By: Michael Xie
Though this year’s Olympic Games were filled with record-breaking athletes, it seems as if another name took the spotlight in Rio: Zika. The Zika virus caused health and safety concerns around the world as spectators and athletes prepared to head to Brazil in the midst of an epidemic. But was the Zika virus’ attendance in Brazil uninvited or one let in by the recent climate trends of our world?
History of Zika
The Zika virus was unintentionally discovered in monkeys of Uganda’s Zika Forest in 1947 while scientists were researching yellow fever, but the virus was not observed in humans until 1952 (1). As a flavivirus related to diseases such as West Nile, dengue, and yellow fever, Zika is transmitted to humans through infected mosquitoes of the Aedes genus, such as Aedes africanus, Aedes aegypti, and Aedes luteocephalus. The viral genome was not sequenced until 2006, at which point there were no documented outbreaks of the virus, with only 14 human cases of Zika isolated within Africa and Asia. The first outbreak occurred in the follwing year on Yap Island, a small Pacific Island within the Federated States of Micronesia (2).
Given the area’s abundance of flavivirus-carrying mosquitoes, Yap was most likely exposed to the virus through migrating mosquito vectors. However, it is also possible that a human with an undetected infection brought the virus to the island, as there was evidence of Zika in the nearby Philippines. In the spring of 2007, doctors observed the disease through symptoms such as rashes, conjunctivitis, fever, and arthritis and joint pain. At first, the disease falsely tested positive as dengue, but further testing by the Center for Disease Control and Prevention (CDC) found that the samples contained Zika virus RNA. Because Yap residents had not developed sufficient immunity, three out of four were infected in this first Zika outbreak (2).
Recently, another outbreak of Zika has sprung up in the Americas, beginning with Brazil. Patient records from early 2015 show “dengue-like symptoms” like rashes and pain reported in the city of Natal (3). The introduction of the disease can be attributed to international visitors that came for the FIFA World Cup in 2014. Rapid travel easily carried both infected vector mosquitoes and diseased humans around the country. During the epidemic, incidences of microcephaly, the condition of having an abnormally small head, seemed to rise. People speculated that the Zika virus could be associated with the condition in fetuses and directed warnings about the disease towards pregnant mothers. Attempts to control the virus through its vector mosquito were complicated by the Aedes mosquito’s efficient adaptation to urban environments, although certain areas of Brazil had a high-elevation climate rendering them unfit for the mosquito and safe from the virus (4).
Zika and Climate Change
Both epidemics show that the spread of the Zika virus is determined by the nature and locations of its vector mosquito. Therefore, climate patterns affecting the distribution of Aedes mosquitoes can also affect the distribution of Zika. It has been found that the Aedes aegypti mosquito, the most common virus vector, survives best in tropical and subtropical regions around the equator and between the 10 °C January isotherms (5). This temperature region roughly covers the area between 45 degrees south and 35 degrees north. Areas of higher humidity and higher rainfall are also more favorable to the mosquitoes, as these characteristics assist in mosquito breeding and survival by preventing adult mosquito desiccation (6).
Since climate has such a large impact on Zika distribution, it follows that the changing climate around the world will also change the scope of the virus. It is projected that the global average for land temperature will rise roughly 3.1—4.8 °C by 2061—2080, with the largest increases at middle to high elevations.5 Land precipitation is expected to increase significantly in most regions.
First, we look at regions currently at risk for Zika. These areas of the world currently have climates suitable for sustaining Aedes mosquitoes. Analyzing future trends in climate, researchers have found that several of these regions will see an increase in the abundance of Aedes mosquitos over the next 40—60 years. Type 1 occurrence patterns, in which the vector is highly abundant year-round, are expected to increase by 44—54% around the world. Type 2 patterns, in which the vector is present year-round but only seasonally abundant, are expected to increase by 15—33% (5).
In regions currently unsuited for the Aedes vector mosquito, seasonal suitability is expected to increase in the next 40—60 years. Type 4 patterns, in which the vector is only seasonally present, will expand into regions that are now mosquito-free. Type 4 patterns are expected to expand by 8—18%, with growth concentrated in mid-latitude regions such as Europe. This expansion, combined with population growth, will increase human exposure to the Aedes mosquito in previously unexposed regions (5). The problem is compounded because people living in these regions would likely not have the immunity and resistance that those more regularly exposed to the mosquito have built. The intertwining of climate change and global health threats extends far beyond the effects on the Zika virus. Models of other diseases, such as malaria and dengue fever, also predict climate-induced changes in transmission. With malaria, it was shown that global temperature rises of 2—3 °C would increase both the length of malaria season and the risk of malaria by 3—5%.6 On top of the diseases, phenomena such as unsustainable heat waves, cyclones, and flooding may cause direct mortality. But there are subtler dangers as well. Asthma incidences in the United States has increased more than fourfold in the past three decades, which can be partially attributed to climate-related factors. For instance, plants such as ragweed can produce roughly 60% more pollen when grown under an abundance of carbon dioxide. Accelerated trade winds over the Atlantic caused by pressure gradients over warming waters even bring air pollutants from expanding African deserts to the Americas. While it may have been originally thought that climate change and human health were unrelated, these trends appear to show otherwise (7).
Looking towards the future, changes in our behavior seem to be necessary to combat the change in the Earth’s environment. For instance, resources can be allocated to improve city conditions so that diseases like Zika do not have a chance to spread as rapidly as they did in Brazil. In the past, the impacts of climate change have been largely underestimated as purely environmental concerns. Considering the bigger picture effects of climate change, clean energy and green thinking may be among the ways toward not only saving the Earth for posterity but also controlling and solving many of today’s global health issues.
Michael Xie ‘20 is a freshman in Thayer Hall.
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 Patz, J. A. et al. In Climate change and human health: risks and responses, McMichael, A. J. et al., Eds.; World Health Organization: Geneva, CH, 2003; pp. 103-132.
 Epstein, P. R. N. Engl. J. Med. 2005, 353, 1433-1436.