New research published in the journal PLOS Currents: Outbreaks shows that the key factors needed to produce a Zika virus outbreak will be present in 50 cities in the United States by this summer. Zika has been making its way across South and Latin America and has already caused outbreaks in 20 countries. And with warmer months on the horizon, conditions are favorable for a widespread outbreak in the United States.
Weather forecasts for this summer predict a 40 to 45 percent chance of warmer-than-average temperatures over most of the continental United States. According to Andrew Monaghan, a scientist at the National Center for Atmospheric Research and lead author of the study, the warmer temperatures could lead to perfect conditions for the Aedes aegypti mosquito (which transmits the virus) in much of the South and East. Monaghan said that in the hottest regions of Texas, Arizona and California, above-normal temps would be less favorable, but travel patterns analyzed show cities in South Texas and South Florida are the most vulnerable.
Computer simulations in the study concluded that the outbreak could occur along the East Coast as far north as New York City, across the southern tier of the country, and as far West as Phoenix and Los Angeles. Even though the mosquitoes that carry Zika are not native to middle American cities, such as Denver and St. Louis, there could still be an introduction of the species via transport. Additionally, the study noted that northern cities could become vulnerable if a related species of mosquito that is more tolerant of cold temperatures begins to carry the virus.
Results of the study show the virus-carrying mosquito population will start to increase in April in the Southeast and some parts of Arizona. By June, nearly all the 50 cities are likely to have low-to-moderate levels of the mosquito, and the most eastern cities are likely to have moderate-to-high levels. By July, August and September, conditions are at their peak for mosquito breeding, and weather conditions will remain favorable until as late as November in cities in the South and West.
Two other factors are also important in assessing where the outbreaks will occur: travel by people into the U.S. from Zika pandemic areas and socio-economic factors.
Photograph by National Center for Atmospheric Research
The researchers estimate that there are regular, direct flights from 22 Latin American countries and territories listed on the CDC's Zika travel advisory. Additionally, almost five times as many people cross the U.S.–Mexico border per month than arrive by air in all 50 cities. Those same border areas tend to have households that are below the poverty line where the use of air-conditioning is not as frequent. These home are also likely to have torn or missing window screens, which increases exposure to mosquito bites.
The majority of people who have been infected with Zika so far have suffered mild, flu-like symptoms that generally clear up in one week. However, pregnant women are advised to be cautious, as contracting the virus can lead to microcephaly, a rare birth defect characterized by an abnormally small head and brain damage.
Monaghan advises pregnant women to continue following warnings to avoid travel to pandemic areas. And if you're pregnant and living in the southern part of the U.S. where the mosquitoes are present seasonally, then take actions to avoid exposure. "It's a day-biting mosquito," Monaghan says.
He also advises that pregnant women should wear long sleeves during the daytime, and pregnancy-safe mosquito repellent if you'll be exposed outdoors.
"This mosquito is quite unique because it breeds in artificial containers of water such as buckets, barrels, tarps, baby swimming pools and bird baths," Monaghan says. To avoid a breeding ground in your yard, he suggests regularly emptying or flushing any containers where mosquitoes could breed so they're unable to complete their lifecycle—which lasts about a week.
Although there is very little risk in the U.S. at the moment because of the lack of mosquitoes due to the current weather, the researchers say as we move into warmer weather seasons, the risk will increase. A local outbreak happens when most or all of the contributing factors are present: weather, travel, poverty and human exposure.
Monaghan says that the study isn't meant to be alarmist; it's a multifaceted look at the risk and all the contributing factors that could lead to widespread outbreaks. "You need to be outdoors quite a bit and have things around your home that can be breeding ground for the mosquitoes," he says.