1 million known infections, and probably several million asymptomatic ones, in Central and South America. It’s nerve-wracking to wonder what that would be like: How many people would fall ill, how serious the effects would be in adults or in babies, and most important, how good a job we would do of protecting ourselves.

But, in fact, we can guess what it would be like. Because we have a good example, not that long ago, of a novel mosquito-borne threat that caused very serious illness arriving in the United States. And the data since its arrival shows that, despite catching on fairly quickly to what was happening, the U.S. didn’t do that good a job.

This possibility became more real Monday when the Pan American Health Organization released a statement that predicts Zika virus, the mosquito-borne disease that is exploding in South and Central America and seems likely to be causing an epidemic of birth defects especially in Brazil, will spread throughout the Americas. PAHO, which is a regional office of the World Health Organization, said:

There are two main reasons for the virus’s rapid spread (to 21 countries and territories): (1) the population of the Americas had not previously been exposed to Zika and therefore lacks immunity, and (2) Aedes mosquitoes—the main vector for Zika transmission—are present in all the region’s countries except Canada and continental Chile.

PAHO anticipates that Zika virus will continue to spread and will likely reach all countries and territories of the region where Aedes mosquitoes are found.

Those “countries and territories where Aedes mosquitoes are found” include a good portion of the United States, as these maps from the Centers for Disease Control and Prevention demonstrate:

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CDC maps of the ranges of two mosquito species that could transmit Zika virus. Graphic from CDC.gov, original here.

The recent history is this: In the summer of 1999, the New York City health department put together reports that had come in from several doctors in the city and realized that an outbreak of encephalitis was moving through the area. Eight people who lived in one neighborhood were ill, four of them so seriously that they had to be put on respirators; five had what their doctors described as “profound muscle weakness.”

Within a month, 37 people had been identified with the perplexing syndrome, which seemed be caused by a virus, and four had died. At the same time, veterinarians at the Bronx Zoo discovered an unusual numbers of dead birds: exotics, like flamingos, and city birds, primarily crows. Their alertness provided the crucial piece for the CDC to realize that a novel disease had landed in the United States: West Nile virus, which was well-known in Europe, but had never been seen in this country before.

West Nile is transmitted by mosquitoes in a complex interplay with birds. It began moving with both birds and bugs down the East Coast and then across the Gulf Coast. As it went, the CDC realized that the neurologic illness that marked the disease’s first arrival had not been a one-time event, but its own looming epidemic within the larger one. “Neuroinvasive” West Nile, which in its worst manifestations caused not transient encephalitis but long-lasting floppy paralysis that resembled polio — and sometimes killed — bloomed in the summer of 2002 east of the Mississippi, and then moved west in the years afterward as the disease exhausted the pool of the vulnerable.

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The CDC’s maps showing the emergence of “neuroinvasive” West Nile virus disease from 2001 to 2004; areas in black had the highest incidence. Graphic by Maryn McKenna using maps by the CDC; originals available here.

So far, so normal, for a newly arrived disease. But here’s where the story gets complicated. By the beginning of this decade, West Nile had become endemic in the lower 48 states. It is not a mysterious new arrival; it is a known, life-altering threat. Its risk waxes and wanes with weather and insect populations, but it has one simple preventative: not allowing yourself to be bitten by a mosquito.

And yet: Here are the CDC’s most recent maps of neuroinvasive West Nile—showing that people are still falling to its most dire complication, 14 years after it was identified.

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The CDC’s maps for 2011-2014 showing the incidence of “neuroinvasive” West Nile virus disease; areas in black had the highest incidence. Graphic by Maryn McKenna using maps by the CDC; originals available here.

The point here is not that people are careless or unthinking; in the early years of West Nile, two of the victims were the husband of the CDC’s then director, and the chief of its mosquito-borne diseases division, who would have been well aware of the risks. (Both recovered fully.) The point is that always behaving in a manner that protects you from a mosquito bite—conscientiously, persistently, faultlessly emptying pots and puddles, putting on long sleeves and repellent, choosing when not to go outdoors—is very difficult to maintain.

Zika is not West Nile. Among other things, Zika is spread by many fewer species of mosquitoes — one or possibly two, compared to 65 for West Nile. And West Nile’s non-human hosts, birds, live in closer proximity to more of us than Zika’s, which appear to be non-human primates. But though the rare, deadly complications of West Nile virus infection are different from those of Zika, they are just as serious and life-altering — and yet we failed to protect ourselves from them. As Zika spreads, we can hope that is a lesson we learn in time.

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