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Global Spread of Zika Linked to Types of Mosquitos that Transmit It

More cities than previously assumed could soon grapple with the Zika virus if two species of mosquitos are found to be equally effective carriers of the disease.

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AUSTIN, Texas — More cities than previously assumed could soon grapple with the Zika virus if two species of mosquitos are found to be equally effective carriers of the disease, a University of Texas at Austin disease ecologist and his colleagues argue in the current edition of The Lancet Infectious Diseases.

Even if only one mosquito species proves capable of spreading Zika, the scientists say cities such as Miami and Houston are at high risk of seeing more of the disease this summer.

Risk map for spread of Zika virus

The circles depict the top 100 cities to which Zika virus might be imported from current infected regions of Latin America and which have suitable habitat for Aedes aegypti or Aedes albopictus in the surrounding region. The size of the circle is the estimated relative risk. Solid red indicates the risk from A aegypti alone. Open circles indicate total risk from both species. The inset shows the status of south Florida, the Caribbean, and nearby areas in more detail. Lauren Gardner

Sahotra Sarkar, a faculty member in the Department of Integrative Biology and Department of Philosophy at The University of Texas at Austin, and his colleagues Lauren Gardner and Nan Chen at the University of New South Wales in Australia, mapped how Zika is likely to spread in 100 cities worldwide under two different scenarios, taking into account air travel to and from affected areas in Latin America and the prevalence of two common mosquito species associated with the disease.

Although both mosquito species have been detected with Zika, the researchers point out that only one type of mosquito is known to be effective at spreading the virus. If the second species also is capable of spreading Zika among people, more places in the United States, Europe and Asia than previously thought will probably experience outbreaks.

“The spread depends critically on if a second mosquito species is a very good transmitter of Zika,” Sarkar says. “If it is, then the risk is even worse than what has been forecast so far.”

Public health officials with the World Health Organization (WHO) and other agencies have forecast the spread of Zika with too little information about the effectiveness of different types of mosquitos in spreading the disease, Sarkar says. Under the first scenario, in which only one mosquito species spreads Zika, existing models significantly overestimate the danger of the Zika virus in most of the world. In the United States, for example, only Florida, Texas and Louisiana — home to the Aedes aegypti mosquito — would probably experience widespread cases of Zika.

“Among urban areas in the U.S., Miami and Houston are at the greatest risk,” Sarkar says. “In general there is greater risk to Florida than Texas because more travel to Latin America and the Caribbean occurs there.”

Under the second scenario, however, in which another species — Aedes albopictus, known to harbor the disease but not yet known to be good at transmitting it — is also capable of spreading Zika, the WHO model underestimates the likelihood of transmission, Sarkar and his colleagues find. Parts of the world previously believed to be at low risk — such as Canada, Chile and many countries in Europe and Asia — would probably experience a spread in Zika. Under this scenario within the United States, New York would face an even greater risk than Houston.

Sarkar and Gardner’s findings came out the same day as a study from the National Center for Atmospheric Research that looked at how Zika would be likely to spread in U.S. cities this summer. In taking into account the ecology of the two mosquito species worldwide and explicitly showing the difference in risk levels if Aedes albopictus proves capable of spreading the virus, the map from Sarkar and Gardner points to the importance of learning immediately whether both mosquito species can effectively spread the Zika virus to ensure an appropriate public health response in different regions, the scientists say.

Regardless, Sarkar says, cities such as Miami, Orlando, Houston, Tampa and New Orleans should step up efforts to sample and monitor local mosquito populations and be prepared to implement drastic mosquito control measures. And travelers to Latin America should continue to take precautions to protect themselves, notes Gardner at the University of New South Wales.

“The risk of additional Zika spread is further heightened,” she says, “by the upcoming 2016 Summer Olympics, which will be hosted in Rio de Janeiro, Brazil, the epicenter of the Zika virus outbreak.”

Click here for an interactive Zika risk map showing relative risks by city.