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Concern grows for spread of Zika in southern US as CDC expert declares the virus 'scarier than we initially thought'

Weather can be a major influence on spread of Zika virus and the life cycle of its mosquito host, especially in areas with warm weather and high humidity. Therefore, in the United States, the virus poses a higher risk in southern areas.

While there has yet to be a case of an infection transmitted locally in the continental United States, the Zika virus, which has already spread across much of South and Central America, has sparked increasing concern from both the Centers for Disease Control and Prevention (CDC) and the World Health Organization (WHO) because of its impacts on pregnant women and their unborn children.

Dr. Anne Schuchat, a deputy director of the CDC, weighed in on increasing concerns of the U.S. risk this week.

"Everything we look at with this virus seems to be a bit scarier than we initially thought," Schuchat said. "And so while we absolutely hope we don't see widespread local transmission in the continental U.S., we need the states to be ready for that."

Candice Burns Hoffmann, a press officer with the National Center for Emerging and Zoonotic Infectious Diseases, said southern areas of the U.S. have the highest risk due to weather conditions.

In a recent map, the CDC highlights the potential range of Aedes aegypti mosquitoes in the U.S. during 2016. Aedes aegypti mosquitoes are the type that can carry Zika virus.

"Changes in weather patterns and the environment may influence vector-borne diseases by affecting how quickly a pathogen such as Zika virus replicates in its mosquito host, Aedes aegypti, the life cycles of mosquito vectors [and] the distribution of disease causing pathogens and animal hosts," according to Hoffmann.

The areas of primary risk for Zika virus in the Americas are restricted mainly to tropical and sub-tropical regions, extending northward to the southern U.S. where the disease carrying mosquito populations exist in numbers high enough to support local transmission.

The mosquito-borne Zika virus can spread from a pregnant woman to her fetus and has been linked to a serious birth defect of the brain, microcephaly. This has made understanding this connection a top research priority, Hoffmann said.

"The Zika virus (ZIKV), a flavivirus related to yellow fever, dengue, West Nile, and Japanese encephalitis, originated in the Zika forest in Uganda and was discovered in a rhesus monkey in 1947. The disease now has 'explosive' pandemic potential, with outbreaks in Africa, Southeast Asia, the Pacific Islands, and the Americas," according to an article published in the Journal of the American Medical Association.

To date 346 infections have been reported in the U.S. which were associated with travel, according to the latest CDC report.

There are known relationships between weather conditions and mosquito-borne transmissions of viruses, Hoffman said. "For example, drought conditions are associated with West Nile virus outbreaks," she said.

Higher temperatures and rain can influence how quickly viruses like Zika can replicate in its mosquito host, she added.

"At warmer temperatures, virus particles replicate faster, leading to higher viral loads, which may contribute to more efficient transmission," she said. "There is, however, an upper limit to temperature at which mosquitoes cannot easily survive and populations are suppressed. This temperature is about 35 C, or 95 F."

In addition, mosquitoes complete development from egg to adult more quickly at higher temperatures, leading to greater transmission risks.

In order for the Zika virus to be transmitted, an adult female mosquito must feed on an infected person.

Once it feeds, the virus must escape the mosquito gut and migrate to the salivary glands so that upon the next bloodmeal the virus can be transmitted. This period is considered the extrinsic incubation period, which occurs more quickly at higher temperatures.

Temperature and humidity can also have a significant influence on mosquito survival and reproduction.

"In the U.S., there is typically a northern-most boundary that defines where a particular mosquito vector can survive. This threshold is usually species specific and defined primarily by temperature. Because of the correlation between altitude and temperature, there is typically also an elevation threshold as well," Hoffmann said.

Aedes aegypti, the host of Zika, has been present in the "New World" for hundreds of years, Hoffmann added, citing historic outbreaks of both dengue and yellow fever from the earliest days of the newly-forged United States.

"Since Aedes aegypti is a container-breeding mosquito, rainfall can also influence mosquito population levels in a given location," Hoffmann added. "Greater amounts of rainfall have been correlated with larger mosquito populations due to increased breeding habitat."

Like with temperatures exceeding 95 F, very high levels of rainfall have also led to smaller populations in mosquito populations that carry West Nile presumably due to flooding of breeding containers and the washing away of larval mosquitoes, she added. Flooding immediately reduces existing mosquito larvae populations.

According to Hoffmann, the occurrence of high winds, flooding, tropical storms and hurricanes may help reduce the infected mosquito populations.

"Natural disasters in the continental United States have rarely been accompanied by epidemics of mosquito-transmitted disease," she said. "Following the event, mosquito eggs hatch and develop and mosquito populations surge (this takes about a week). New adult mosquitoes, while often numerous, are not infected with virus until they bite an infected person or animal."

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In addition, it is unlikely high winds could transport infected mosquitoes from one area to another.

"There is no evidence that high winds can successfully transport mosquitoes into new areas where they will then survive," Hoffmann said. "Mosquitoes cannot survive high winds; they dry out and die."