New Map Shows Where Zika Mosquitoes Live in the US

As summer heats up, new maps from the Centers for Disease Control and Prevention (CDC) gives us our best guess at where Zika-carrying mosquitoes could be hanging out this year in the US.

The Zika virus is a potentially serious virus carried by mosquitoes. As of June 2017, the total confirmed case count for all states and the District of Columbia was 1,883. For the past two years, Zika has spread in South America and into the US and so has our knowledge of the infection, its dangers, and how people can protect themselves, their partners, and unborn children.

Zika is spread through the bite of an infected mosquito. Although there are multiple species of mosquitoes in the US that can theoretically spread the disease, the primary mosquitoes of concern are Aedes aegypti and Aedes albopictus.

Identified in 1947 in the Zika Forest in Uganda, the Zika virus, transmitted mostly by A. aegypti it has been circulating in equatorial Asia and Africa, before being identified as the cause of flu-like outbreaks in the Pacific Islands. Moving to South America, the virus had a virulent emergence in Brazil, with more than one million Brazilians infected. The virus targets the brain of unborn children, causing microcephaly, and other congenital disabilities.

A. aegypti is one nasty, aggressive character. The mosquito has long been a carrier of deadly Yellow Fever, Dengue fever, and Chikungunya. Like other mosquitoes, it is the females that are human bloodsuckers, and A. aegypti is particularly fond of human habitat. It isn't particular about where it lays its eggs, and it is more likely to hunt humans all day long, rather than merely at dawn and dusk.

In addition to the habits of this mosquito, the flavivirus that causes Zika replicates quickly in A. aegypti, making it more likely that an infected mosquito can transmit the pathogen.

A female A. aegypti during dinner hour. The red coloring in the feeding tube, and her abdomen is from human blood

Unfortunately for humans, A. aegypti are not only aggressive, and partial to human blood, but a female can lay about five batches of around 100 eggs each, if she is well fed during her two-week life span.

Where in the US Can You Find A. Aegypti?

In addition to A. aegypti, the mosquito A. albopictus can also transmit Zika. Together, the two mosquitoes cover a range in the US considerably larger than many will be comfortable with. In a report from the CDC published in the Journal of Medical Entomology, officials confirm the presence of A. aegypti in 220 counties in 28 states, plus the District of Columbia. Researchers found the secondary vector, A. albopictus in 1,368 counties in 40 states and DC.

The survey, conducted at the end of 2016, updates surveillance from early in the same year. The more recent survey saw a 21% increase (38 new counties) and 10% (127 new counties) in counties reporting the presence of A. aegypti and A. albopictus, respectively.

While this survey, which analyzed historical data between 1995 and late 2016, showed recent significant jumps in the presence of these species, study authors felt those numbers could be partially due to increased attention due to the Zika virus emergence in Florida and Brazil. It is possible either mosquito could be living in other areas of the US, but has not yet come to the attention of health authorities.

The study does not illustrate the current danger of Zika in a particular area but offers information on where the mosquitoes are already present, areas in which we should worry about Zika's potential. The research also highlights surveillance gaps where the mosquitoes are likely to be present but remain unreported.

A county-level survey conducted in 2015 and 2016 by the US Centers for Disease Control and Prevention shows the historic occurrence of Aedes albopictus mosquitoes between January 1995 and December 2016 in the United States. Counties with black dots had new, previously unreported surveillance records in the second round of the survey in 2016, reflecting increased attention on surveillance for mosquitoes.

In a press release, Rebecca Eisen, a research biologist with the CDC, remarked: "This information will help to target limited public health surveillance resources and help to improve our understanding of how widespread these mosquitoes are."

A county-level survey conducted in 2015 and 2016 by the US Centers for Disease Control and Prevention shows the historic occurrence of Aedes aegypti mosquitoes between January 1995 and December 2016 in the United States. Counties with black dots had new, previously unreported surveillance records in the second round of the survey in 2016, reflecting increased attention on surveillance for mosquitoes

The study authors note the presence of the mosquito species creates the possibility that the Dengue and chikungunya viruses could be transmitted in the US if those viruses emerged here. These maps closely track with the current estimated US range map of A. aegypti and A. albopictus, that graphically illustrate the presence of these mosquito species in the US.

This map shows the estimated range of these two species.

Again, these maps do not indicate that Zika is spreading wherever these mosquitoes are present. Southern Florida and Brownsville, Texas are the only two locations where local transmission of Zika has occurred. Some biologists believe the range of A. aegypti will not advance quickly, but that A. albopictus is already endemic as far north as New Jersey, and possibly Connecticut.

A relatively large percentage of the population of the US lives in an area where one, or both, of these mosquito species reside. While it is frightening to consider Zika spreading further into the US, awareness of where these mosquitoes, or "disease vectors" are endemic helps officials and agencies understand where to deepen surveillance efforts.

For the rest of us, this information is a heads-up to be sure you use the right mosquito repellent, drain standing water on your property, and be mosquito-aware even during daylight areas in these regions of the US.

Yes, the mosquitoes may be coming — but we will be ready for them.

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Cover image by Lauren Bishop/CDC