Do vacant properties explain Miami's Zika outbreak?

Property for rent in Wynwood
Vacancy rates in Miami's Wynwood district are among the county's highest.

For months, experts predicted that local Zika transmission would hit the U.S. When mosquitos began transmitting the virus from infected to uninfected people, the threat of a major epidemic would rise significantly.

Then it happened. The first cases were detected in Miami, with a large cluster found in a one-square-mile patch on the north side of the city, in a hip, artsy neighborhood called Wynwood.

As expected, Zika appeared in a major city along the Gulf Coast, where the Aedes aegypti mosquito thrives. But why Wynwood?

The neighborhood, long known as “Little San Juan” for its large Puerto Rican community, has been quickly gentrifying over the last ten years (according to Wikipedia). Now it's also known for its street art, including the Wynwood Walls, which has driven a hipster feel and growing fashion scene.

But, as the New York Times reported, while bustling and visually appealing, Wynwood includes a “still-tattered section of run-down buildings where residents struggle in poverty.” The Times noted that the mix of housing, businesses and warehouses made fighting mosquitos particularly tough.

I ran my own analysis on data collected by the U.S. Postal Service to assess a possible link with vacant housing. Vacant and foreclosed homes have been identified as major potential breeding sites for mosquitos: as Sonia Shah wrote recently for the Washington Post, swimming pools in foreclosed homes were implicated in Florida’s 2009 dengue outbreak and Bakersfield, California’s 2007 West Nile virus outbreak.

I found a strong link. Out of Miami-Dade County’s 517 census tracts (geographical groupings of roughly 1,200-8,000 people) the three highest rates of residential vacancy are tracts 28, 31 and 22.02. One of these tracts represents Wynwood’s southern half; the others lie directly adjacent to Wynwood’s north and south limits.

Wynwood residential vacancy

In each of these areas, around 10% of homes (as represented by a USPS mailing address) had been vacant for at least 90 days, as of June 30. That’s far higher than the county average of 2.1% and median of 1.4%. These census tracts aren’t the biggest in Miami-Dade County, nor have they got the most vacancies, but they’ve got the steepest concentration of vacant homes:

Vacancy chart by tract

Each area has also got a business vacancy rate over 10%, placing them in the top quintile of Miami-Dade census tracts. On a quick look at Google Street View, you can hardly miss all the available commercial spaces, like this one:

Wynwood block for sale

When nobody’s using a property, it’s a lot less likely to stay clean. That could mean more debris and standing water, where Ae. aegypti mosquitos like to breed. As health officials scramble to prepare cities all along the Gulf Coast, residential vacancy rates could prove a useful statistic for predicting neighborhood-level risk.

(You may be wondering, what about the other early site of local transmission, South Beach? Vacancy is lower; the concentration of infected travelers is likely much higher.) 

Since the mosquitos don’t fly far—WHO says they travel about 400 meters in their lifetimes—and take 90% of their meals from human hosts, they also need environments with plenty of access to human residents and/or passersby. Wynwood’s active street life might have added fuel to the mix, especially since Ae. aegypti likes to bite during daylight hours.

But poverty is likely a driver, too. The southern Wynwood tract has Miami-Dade’s third-highest rates of poverty—a staggering two-thirds of the population lives below the poverty line, according to 2014 U.S. Census estimates:

ACS poverty MIA

Poverty contributes to numerous Zika risk factors: inadequate housing conditions make good breeding sites; people might be less likely to have protective measures like window screens, air conditioning and mosquito repellent; they may be less able to avoid outdoor work and foot transit; and they may have lesser access to family planning and antenatal care services. Indeed, socioeconomics could make a widespread Zika epidemic the next Hurricane Katrina in its vastly disproportionate impact on the urban poor.

Based on these initial findings around Wynwood, I think health officials should be looking for neighborhoods—like Wynwood—where vacant spaces meet urban poverty.

In the coming weeks, I’ll be looking at more data from Miami and from cities like Houston and New Orleans to try to see where conditions look worst for Zika transmission.

Please stay tuned and share your thoughts.