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In February of 2016, a formerly little-known tropical fever entered the collective consciousness in a big way. Zika is a member of the genus Flavivirus—which includes dengue and West Nile—but had up until then been considered the mildest of the group. Outbreaks of the fever had occurred infrequently since its identification in the middle of the twentieth century, and its relatively mild symptoms meant that little to no effort was put towards finding a vaccine. Now, Zika is being blamed for a spike in cases of microcephaly in Brazilian babies, and the world has taken notice. As new Zika cases are reported daily, doctors and scientists are racing to create a vaccine.
Zika has currently reached pandemic levels throughout Central and South America and the Caribbean. More and more cases are being reported, and the information coming to light about the virus—most recently that it can be transmitted sexually—has startled researchers. So far, Zika has mostly stayed in the equatorial zone, but travelers are increasingly bringing it home with them. And although cases of Zika in the Southern United States are relatively few, there is concern that Zika could eventually reach pandemic levels right here at home.
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It is difficult to chart the evolution and effect of a disease as an epidemic unfolds. Many elements go into the making of a true pandemic. For yellow fever and Zika, as well as for other flaviviruses, the disease itself, its vectors, and the climate have to align to create the perfect storm.
Zika originated in the equatorial zone, where it still holds a grip. Its name comes from Uganda's Zika Forest, where it was first isolated in a sentinel rhesus monkey in 1947. Within a year, the virus was identified in the Aedes africanus mosquito. The first human cases were documented in the early 1950s in Africa. Like its cousin flaviviruses, it probably arrived in Central/South America and the Caribbean when it was brought by insects, infected humans, or animals. Here, its primary mosquito vector became the Ae. aegypti mosquito (also known as the "Yellow Fever Mosquito," another species that originated in Africa but is prevalent throughout the world's equatorial zones). Ae. aegypti is the same primary mosquito vector for the other flaviviruses.
The last major outbreak of Zika was in French Polynesia in 2013-14. Because of its relatively mild symptoms, it didn't receive a lot of attention. This new outbreak has gained critical attention in part because of sheer numbers—almost 2 million infections have been reported, as well as a much higher rate of microcephaly in newborns. So far cases of microcephaly seem to be isolated to Brazil, but many are concerned that rates of Zika infection and its devastating effects on unborn children and infants will spread outside of the tropical zone due to environmental and social factors.
The changing climate is one such factor. As average temperatures across the globe continue a steady, frightening rise, some experts have turned their attention to the Southern United States—where average temperatures have risen by 2 degrees Fahrenheit since 1970—as a possible site of disease epidemics. The US (among many other places) is currently experiencing an increase in the populations of disease-carrying insects like ticks, which are responsible for spreading Lyme disease and Rocky Mountain spotted fever to humans. Habitats previously free from these illnesses are now under scrutiny as climate change continues to enable the northward expansion of tick populations. Following this logic, it is reasonable to predict that other insect vectors, including mosquitoes currently found in equatorial zones, will begin to move northward as the climate becomes more hospitable for them.
While Ae. aegpyti are found in South Florida and along the Gulf Coast, the most common disease-carrying mosquito found in North America is Ae. albopictus (known commonly as the "tiger mosquito"). Like Ae. aegpyti, it is a vector for the flaviviruses, but unlike the other species, it is less effective in transmitting these viruses. Although both mosquitoes are considered urban, Ae. aegptyi is more often found in populated areas and strongly prefers to feed on humans, factors that increase the likelihood of transmission. But shifts in climate could shift mosquito populations as well, bringing Ae. aegypti closer to the Southern US. The mosquito has been recorded in warmer areas of Georgia, as well as in warmer sections of the Carolinas. All mosquitoes prefer warmer temperatures and standing water, which the South has in abundance—and which may become more prevalent as precipitation patterns shift.
Despite the predictions of climate models, though, the likelihood of a flavivirus epidemic is still not too great of a concern for the US. There are some major differences in lifestyle between the United States and areas of Latin America and the Caribbean where these viruses are hitting the hardest: These include the widespread use of window screens and the prevalence of air conditioning and climate control in homes, cars and public transport, and workplaces. "Unless we're having a barbecue," says Dr. Christopher Ohl, Professor of Infectious Disease at Wake Forest Medical School, "we're not really being exposed to mosquitoes, and those of us who are tend to try not to get bitten." In areas where these diseases are endemic, they often hit the poorest members of a community, who are far less likely to have amenities like screens, air conditioning, or even bug sprays.
Chikungunya, another of the flaviviruses also transmitted by Ae. aegypti and Ae. albopictus provides "the best model for Zika transmission in the Southeastern US," says Ohl. In 2014, multiple cases of the virus were reported in North and South Carolina. All of the patients contracted the virus while in the Caribbean. "When chikungunya happened," says Ohl, "we were concerned it might set up shop in the Southeastern US. We tried to diagnose all the chikungunya cases imported to the US from endemic areas (mostly the Caribbean at that time). When we found them, if the person had been outside for any appreciable amount of time we would spray and use larvicides, and we would tell people who had symptomatic chikungunya to stay indoors for seven days, use repellant, and try not to get bitten. We were worried that chikungunya might become established, and we had traveler cases in North and South Carolina but never any local endemic transmission in these areas." He credits preventative measures, sociocultural differences in lifestyle, and the behavior of the mosquitoes themselves as reasons that no outbreak occurred, and posits that "using the model of chikungunya would predict that Zika would also probably not be established and become endemic."
The recent revelation that Zika can be sexually transmitted, though, is perhaps a more significant cause for concern. According to Ohl, sexual transmission through the semen of an infected individual was "a curve ball." The question then becomes how to counsel men who may be able to transmit the virus sexually. Advice ranges from using condoms to abstaining entirely from intercourse with a pregnant partner for the duration of the pregnancy. While Zika can be isolated through molecular methods in semen, scientists and doctors are still unsure how long it remains contagious.
Zika is also of concern for people who aren't (or aren't planning to get) pregnant. Although the symptoms of the virus itself are relatively mild and tend to resolve within a few weeks, it has been associated with Guillain-Barré Syndrome, an affliction of the motor nerves. The likelihood that anyone who comes down with Zika will get Guillain-Barré is relatively low, but the risk does exist and has been brought to the attention of the media through high-profile cases, like that of actress Kelly-Marie Stewart, who believes Zika was responsible both for her Guillain-Barré and the miscarriage of her baby nine years ago.
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But Zika's alleged effects on children are the most serious by far. Some researchers are questioning the link between Zika and microcephaly: In a controversial report, a group of doctors from the Argentine group Physicians in the Crop-Sprayed Towns (PCST) blame a larvicide added to drinking water on the spike in microcephaly. But the suggested culprit, pyriproxyfen, is a common ingredient in bug spray and has been declared safe to use, even in drinking water. There has been no previous link between pyriproxyfen and birth defects in animals or humans. While pyriproxyfen has a very low chance of being linked to microcephaly, other environmental factors have not yet been ruled out. What's undeniable is that those most affected by Zika and microcephaly in Brazil are the underserved—the same population that doesn't have access to air conditioning and that is likelier to be exposed to toxins and pollutants.
Zika has received the most attention in the media recently because of its link to potentially devastating birth defects, but other diseases, both vector-borne and not, are creeping closer to the American South. Climate change has increased the geographic ranges of many of these diseases, and has also increased their seasons (the time periods when they are most likely to be recorded and transmitted). "There are diseases we have seen change," says Jesse Bell, PhD, who works with the Cooperative Institute for Climate and Satellites in North Carolina, "and we know the correlations between them and overall warming trends are pretty strong." One of these is Vibrio, a genus that includes the species vulnificus, a common pathogen along the Atlantic and Gulf Coasts. One type of Vibrio causes tissue lysis in coral, and another is responsible for a lot of shellfish poisoning. Exposing a cut or open wound to Vibrio can result in its infecting your skin, where is has flesh-eating characteristics. The expanding season and increased spread of Vibrio are directly correlated with warming temperatures.
Another illness that seems to be gaining ground because of warmer waters is Naegleria fowleri, a particularly nasty brain-eating amoeba that travels through the nasal cavity. Cases of this amoeba are being recorded farther north, too.
Even something as seemingly benign as pollen can threaten human health as the temperature rises. "We always think of allergies as not that big of a deal," says Bell, "but people who have respiratory issues can have asthma attacks. With climate change comes an extension of the growing season, which means more opportunities for pollen as well as increased CO2 concentration in the atmosphere, which just heightens the whole process." And, as Bell reminds me, "on average, extreme heat kills more people in the United States than any other extreme weather event."
Zika may not spread widely in the South. But other diseases and health conditions will. One way to combat their spread is to be aware of what's happening with the climate—not just on the global level, but in the realm of the hyperlocal. More people are starting to put together the pieces that link environmental and climactic shifts and human health. "Awareness is the biggest thing, just awareness and education," says Bell. "I think people think of climate change not in the context of what will happen to them, but in terms of whether or not the Arctic is warming. They need to realize that this is something that's going to impact human health."