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A third of the world’s population is at risk of catching the Zika virus, according to a new study led by scientists at Oxford University in England.
The study does not guarantee that this number of people will actually contract the mosquito-borne virus, which was causally linked last week to microcephaly and other birth defects. Instead, the researchers combine climate data, mosquito prevalence and the socioeconomic makeup of each region to chart the likelihood of the disease getting a foothold in a particular location. In three maps published today in the journal eLife, the team charts the past, present and future of this mosquito-borne virus.
“Our findings that a global area inhabited by over 2.17 billion people is highly suitable for transmission of Zika virus…emphasize why the World Health Organization has declared the current outbreak a Public Health Emergency of International Concern,” epidemiologist and study co-author David Pigott from Seattle’s Institute for Health Metrics and Evaluation said in a statement.
Zika was discovered in Uganda in 1947, and no outbreak occurred outside of Asia and Africa until 2007. That year, the virus dipped into Micronesia, causing a large outbreak on Yap Island before spreading into Oceania. Scientists suspect the virus landed in Latin America between May and December of 2013, though the first case wasn’t reported until 2015. Illustration by Messina et al., eLife, (2016)
Earlier prediction maps by the Centers for Disease Control and Prevention and other public health researchers focused on the locations of the mosquitoes responsible for transmitting the virus. So far, researchers have found 19 species of Aedes mosquitoes capable of carrying Zika virus, but the most common is the tropical pirate, Aedes aegypti. Plus in the relatively recent past, its cousin — the tiger mosquito Aedes albopictus — has blossomed, expanding its range globally.
Based on mosquito location alone, much of the U.S. would be at risk during this summer’s mosquito season, but Messina and her colleagues offer an alternative scenario. The researchers argue that only Florida and Texas are suitable for high levels of Zika transmission because of their population density and climate. Mosquito-borne viruses fester in urban areas, where they can quickly hop between the insects and humans.
That’s why, according to the new study, Brazil’s coastal cities have been hit hard by the virus, while central Brazil and its low densities of people and mosquitoes have been somewhat spared.
“We have comprehensively assembled all the data for Zika occurrence in humans, displayed it as a map and combined it with detailed predictions of where the virus could spread next,” University of Oxford epidemiologist and lead author Janey Messina said in a statement.
Climate plays a deciding role because of temperature and rainfall, and on this point, the researchers make an assumption. Mosquito-borne viruses such as Zika are like pies. The viruses need a certain amount of time inside mosquitoes at a precise temperature and humidity before they can infect humans. Scientists haven’t resolved this climate suitability window for Zika virus, but these conditions are well known for its cousin, Dengue. Dengue virus needs 10 consistent days of balmy weather to incubate inside a mosquito before becoming infectious.
Status of Zika virus reporting as of 2016 by country, showing countries that are highly environmentally suitable (having a suitable area of more than 3,800 square miles) but which have not yet reported symptomatic cases of Zika virus in humans. “Currently reporting” countries are those having reported cases since 2015. Illustration and caption by Messina et al., eLife, (2016)
By taking this temperature restriction and mixing it with what’s known about how Zika virus spreads in urban and rural habitats, the team paints a picture of possible locations for future outbreaks.
The potential risk is high in Sub-Saharan Africa, namely in the Democratic Republic of Congo, its surrounding areas and West African nations like Nigeria. India has 400 million people living in coastal and northeast areas that are suitable for Zika, based on the findings, though the country has yet to record a case. China and Oceania, likewise, harbor large regions of possible breeding grounds for Zika, while only the northernmost parts of Australia might suit the virus.
It’s unclear if these areas could also suffer from high levels of microcephaly or other neurological conditions linked to Zika. So far, only Brazil has witnessed a large number of Zika-mediated microcephaly cases (1,113). Fewer than 30 cases have been confirmed throughout the rest of the Americas.
Regardless, public health officials may use the new maps to prepare for the future.
“Our global risk map reveals priority regions where authorities could intervene to control the vector population and where surveillance of the virus should be concentrated in order to improve rapid outbreak response and clinical diagnosis,” Messina said.
Nsikan Akpan is the digital science producer for PBS NewsHour and co-creator of the award-winning, NewsHour digital series ScienceScope.
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