UPDATE (Aug. 1, 2:20 p.m.): Florida Gov. Rick Scott announced Monday that state health officials have found 10 more people with Zika virus who are believed to have been infected by mosquitoes in a Miami neighborhood, for a total of 14 people. The U.S. Centers for Disease Control and Prevention has recommended that pregnant women avoid traveling to an area that includes part of the city’s Wynwood neighborhood.
When the news broke Friday that four people with Zika virus in southern Florida had likely been infected by mosquitoes in a tiny section of Miami — in what would be the first known Zika infection via mosquito in the continental U.S. — one of the editors here at FiveThirtyEight asked how sure officials could be that the Zika came from mainland mosquitoes. None of these winged menaces has been caught with the virus yet; shouldn’t we wait until that happens before drawing any conclusions? And, we wondered, why is the state only considering a small neighborhood an active risk for transmitting the virus?
Finding mosquitoes with Zika is not an easy task. In Brazil, there have been more than 66,000 confirmed cases of the virus, which can be dangerous for pregnant women, who are at risk of giving birth to infants with neurological defects as a result of the infection. Brazil has had 1,687 confirmed cases of brain-related birth defects in newborns associated with Zika since the first case was identified in April 2015. But it wasn’t until May of this year that researchers found a mosquito in the wild that was carrying Zika. As Tom Frieden, director of the Centers for Disease Control and Prevention, said during a call with the media Friday, it’s like looking for a needle in a haystack.
Saying with certainty how someone with Zika acquired it is nearly impossible. Until now, most Zika cases reported in the U.S. have been in people who had traveled to or lived in a Zika-affected country. There have also been reported cases of sexual transmission. But in all four of the Florida cases — which were confirmed earlier this month — sex and travel have been ruled out as causes, according to the CDC, and public health officials are operating under the assumption that the infections took place in Miami.
That the state investigation of active transmission of Zika is focusing on a narrow area just north of downtown Miami was striking as well. It mostly consists of a small neighborhood called Wynwood, a rapidly gentrifying area that’s home to art galleries, restaurants, bars, businesses and residences and thus attracts visitors from many parts of the city. Although the CDC and the Florida Department of Health have provided only limited information so far, Frieden offered some explanation for why they have homed in on that area. For two of the infected people, Wynwood is the only known location where their paths coincided. That connection and the fact that Aedes aegypti mosquitoes, believed to be the main type that transmits Zika, travel a maximum of a few blocks in their lifetime have led state epidemiologists to believe the infection occurred in that area.
Because Florida sprays for mosquitoes near the residence of anyone identified as having Zika, even those who acquired it abroad, the state has been spraying in Wynwood for several weeks. Frieden said that for now, health officials believe the risk in the neighborhood is under control; they will be concerned only if they start seeing more cases in a few weeks.
Frieden stressed that we should expect to see more cases of locally acquired Zika in the future but that it’s unlikely that the U.S. will experience a major outbreak similar to those that Brazil and other Latin American countries have had. One reason may be that people here spend more time indoors with window screens and air conditioners than in those other countries. Also, past experience with viruses such as chikungunya and dengue show that the U.S. is not generally susceptible to major outbreaks of mosquito-borne viruses.
That is, of course, except for Puerto Rico, which has had locally acquired Zika since December. As of July 7, at least 5,582 people had been diagnosed with the virus there, including 672 pregnant women.