A new study suggests southeastern Texas and Florida are the most vulnerable parts of the continental United States to incursion by the Zika virus, believed to be the cause of an epidemic of birth defects in Brazil.
Researchers analyzed international travel patterns from Brazil’s Zika hot spots, plotting traveler destinations that also had populations of mosquitos that can transmit the virus. The combination of infected travelers and the right types of mosquitoes could ignite the spread of Zika virus in new regions, especially those where mosquitoes are active year round.
Significant parts of Mexico, Central America, and the Caribbean would be at risk of seeing imported Zika touch off a spread of the virus at any point in the year, the researchers reported in the journal The Lancet.
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In the United States, year-round risk lies on Florida’s peninsula and in a diamond-shaped part of Texas between San Antonio and the Mexican border, according to the authors, from Canada, the United States, and Britain.
Zika virus transmits in a cycle involving mosquitoes and people. A mosquito becomes infected by taking blood meals from people with virus in their blood. When it takes future meals the infected mosquito passes the virus along.
The virus spreads geographically when a person who is infected travels to another setting and is bitten there, potentially starting a transmission cycle in the new locale. It could also happen if an infected mosquito was transported to a new location, in shipped goods for instance.
Zika virus is transmitted by Aedes mosquitoes — Aedes aegypti and possibly Aedes albopictus. The former are found in the southern United States while the latter are hardier and have been found as far north as New York.
Zika virus causes mild disease. But there is mounting evidence that infection during pregnancy can harm a developing fetus. Since October, Brazil has seen a sharp increase in cases of microcephaly — infants born with small heads and underdeveloped brains. Zika virus started infecting huge numbers of people in Brazil last May.
The senior author of the paper, Dr. Kamran Khan, uses international air traffic data to analyze where the risks for disease spread lie.
Concerned about Zika’s recent spread through Brazil, Khan — an infectious diseases physician and scientist at St. Michael’s Hospital in Toronto — teamed up with researchers at Oxford University, Harvard, the University of Washington, and other institutions to construct a mathematical model factoring in where Aedes mosquitoes are found and where the local climate might favor year-round or merely seasonal spread of Zika virus.
The model assumes albopictus mosquitoes can transmit the virus as effectively as aegypti mosquitoes can, Khan told STAT. If they cannot, the risk of local spread outside of Florida and southeastern Texas would decline.
Experts at the Centers for Disease Control and Prevention have said the United States is likely to experience some local spread of Zika virus. There has already been a case in Puerto Rico involving a person who had not traveled.
“I think the bottom line is, yes, local transmission can occur, particularly in the areas of the southern United States where vector mosquitoes are present. But we don’t expect very large outbreaks,” Dr. Lyle Petersen, director of the CDC’s division of vector-borne diseases, told STAT.
Petersen said the US experience with related viruses — the ones that cause dengue fever and chikungunya — suggest small clusters of cases are more likely.
The authors estimate more than 60 percent of Americans live in parts of the country where local Zika transmission might be able to take place, if the virus were imported at a time of year when mosquitoes are active.
Khan admitted that is a worse-case scenario, though, because it is dependent upon albopictus mosquitoes playing a real role in the spread of the virus.
Still, researchers said 22.7 million people live in parts of the United States where the climate could allow for mosquito activity year-round, meaning an imported infection at any time might trigger some local cases.
None of this predicts that the virus would become entrenched and spread continuous here. And even if someone came home sick with Zika infection, it doesn’t necessarily follow that local transmission will take place.
Still, the Zika virus is going to pose significant challenges, said Michael Osterholm, director of the Center for Infectious Diseases Research and Policy at the University of Minnesota.
Osterholm, who was not involved in Khan’s study, said Aedes mosquitoes are hard to control. They are like the rats of the mosquito world, he said — highly adapted to living in urban settings. “They do well in human environments.”