Last week, the Centers for Disease Control and Prevention reported something it had been anticipating for months: Zika is now spreading locally in the United States.
The announcement came after the agency and the Florida Department of Health identified four individuals that officials believe contracted the disease from mosquitos in Miami carrying the virus.
By Monday, Florida officials had confirmed another 10 cases of Zika in the Miami area, leading the CDC to take the unusual step of issuing a travel advisory for pregnant women, or women hoping to become pregnant, to avoid a 1-square-mile area north of the city’s downtown. It was a first time the CDC has ever told pregnant women to steer clear of a specific place in the continental United States.
A total of 15 cases have now been reported in Florida, and officials are continuing to investigate the extent of the outbreak, testing contacts of those affected and community members for the disease.
The threat of the Zika virus spreading beyond this tiny area lingers. It is, however, a risk constrained by geography and climate — there are only so many places where the mosquitos that transmit the virus can live. And some places are more at risk than others.
Let’s break it down.
The CDC issued a travel advisory for a very tiny slice of Miami
Here’s a map of Miami.
Let’s zoom into Wynwood.
Outlined in black here is the boundary of the area where the CDC has issued a travel advisory.
The CDC believes that 12 of the cases can be pinpointed down to a 150-square-meter area within this district. Mosquito control has been difficult in this area, Tom Frieden, the CDC director, admitted Monday.
"Despite the daily use of spraying, the vector control experts there are seeing new larval mosquitos, and moderately high Aedes aegypti counts," Frieden said, referring to the mosquitos that transmit the virus.
This is the first incidence of local transmission, but travel-related Zika cases have been popping up all over the country.
Much of Central American, South America, and the Caribbean has seen active local transmission of Zika. Many Americans travel to these countries throughout the year, and some have brought back Zika infections with them.
As of the end of July, only four states — Idaho, Wyoming, South Dakota, and Alaska — had not seen any travel-related Zika cases. On its latest count, the CDC reports 1,657 travel-related Zika cases within the US (not including Puerto Rico, where the number of Zika cases have been exploding. More on that below).
For a local outbreak to begin, a mosquito has to bite a Zika-infected person, and then bite another person. If this happens in an area with a huge number of Aedes aegypti mosquitos, Zika’s spread can explode rapidly.
Where do Zika friendly mosquitos live?
There are two species of mosquito that are believed to be able to transmit Zika — the Aedes aegypti and Aedes albopictus. The bigger concern is Aedes aegypti, which has a particular fondness for biting humans.
Here is where Aedes aegypti is thought to be able to live within the United States, according to the CDC.
And here’s the range of Aedes albopictus, the bug that's believed to also transmit the disease, but hasn't yet figured into a major outbreak here or abroad.
These maps only indicate where the climate favors the mosquitos’ survival.
Some US cities are more at risk than others
Generally, cities with the highest levels of travel, the highest numbers of mosquitoes in peak summer months, and those that are in the climate range in which the mosquitoes thrive are the most at risk.
The National Center for Atmospheric Research produced the map above after analyzing the relative Zika risk of 50 US cities.
Its research, which was published in PLOS Current Outbreaks in March, combines meteorological records, simulation of Aedes aegypti mosquito population growth, estimates of human-mosquito exposure, and transportation data.
While the Aedes aegypti, the mosquito most likely to transmit the virus in the US, can't tolerate the cold of winter in many US cities, its numbers start to increase as temperatures rise. The exceptions are Florida and Texas, where the mosquitoes can live year-round. The risk decreases for the more Northern cities as summer turns to fall.
The map is not an absolute ranking. Rather, the map shows where the most critical factors align to cause an outbreak. Overall, the findings are pretty simple.
(And though peak mosquito season is ending across the country, there’s always next year to worry about. Zika is now feared to become endemic in some Central and South American countries. As long as travel remains, so does the risk of bringing Zika to the United States).
We should be concerned on the mainland, but the bigger story is in Puerto Rico
Fifteen cases of Zika in Florida is concerning. But there’s a place in the United States where the Zika outbreak has already reached epidemic proportions: Puerto Rico. In the case of Puerto Rico, health officials are dealing with a major outbreak that is much more difficult to control.
As of July 7, some 5,582 people — including 672 pregnant women — have been diagnosed with the virus, the CDC said in its latest Morbidity and Mortality Weekly Report.
“This could lead to hundreds of infants being born with microcephaly or other birth defects in the coming year,” Lyle Peterson, a member of the CDC’s Zika Response team, said in a statement.