Katherine J. Wu is a Digital Editor at NOVA and Story Collider producer. She holds a Ph.D. in Microbiology and Immunobiology from Harvard University. Previously, she served as a AAAS Mass Media Fellow at Smithsonian magazine.

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The teeth of Shanidar 1, a male Neanderthal unearthed from Shanidar Cave in Iraqi Kurdistan. Shanidar 1 lost his right arm at the elbow, possibly due to a congenital or childhood disease or an amputation, and used his front teeth—which are unusually worn down—as a compensatory mechanism. Image Credit and Copyright: Erik Trinkaus, Washington University in St. Louis

Like Neanderthals, early modern humans used their teeth as tools

Though often hidden behind closed lips, teeth do a lot of heavy lifting in daily life. They’re the pearly whites flashed for smiles, the sharp powerhouses that grind our food.Our teeth can also take the place of tools, removing pen caps, tearing open bags of chips, and snipping tags off new clothes. Dentists <a href="https://www.deltadental.com/us/en/protect-my-smile/overall-health/oral-habits---safety/your-teeth-are-not-tools.html">decry the practice</a>, but it really comes in handy—literally: Sharp-edged and stable, teeth often function as well as fingers, earning the mouth the moniker of “<a href="https://www.nature.com/articles/sj.bdj.2012.902">third hand</a>.”“I use my mouth all the time when both my [actual] hands are tied up,” says biological anthropologist <a href="https://www.luc.edu/anthropology/faculty/drkristinlkrueger/">Kristin Krueger</a> of Loyola University Chicago. “Sometimes, it’s the only way to get things done.”Krueger’s not alone. Plenty of modern humans join her in this convenient form of dental dexterity—and that’s apparently been the case for tens of thousands of years.
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<a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0224573">Reporting today</a> in the journal PLOS ONE, Krueger and her colleagues have found that our early modern human ancestors also treated their teeth as tools, perhaps using them to support their hands in grasping objects, softening wood or fibers, or preparing animal hides for clothing.Though this strategy has long been known in Neanderthals, it’s often been dismissed as a “primitive” behavior in the fossil record, restricted to lineages without the sophistication to fashion external tools. But the new study, which shows similar wear patterns on Neanderthal and early modern human canines and incisors (teeth at the front of the jaw) takes a big bite out of that antiquated idea.“In many cases...using teeth as tools was likely a clever, expedient thing to do,” says <a href="https://anthropology.osu.edu/people/guatelli-steinbe.1">Debbie Guatelli-Steinberg</a>, an anthropologist at Ohio State University who wasn’t involved in the study.The findings also add to the growing list of traits shared by Neanderthals and our early modern human ancestors—two groups that, despite their radically different evolutionary fates, appear to have interacted with their surroundings in very similar ways.“We’re still caught up with this idea of the behavioral revolution: that anatomically modern humans are so much better [than other lineages that died out],” says <a href="https://www.kenyon.edu/directories/campus-directory/biography/bruce-hardy/">Bruce Hardy</a>, a paleoanthropologist and Neanderthal expert at Kenyon College who wasn’t involved in the study. “But it’s becoming increasingly difficult to stick with that line.”
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<figcaption>Modern humans (left) and Neanderthals (right) had their fair share of anatomical differences (including the size, shape, and features of their skulls). But years of research have shown that, contrary to prior belief, they had many behavioral similarities. Image Credit: hairymuseummatt, DrMikeBaxter, Wikimedia Commons
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Neanderthals first became known to researchers in the mid-1800s, after a partial, stocky skeleton bearing a thick brow ridge on its skull was unearthed in Germany’s Neander Valley. Scientists were quick to describe the newly dubbed Homo neanderthalensis as a species of hunched, barbaric beasts—a “lesser” lineage that had been quashed in the evolutionary race against the well-postured, intellectually savvy Homo sapiens.The Neanderthal narrative <a href="https://www.pbs.org/wgbh/nova/article/neanderthal-rib-cage-moved-breathed/">has since evolved</a>. In recent years, a deluge of studies have shown that Neanderthals weren’t the boorish brutes they were once thought to be. Like early modern humans, they <a href="https://www.amnh.org/exhibitions/permanent/human-origins/neanderthal-tools">crafted stone tools</a> and harnessed fire to <a href="https://www.nationalgeographic.com/news/2014/5/140430-neanderthals-cook-food-evolution-science/">cook complex meals</a>. They <a href="https://www.theatlantic.com/health/archive/2018/10/neanderthal-medicine/573028/">concocted medicines</a> to treat the injured and sick, and may have even gone to great lengths to <a href="https://www.scientificamerican.com/article/new-evidence-suggest-neandertals-buried-their-dead/">bury their dead</a>.Some researchers even believe Neanderthals and modern humans <a href="https://www.pbs.org/wgbh/nova/article/are-neanderthals-human/">aren’t separate species at all</a>. Whatever their differences, the two groups were certainly close enough to interact, and <a href="https://www.pbs.org/wgbh/nova/article/ancient-dna-neanderthal-complex/">even interbreed</a>—ancient rendezvous that can still be <a href="https://ghr.nlm.nih.gov/primer/dtcgenetictesting/neanderthaldna">detected in the DNA</a> of people living today.
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But old stigmas die hard. Part of the longstanding stereotype of Neanderthals is their “uncouth” practice of grasping and clamping objects with their teeth, evidenced by the severe amount of wear and tear on their fossilized canines and incisors. “They had tools, but the idea was that they weren’t sophisticated enough to not use their teeth, too,” Krueger says. Because we’re still around and they’re not, she says, many researchers assumed that early modern humans—our direct ancestors—must have been doing “something better.”Until now, though, little had been done to actually check if that was true. So Krueger and her colleagues put the theory to the test.
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<figcaption>The upper jaw and teeth of an infant Neanderthal, unearthed from the Schmerling Cave in Belgium. Image Credit: Tanya Smith, Griffith University
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When she and her team compared casts from 45 Neanderthal and 30 early modern human teeth under a microscope, they found that the two groups’ dental wear patterns were pretty much identical. Both sets of chompers carried haphazard scratch marks: some vertical, from the up-and-down motion of biting into food, but also some horizontal, likely from grasping objects that were tugged back and forth.On average, the two sets of teeth also bore similarly sized grooves and divots. The depth of these pockmarks, Krueger says, is a proxy for bite force, which increases as teeth take on more heavy-duty functions.All told, this suggests that on a day-to-day basis early modern humans and Neanderthals were doing comparable things with their teeth—results that came as a surprise to Krueger. “I was expecting to see that early modern humans weren’t [doing this],” she says. “But when I ran the numbers...I really had to shift my frame of reference.”Actually nailing down what our predecessors were sinking their teeth into, however, is a bit complicated. Animal hides and plant fibers don’t have the same staying power as skeletal fossils—which means the nature of these materials, and how they were treated, has to be inferred from only the marks they left behind.But Krueger and her team found that the marks on the ancient teeth resembled those from the dentition of the Tigara, an indigenous Arctic population that inhabited what’s now Point Hope, Alaska, between 750 and 250 years ago. The Tigara are one of many modern groups known to have frequently employed the “third hand,” using their teeth while processing animal hides or softening sinew threads for weaving. Though it’ll be tough to confirm, it’s possible that Neanderthals and early modern humans engaged in similar practices long ago—or at least close analogs befitting of the time and place they lived in, Krueger says.
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<figcaption>A set of recent modern human teeth from Amarna, Egypt. Image Credit: Courtesy of Kristin Krueger, Loyola University Chicago
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Though these behaviors are well-documented in ethnographic records of the Tigara and several other populations from around the globe, these people—and the innovative things they did with their teeth—haven’t typically been considered representative of modern humans. This oversight is perhaps indicative of scientific racism, the failure of anthropologists to be culturally sensitive and inclusive to non-Western practices, Krueger says.“We’re educated to not put [non-food] items in our mouths and bite down on them,” says <a href="http://www.drtanyamsmith.com/">Tanya Smith</a>, a biological anthropologist and teeth expert at Griffith University in Australia who wasn’t involved in the study. “Doing that contradicts what we think of as hygienic...we imagine it as a verboten behavior.”As such, the findings might be viewed as progressive, rather than regressive, and elevate the idea of teeth as tools, Krueger suggests. (As of the afternoon of November 27, representatives of Alaska’s North Slope Borough could not be reached for comment.)Uncovering and analyzing more dental samples will continue to build upon the picture that’s emerging, Smith says. While the Neanderthal teeth used in the study come from all over western Eurasia, spanning a period of roughly 200,000 to 40,000 years ago, the early modern human dental set is sparser, clustering mostly within European sites from the last 40,000 years. It would be fascinating, Smith says, to get a glimpse of the teeth of early modern humans before they left Africa—a time more contemporary with Neanderthals.It’s still unclear why the Neanderthal lineage fizzled out while ours persisted. And the use of teeth might not have had anything to do with it. “There may be many other behavioral differences [between us and Neanderthals],” Guatelli-Steinberg says, “but this wasn’t one of them.”
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New findings bolster the argument that the two groups of early humans had a lot of behavioral similarities.

Working with polar bears "isn't about what you do when you have a bad encounter," says polar bear researcher Eric Regehr, "but what you do to prevent them." Image Credit: Michael Ginzburg

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A glimpse into the lives of the MOSAiC mission’s polar bear guards—and the powerful predators they watch for.

Engineers Shanhui Fan (left) and Zhen Chen (right) with their dual-action device, which simultaneously harvests solar energy and cools surfaces without the need for electricity. Photo Credit: Linda Cicero, Stanford News

The Forecast on Energy-Efficient Cooling Just Got a Bit Sunnier

As autumn gives way to winter in the northern hemisphere, it’s sometimes easy to forget that the planet is getting hotter. Air conditioners and electric fans account for <a href="https://www.iea.org/newsroom/news/2018/may/air-conditioning-use-emerges-as-one-of-the-key-drivers-of-global-electricity-dema.html">about 10 percent of global electricity consumption today</a>—and as average global temperatures continue to rise, the world’s reliance on air conditioners will likely more than triple by 2050.This has created a bit of a vicious cycle. Homes in the United States alone expel <a href="https://www.energy.gov/energysaver/home-cooling-systems/air-conditioning">over 100 million tons of carbon dioxide</a> into the atmosphere each year from AC alone, nudging temps further upwards—and prompting people around the world to seek even more solace in temperature-controlled buildings.But researchers are now developing tools that allow us to boot unwanted heat into outer space without using any of the power air conditioning requires. Today, in the journal Joule, a team from Stanford University takes the innovation <a href="https://www.cell.com/joule/fulltext/S2542-4351(18)30471-9">one step further</a> with a device that could not only keep us cool in the absence of air conditioning, but also simultaneously harness <a href="https://www.pbs.org/wgbh/nova/tech/how-solar-cell-works.html">solar power</a> to meet energy demands elsewhere in the grid.
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The cooling half of this new technology, pioneered by electrical engineer <a href="https://web.stanford.edu/group/fan/">Shanhui Fan</a> of Stanford University, takes advantage of the natural ability of all objects and organisms to emanate heat through radiation. Unlike conduction and convection—the other methods through which <a href="https://mass.pbslearningmedia.org/resource/lsps07-sci-phys-thermalenergy/thermal-energy-transfer/">thermal energy is transferred</a>—radiation doesn’t require direct contact or the movement of fluids to move heat from place to place: It simply emanates from all matter that’s at a temperature above absolute zero (or -459.67 degrees Fahrenheit… so, for all intents and purposes, let’s call it “all matter”). For example, radiation is responsible for the heat you feel wafting out from a toasty oven even when its door is shut.Through radiation, heat can be jettisoned spaceward—and if it clears the hurdle of the atmosphere, then it’s past the point of no return: Outer space is pretty much a vast, insatiable heat sink. Unfortunately, though, outbound radiation of most wavelengths doesn’t always make it that far. Some of it ricochets off water molecules in the atmosphere and boomerangs right back to Earth; another portion in the is absorbed and retained in the atmosphere by carbon dioxide and other greenhouse gases.But not all materials emit radiation at the same wavelength. If a surface emits radiation with a wavelength between eight and 13 micrometers, it hits a sweet spot that bypasses many of these atmospheric hurdles. By slipping through this mid-infrared “window,” heat can finally enter cold, cold space—allowing the radiating object to dip below the temperature of the surrounding air. Voilà: cooling achievement unlocked.
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Fan and his colleagues first exploited this window and its associated chill factor in 2014, with the design of a so-called radiative cooler. Because the device was engineered with silicon compounds and other materials that emit in the mid-infrared range, it was able to remain <a href="https://www.nature.com/articles/nature13883?foxtrotcallback=true">around 5 degrees Celsius (or 9 degrees Fahrenheit) below</a> ambient air temperatures while atop the roof of a building in sunny California. Two years later, Fan’s lab published a follow-up that used a similar setup to <a href="https://www.nature.com/articles/nenergy2017143">cool flowing water</a>—a breakthrough that, if scaled up appropriately, could reduce the reliance of air conditioning and refrigeration on electricity.For Fan, outer space is the ultimate untapped well of renewable energy. One of the most exciting potential applications of this technology involves speckling the world’s rooftops with radiative coolers. Unfortunately, this move could ruffle the feathers of those attempting to do the same with solar absorbers and solar cells, which typically colonize that same space to harvest energy from the Sun.At first glance, the two strategies seem incompatible: one hungers for heat, while the other is keen on keeping cool: Placing conventional radiative coolers and solar panels side-by-side could even reduce the efficiency of both processes. But as humankind struggles to curb carbon emissions, it’s pretty clear we need all the help we can get. Together with lead author <a href="https://sites.google.com/site/zchen0203/">Zhen Chen</a>, a mechanical engineer at the Southeast University of China, Fan assembled a team of researchers to engineer a peaceful coexistence between the two technologies.
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“The solar energy that’s coming into Earth must eventually come out of Earth,” Fan explains. “So these technologies are complementary: Solar energy harvesting is trying to benefit from the incoming flux of energy, while radiative cooling is trying to benefit from outgoing flux.”In fact, the spectrum of the Sun’s energy most relevant for a solar panel doesn’t actually overlap with the mid-infrared window—meaning that even if the two devices were stacked atop one another, they wouldn’t have to cancel each other out. To put this idea to the test, Chen, Fan, and their colleagues slid a silicon-based radiative cooler beneath a germanium solar absorber, which soaks in sunlight for the purposes of energy-efficient heating (not to be confused with a solar cell, which adds in the extra step of converting this energy into electricity). Unlike conventional solar absorbers, however, this particular panel is transparent to radiation in that mid-infrared range of eight to 13 micrometers. This allows the cooler to pass infrared heat through the solar absorber and eject it into space unimpeded.Importantly, the two layers of this device operate completely independently of each other. The solar absorber hoards heat—but none of this energy needs to be funneled to the radiative cooler below, which, by definition, works power-free. This means that the solar payload collected above can, theoretically, be routed elsewhere.
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<figcaption>The researchers' double-layered rooftop panel. On the bottom is a radiative cooler that emits heat in the mid-infrared range into outer space. On top is a solar absorber that is transparent in the aforementioned infrared range. Photo Credit: Linda Cicero, Stanford News
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While the two halves of this device aren’t interdependent, they can still benefit from each other’s company. Many conventional radiative coolers function best at night, out of the heat of the Sun; for efficient use in the daytime, the devices need <a href="https://www.nature.com/articles/nature13883?foxtrotcallback=true">highly reflective surfaces</a> capable of deflecting light. While this keeps the temperature down, it also wastes what could otherwise be a serious kick of solar power. With the solar panel on top sponging up sunlight, though, the cooler doesn’t need this reflective boost.Shielded by the solar panel, the temperature of the team’s radiative cooler plunged as low as 29 degrees Celsius (52 degrees Fahrenheit) below ambient temperature, while the absorber roasted at a pleasant 24 degrees Celsius (43 degrees Fahrenheit) above the surrounding air—an enormous differential. One immediate application, Chen says, has already revealed itself: The temperature gradient created by these two extremes could eventually be harnessed to power <a href="https://www.youtube.com/watch?v=p1woKh2mdVQ">a thermal engine</a>.Even more exciting to Chen and Fan is the idea of replacing the solar absorber half of their device with an electricity-generating solar cell. If this switch pans out, the dual-action technology could serve as a supplement to our conventional, electricity-sapping cooling methods while simultaneously supplying a source of renewable energy.“We’re trying to change how people view rooftops to harvest renewable energy,” Chen explains. “On hot summer days, you could continuously harvest electricity, and at the same time, you could use the device to cool down your house.”
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However, Chen doesn’t think radiative cooling will ever progress to the point of completely replacing air conditioning or refrigeration. Even the <a href="http://science.sciencemag.org/content/early/2017/02/08/science.aai7899">highest-efficiency coolers</a> would probably need to be scaled up to cover entire rooftops or more—probably an untenable engineering ask, especially given the costs of the materials the team used in their prototype. The mid-infrared window also bangs shut on cloudy or rainy days, making this technology the most literal of fair-weather friends. But, Chen explains, radiative cooling could still have its day in the Sun (on a small scale, at least) in arid climes or regions with inconsistent access to power. The researchers are also looking into rebuilding their apparatus with more cost-effective materials.In the meantime, there are still many technological roadblocks to clear. While the theory behind the methods is elegant, explains <a href="https://daedalus.caltech.edu/team-member/dr-emily-warmann/">Emily Warmann</a>, a mechanical engineer studying solar cells at the California Institute of Technology who did not participate in the study, “any practical implications are very, very far away.” Simple though they sound, the necessary tweaks to boost the device’s efficiency, or make the swap from solar absorber to solar cell, could very well compromise the panel’s critical infrared transparency.Furthermore, the prototype’s radiative cooler is sealed up in a vacuum chamber to minimize unwanted heat loss through conduction and convection. To function as an air conditioning or refrigeration alternative, as in their previous work, it would need access to water—but the exposure to the surrounding air would slash its cooling capabilities <a href="https://www.nature.com/articles/nenergy2017143">back down to about 5 degrees Celsius (9 degrees Fahrenheit) below ambient temperature</a>, as opposed to the 29 degrees Celsius (52 degrees Fahrenheit) achieved within the vacuum.
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Despite these obstacles, Chen, Fan, and their colleagues have demonstrated something new and important, says <a href="https://www.researchgate.net/scientific-contributions/2030818528_Mark_A_Nurge">Mark Nurge</a>, a physicist at NASA’s Applied Physics Lab who did not contribute to the new finding. For the first time, it’s clear from the one-two punch of this device that these two sources of renewable energy don’t have to jostle for space—and with radiative cooling on our side, even the sky is no longer the limit.“This is a creative idea,” says <a href="http://www.schlaucohenlab.com/">Gabriela Schlau-Cohen</a>, a chemist studying light harvesting at the Massachusetts Institute of Technology who was not involved in the study. “If we want to actually address our growing energy needs, we need to do it in a way that’s paradigm shifting. And that means ideas that are different than incremental improvements on our current structure.”

Scientists are pioneering technology that may someday cool buildings without electricity, while harvesting solar energy at the same time.

All infectious diseases may be seasonal, according to a new scientific report—which means that every season may now come with its own set of symptoms. Photo Credit: Valerii Tkachenko, Wikimedia Commons

It's Fall, Which Means It's Time for Gonorrhea

The transition from winter to spring signals the end of months of cold snaps, sweater weather, and <a href="https://www.pbs.org/wgbh/nova/article/llama-antibodies-flu-fighting/">flu season</a>. But even as the roads clear and flowers bloom anew, it may not yet be time to shelve your sick days for the year.According to <a href="https://journals.plos.org/plospathogens/article?id=10.1371/journal.ppat.1007327">a new article</a>, published today in the journal PLoS Pathogens, all infectious diseases may be seasonal—and there’s at least one for every time of the year.Study author <a href="https://www.mailman.columbia.edu/people/our-faculty/mem2352">Micaela Martinez</a>, an infectious disease ecologist at Columbia University’s Mailman School of Public Health, presents a compendium of 69 infectious diseases that run the gamut from rare to common, neglected to notorious, innocuous to deadly. Despite their differences, all the syndromes have one thing in common: They rise and fall with changing seasons.
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Martinez, a conservation ecologist by training, initially set out to track the seasonality of acute, or short-term, infections like influenza and chickenpox after noting similarities between the disease states of humans and wildlife. But as she began to compile a list of infectious diseases that tend to plague humans, Martinez found that the trend also held true for chronic, or long-term, diseases like gonorrhea and leprosy.“There’s documented seasonality for all infectious diseases, which is not what I was expecting,” Martinez explains. “It’s an even more widespread phenomenon than we thought.”A quick look at the infectious disease calendar paints quite the sobering picture. As autumn leaves turn from green to red, gonorrhea and yellow fever rear their ugly heads in some parts of the world. The winds of winter are <a href="https://www.popsci.com/flu-season-paid-sick-leave?src=SOC&amp;dom=fb&amp;linkId=57776260">famous for flu</a>, but also bring bouts of pneumonia. Refreshing springtime blooms breathe new life into outbreaks of chickenpox and salmonella. And last but certainly not least, summer months pack the heat with spikes in Lyme disease, polio, syphilis, tetanus, tuberculosis, and more.
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<figcaption>Female Aedes albopictus mosquitoes, which can carry yellow fever, dengue fever, and Chikungunya fever, are sensitive to seasonal changes. Ebbs and flows in their population can affect disease transmission. Photo Credit: FotoshopTofs, Pixabay
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Other diseases are a bit less finicky: They’ll take any period of natural warmth, spreading the joy over many months at a time—often in regions of the world that don’t experience four seasons of climactic change. For instance, Chagas disease, diphtheria, and genital herpes all flourish over both spring and summer, while their prevalence takes a welcome dip when temperatures fall. Other infectious diseases fare better during rainy seasons, or when climes are particularly dry.Though the diseases she linked together shared little in common, including even their seasons of prevalence, Martinez theorized there might be similar reasons driving their cyclic nature. To tease apart the connections, Martinez amassed data from 100 previously published studies, zeroing in on several factors that shape seasonality.For instance, environmental factors obviously come into play. Climate conditions such as temperature, humidity, and rainfall impact the wellbeing of infectious microbes, as well as the humans and wildlife they plague. In diseases that are ferried to humans via an insect vector like a <a href="https://www.smithsonianmag.com/science-nature/do-not-fear-drones-air-dropping-50000-mosquitoes-above-180970068/">mosquito</a> or fly, the seasonal ebb and flow of these pest populations may also play a role.
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The behaviors of hosts of disease, both human and wildlife, also appear to fluctuate from month to month. This has been infamously exemplified by congregations of kids in schools begetting measles outbreaks. But there’s also something to be learned, Martinez says, by studying flux in the habits of non-human animals, who go through bouts of seasonal breeding, territoriality, and migration, and how these changes affect the spread of disease. Humans may not be seasonal maters—at least, not to the same degree—but even a subtle shift, like an uptick in sex during the summer months, could spur outbreaks of <a href="https://sti.bmj.com/content/sextrans/54/6/433.full.pdf">gonorrhea</a>, <a href="https://sti.bmj.com/content/sextrans/54/6/433.full.pdf">genital herpes</a>, or <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0149401">syphilis</a>.“It’s not that we are vulnerable at a particular time of year and healthy at another,” Martinez explains. “We’re restructuring throughout the year. And the identity of the thing we’re vulnerable to changes with the seasons.”The idea that some infectious diseases are seasonal isn’t new, says <a href="https://med.stanford.edu/profiles/shanthi-kappagoda">Shanthi Kappagoda</a>, an infectious disease physician and epidemiologist at Stanford University who was not involved in Martinez’ work. However, Kappagoda adds, Martinez’ framework is unique in that it includes some infections that haven’t traditionally been considered seasonal—including several sexually-transmitted infections—and may change how clinicians and researchers approach future epidemics.
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<figcaption>As climate change progresses, increasingly warmer locales may favor the transmission of infectious diseases like cholera, or mosquito-borne infections. Photo Credit: Martijn Meijerink, Pexels
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According to Martinez, knowing when certain diseases are in season could help doctors quickly and effectively treat chronic infections that tend to flare up at certain times of the year, like herpes. With this sort of predictive power, patients may be able to minimize the amount of time they’re forced to deal with seasonal symptoms. The same line of thinking might even be applied to scheduling vaccines, which are ideally administered prior to the onset of an outbreak.Understanding the drivers of infectious diseases’ seasonality may also be helpful on much broader scales, including the <a href="https://www.pbs.org/wgbh/nova/article/disease-outbreaks/">forecasting of epidemics worldwide</a>, adds <a href="https://www.jhsph.edu/faculty/directory/profile/3546/amy-wesolowski">Amy Wesolowski</a>, an infectious disease epidemiologist at the Johns Hopkins Bloomberg School of Public Health who did not participate in the new analysis.
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Such global thinking is especially relevant in a rapidly changing world. As <a href="https://www.pbs.org/wgbh/nova/video/understanding-the-climate-clock/">climate change</a> progresses, the landscape of infectious diseases will shift, explains Kappagoda. Increasingly warmer locales may favor the transmission of summer-loving diseases like cholera. Humankind may also witness a boom in populations of insects like mosquitoes, which are likely to expand their habitats as temperatures climb, chauffeuring with them outbreaks of <a href="https://www.pbs.org/wgbh/nova/article/dogs-diagnose-malaria-socks/">malaria</a>, <a href="https://www.pbs.org/wgbh/nova/article/post-infection-symptoms/">West Nile virus</a>, and more. Additionally, human sensitivity to heat shouldn’t be underestimated in the context of infectious disease, Kappagoda points out: Climate change will continue to displace large populations of people, spurring the onset of epidemics.Whether we like it or not, the spread and severity of infectious diseases are inextricably tied to both biology and behavior—which, in turn, cycle with the seasons. According to Martinez, the implications of this go far beyond fingering a malady for all seasons; rather, these patterns could change how we view our own bodies… and give new meaning to the phrase, “feeling under the weather.”“This isn’t just about transmission—seasonality is also in the human body itself,” she explains. “There’s something happening in our bodies we don’t quite understand yet. Seasonality in infectious disease is just an enticing little piece of the puzzle.”

The flu isn&#x27;t alone: All infectious diseases might be seasonal, according to a new report.

The Cabbage tree emperor moth (Bunaea alcinoe) decorates its wings with tiny, sound-absorbing scales. By blocking echolocation, these scales lower the chances of the moth being pinpointed and eaten by a bat. Photo Credit: Bart Wursten, flickr

Moths Muffle Bat Sonar with Sound-Absorbing Wings

It’s not easy being deaf in the dark—especially when your greatest enemy is a master of sound.Such is the twilight plight of the humble cabbage tree emperor moth (Bunaea alcinoe): It’s all these umber underdogs can do to avoid becoming a hungry <a href="http://www.pbs.org/wnet/nature/group/mammals/bat/">bat’s</a> dinner. Bat sonar is so sensitive that these predators attain an impressive intimacy with their surroundings—one they manipulate to easily locate and devour the minute, wriggling bodies of their prey. But even moths that can’t hear their stalkers’ telltale screech aren’t as powerless as you might expect.Today, in the journal PNAS, researchers <a href="http://www.pnas.org/cgi/doi/10.1073/pnas.1810025115">report</a> that the wings of cabbage tree emperor moths are flecked with tiny scales capable of absorbing the sounds of echolocating bats, dampening reverberations that might give away their location. The evolution of this stealth-mode strategy may have helped moths evade detection by their nocturnal predators.
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Often dismissed as drab <a href="https://www.pbs.org/wgbh/nova/nature/journey-butterflies.html">butterfly</a> alternates or light-loving nuisances, moths might just be one of the most underappreciated members of the insect world. Like their butterfly relatives, moths are essential pollinators and sources of food, supporting diverse ecosystems worldwide. Even in the face of danger, the modest moth is mighty: In recent years, researchers have uncovered a dizzying array of anti-bat defenses in these little insects. Because bats rely heavily on sonar to navigate the world around them, much of the moth’s modus operandi is auditory. Some moths will produce their own ultrasonic clicks to <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4443353/">jam bats’ sonar</a> or <a href="https://link.springer.com/article/10.1007%2Fs00359-014-0924-0">vocally advertise their toxicity</a>, while others have adopted a strategy of constant vigilance by <a href="https://academic.oup.com/bioscience/article/51/7/570/268256">evolving the insect equivalent of ears</a>, which are sensitive to the audio of approaching bats.However, one group of oddballs exhibits none of the aforementioned tricks of the trade—and it was these miscellaneous moths that piqued the interest of <a href="http://www.bristol.ac.uk/biology/people/marc-w-holderied/">Marc Holderied</a>, a sensory ecologist at the University of Bristol in the United Kingdom.<a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027190">Previous research</a> had hinted that these mysterious moths, while effectively “deaf” to bats, were literally flying under the radar: They absorbed the sounds of sonar using the <a href="https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0027190">exquisite architecture</a> of tiny, leaf-shaped scales that decorate their <a href="https://www.jstor.org/stable/1539496?seq=1#page_scan_tab_contents">wings</a>. These scales might act like the auditory equivalent of an invisibility cloak, allowing moths to pass through bat territory neither seen nor heard. But no one had yet figured out exactly how these little scales were accomplishing such an acoustic feat.
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<figcaption>The Cape serotine (Neoromicia capensis) is a sub-Saharan species of bat that feasts on insects like moths, using echolocation to home in on its prey's location. Photo Credit: Bernard Dupont, flickr
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To puzzle out the mechanics of the muffling, Holderied and his colleagues, led by acoustician <a href="http://www.bristol.ac.uk/biology/people/zhiyuan-shen/overview.html">Zhiyuan Shen</a>, plucked individual scales from cabbage tree emperor moths and constructed 3D models of their structure to see how they would behave in the presence of sound waves.When bat sonar hits a typical object, some of the sound waves will ricochet off the surface and boomerang batward. This outcome is ideal for the bat, which can harvest the information contained in the echo and home in on its prey. All objects, however, have what’s called a resonant frequency, or a natural frequency at which they vibrate. (This is the phenomenon behind the common trope of opera singers shattering wine glasses—the sound hits the glass at its resonant frequency, causing the poor vessel to vibrate itself to pieces.) If incoming sonar meets its target at its resonant frequency, the energy of the sound wave can be converted into motion. This curbs the sound waves’ ability to reverberate and muddies the bat’s estimation of the vibrating object’s whereabouts.For this strategy to be viable for a moth in a bat’s crosshairs, the resonant frequency of its scales would need to overlap with the frequency of bat sonar, typically between 20 to 150 kilohertz. When the researchers analyzed their model scale, they found one resonant frequency. And then another. And then another. To Holderied’s amazement, an individual insect scale was capable of vibrating at not one, but three specific frequencies—28.4, 65.2, and 153.1 kilohertz—depending on how exactly the scale trembled in space.
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“They were perfectly spaced to cover the bat echolocation range,” Holderied explains. “At that moment, we know this wasn’t a coincidence: They’re there for a reason.”As a point of comparison, the researchers repeated their analysis with a modified scale resembling that of a butterfly, a close relative of the moth. Like moths, butterflies adorn their wings with scales—but because most butterflies are most active during the day, as a group, they probably have much less in the way of anti-bat defenses, Shen explains.Like the B. alcinoe model scale, the butterfly-like scale exhibited three resonant frequencies—but this time, they fell less cleanly within the relevant range of bat sonar, clocking in at 88.4, 150.9, and 406.0 kilohertz. It seemed that, under pressure from their bat predators, moths had embarked on this specific evolutionary trajectory alone.
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<figcaption>A single scale from the wing of a cabbage tree emperor moth (Bunaea alcinoe). Typical scales are less than 0.25 millimeters wide. Photo Credit: Marc W. Holderied and Zhiyuan Shen, University of Bristol
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But bats are not to be outdone. They wield a <a href="https://www.nature.com/articles/nature11664">high degree of control</a> <a href="http://www.pnas.org/content/114/41/10978">over the frequency</a> of their own sonar, meaning there’s a chance they could pinpoint gaps in the moth’s acoustic armor. However, not all moth scales are created equal, and every wing is dotted with units of different shapes and sizes, Holderied explains. With such diversity, B. alcinoe wings may stifle an even broader range of frequencies. It’s not yet clear exactly how much these moths compromise bat echolocation in a more natural context—but in the arms race between predator and prey, any amount of sound absorption could tip the scales in favor of Team Moth.Accordingly, one of the clearest next steps is to return to the field and see if the findings hold true for moths on the run from actual bats, says <a href="https://eeb.boisestate.edu/home/current-students/">Juliette Rubin</a>, a sensory ecologist at Boise State University who also studies anti-bat strategies in moths, but did not contribute to the newest finding. For instance, it will be important to test if sound-absorbing moths actually fall prey to bats less often than their not-so-covert cousins—or if shaving scales off B. alcinoe makes them more vulnerable to the hunt. After all, subtle sculpting of scales doesn’t guarantee that B. alcinoe can actually flummox an experienced, living predator. But if these wings are truly as functional as they seem, moths may just fly in the face of bats’ acoustic aptitude.“If the scales are really functioning to absorb sonar, this shows you the incredible adaptations that moths have made to bats,” says <a href="https://www.floridamuseum.ufl.edu/mcguire/kawahara/akito-kawahara/">Akito Kawahara</a>, a sensory ecologist at the Florida Museum of Natural History who did not participate in the research. “It really highlights the importance of bats in the nocturnal landscape and how much impact they have on insects.”
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The researchers are also currently investigating whether these scaly structures are present on other moths—especially those known to engage in other anti-bat antics. Doubling up on defenses could pay some serious dividends, but these adaptations often come with a cost. For instance, Holderied explains, soundproofing scales may have made moth wings heavier, potentially impeding flight.On the other hand, tinkering with the microstructure of a tiny scale is nothing compared to some of the evolutionary triumphs of some other moths—like growing a set of ears, explains biologist <a href="https://sites.google.com/site/cwrufoxlab/people">Jessica Fox</a>, who studies insect flight at Case Western Reserve University but was not involved in the new research.This is not to undersell the exquisite nature of these little scales. While humans continue to painstakingly construct more lightweight, effective resonant absorbers to soundproof buildings and conceal military operations, moths have executed a feat of engineering that’s eluded us for years. Evolution, it seems, operates on even the smallest of scales in the war waged between moth and bat.“Natural evolution and human engineering are converging on the same point,” Shen adds. “But nature is better than engineering.”

To shield themselves from bat echolocation, moths don an acoustic cloak of invisibility—using the sound-absorbing scales on their wings.

Four decades of community-based conservation in Brazil have increased the number of giant South American turtle hatchlings nine-fold along 1,500 kilometers of the Juruá River, a major tributary of the Amazon. Photo Credit: Camila Ferrara, Wildlife Conservation Society

Amazon Turtles Recovering, Thanks to Local Volunteers

Every year, as the late July waters ebb from the banks of the Brazilian Amazon, a bale of female<a href="https://www.arkive.org/giant-south-american-turtle/podocnemis-expansa/"> giant South American turtles</a> creeps shyly from the river’s edge. Here, they will burrow into the soft, speckled sand, excavating the pits in which they’ll lay their eggs. If the young are lucky, they’ll hatch just before the rains return to flood the river anew and amble toward the water, following <a href="http://www.hljournals.org/doi/abs/10.1655/HERPETOLOGICA-D-13-00050R2">the maternal calls</a> that beckon them home.But along portions of the Amazon’s Juruá River, most of these little turtles never make it out of their shells. Nesting areas on unprotected beaches remain vulnerable to raids by poachers, further imperiling a species already threatened by years of habitat destruction.In recent years, however, turtle populations have experienced a miraculous uptick, according to <a href="http://dx.doi.org/10.1038/s41893-018-0170-5">a new report</a> in the journal Nature Sustainability. The turtles’ recovery appears to have also buoyed beach biodiversity in these riverside habitats—and it’s all thanks to four decades of conservation efforts made by local communities.
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“Community engagement is an essential ingredient in wildlife and natural conservation,” says <a href="https://www.cpp.edu/~kconwaygomez/">Kristen Conway-Gómez</a>, a geographer and conservationist at California State Polytechnic University, Pomona, who did not participate in the research. “We can’t achieve [the same amount of] conservation without including local stakeholders.”Freshwater turtles of the Brazilian Amazon have long been sustainably harvested by indigenous peoples and remain a crucial aspect of traditional cultures. But the arrival of European settlers marked a turning point for these river populations, and foreign commercialization of the turtle trade quickly began to <a href="http://www.bioone.org/doi/abs/10.2744/CCB-0926.1?journalCode=ccab">take its toll</a> on local wildlife. During the Amazon rubber boom that began in the late 19th century, European colonists ravaged riverside habitats for natural resources, decimating fragile populations of turtles that once thrived in the waters. New legislation enacted by the Brazilian government, beginning in 1967, did little to curb poaching efforts. By the 1980s, P. expansa had found itself on the <a href="https://www.iucnredlist.org/species/17822/97397263">endangered species list</a>.“These river systems in the Amazon today are a pale shadow of what they were 150 years ago,” explains study author <a href="https://people.uea.ac.uk/en/persons/c-peres">Carlos Peres</a>, a conservation ecologist at the University of East Anglia in the United Kingdom. “P. expansa’s original habitat range has been contracted to perhaps 2 percent of what it used to be.”
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<figcaption>Due to centuries of over-exploitation, populations of the giant South American turtle (Podocnemis expansa) dwindled to concerning levels, particularly after the Amazon rubber boom in the late 19th century. Photo Credit: Priscila Miorando, Federal University of Western Pará
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But in the middle of the chaos, a small movement began to take root. In the mid-1900s, concerned Brazilian locals self-organized, recruiting volunteers to stake out turtle nesting beaches, or tabuleiros, to ward off poachers during breeding season. Today, several tabuleiros are protected by round-the-clock local surveillance for up to six months each year.This self-sustaining system has been in place for decades—but it wasn’t until recently that a team of researchers led by Peres and <a href="https://www.researchgate.net/profile/Joao_Campos-Silva2">João Campos-Silva</a> of the Federal University of Alagoas in Brazil finally quantified their impact on turtle viability.To track the wellbeing of Amazon turtle populations, the team collected 40 years of nesting data between 1977 and 2016—and it quickly became clear just how astoundingly successful the locals’ efforts had been. Four decades of community-based conservation had increased the number of P. expansa nests 11-fold along the Juruá River; the number of hatchlings had also increased nine-fold, averaging out to more than 70,000 additional hatchlings released on beaches each year.
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“This is a very important study,” says <a href="https://eohsi.rutgers.edu/eohsi-directory/name/joanna-burger/">Joanna Burger</a>, an ecologist and conservationist at Rutgers University who did not participate in the research. “There are so few long-term studies in conservation… so it’s hard to show that these work.”When the team compared wildlife inhabiting protected and unprotected beaches along a 1,500-kilometer (over 900-mile) stretch of the Juruá, they found that the locals’ vigilance had effectively deterred poachers from not only P. expansa nests, but also the burrows of their close relatives—yellow-spotted river turtles (Podocnemis unifilis) and six-tubercled river turtles (Podocnemis sextuberculata). While a staggering 99 percent of the 200 nests on unguarded beaches suffered poaching raids, only 2 percent of the over 2,000 nests on monitored beaches had been harvested. When the researchers took all local factors into account, they found that the number of years a beach had been protected was the most important contribution to the turtles’ reproductive success.
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<figcaption>Several freshwater turtle species, including the giant South American turtle (Podocnemis expansa), the yellow-spotted river turtle (Podocnemis unifilis), and the six-tubercled river turtle (Podocnemis sextuberculata), have been affected by human over-exploitation, but community-based efforts have brought populations back from the brink. Photo Credit: Camila Ferrara, Wildlife Conservation Society
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And the turtles weren’t the only beneficiaries of local generosity. The researchers were surprised to find that many diverse species, including other reptiles, birds, fish, and insects, were also thriving on protected beaches and in the waters they bordered—a striking contrast with the “deafening silence” of the unprotected beaches nearby, Peres explains. In this way, Campos-Silva says, the turtles were acting as a bit of an umbrella species, delivering collateral benefits to their neighbors.“The greatest contribution of this paper is all these positive benefits to [other] species,” explains <a href="https://bsc.ua.edu/profiles/jennifer-howeth/">Jennifer Howeth</a>, a conservation ecologist at the University of Alabama who did not contribute to the research. “From a conservation perspective, you’re getting a really good bang for your buck.”To round out their analysis of the program, the researchers interviewed beach guards who had participated in the efforts over the years. These results, unfortunately, were more sobering. While locals were enthusiastic about the recovery of biodiversity in the area, they expressed concerns about the dearth of monetary support and a lack of appreciation from government agencies for work that was harrowing, time-consuming, and often dangerous due to threats from particularly recalcitrant poachers.
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The sole compensation the guards receive is a monthly food hamper worth about $110, paid only during turtle breeding season. In spite of this, some of the most stalwart participants had been on the job for over 35 years, citing a moral obligation to continue the work they’d inherited from their parents and grandparents. “The guards are doing it out of love,” Campos-Silva says. “But we need a better solution for economic sustainability.”About 390 protected tabuleiros exist throughout the Brazilian Amazon. According to the researchers’ estimates, maintaining surveillance across all locales would incur an annual cost of about $833,000, a sum that will require a consistent, independent stream of financial support.In the meantime, however, Campos-Silva, Peres, and their colleagues hope that these early successes can inspire other conservation efforts worldwide—especially those lacking in funding or dedicated staff.
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<figcaption>Turtle conservation along the Juruá River has been almost entirely spearheaded by local volunteers in a labor of love. Guards monitor turtle breeding grounds 24/7 during the dry season for only a monthly food stipend, and some of the most devout individuals have been keeping watch for nearly four decades. Photo Credit: Joseph Hawes, Anglia Ruskin University
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But Conway-Gómez cautions that, while the statistics from the Juruá region’s conservation efforts are “striking,” it’s unclear how much—or if—they will be replicated in other communities around the world, where cultural resources and local enthusiasm may be less abundant. Future work will require tailoring each action plan to individual wildlife species and human communities and, as Howeth points out, identifying consistent sources of funding, management, and oversight.Once empowered with the right resources, local investment can be one of most critical aspects of conservation. A handful of passionate individuals might be all it takes to bring a species back from the brink.“In some ways, communities have better ways to do this work than outside biologists and conservationists,” Conway-Gómez adds. “It’s important to give them a place at the table.”

Forty years of community engagement has brought the freshwater turtles of Brazil back from the brink—but lack of funding may imperil conservation.

Aedes aegypti mosquitoes thrive in the world's tropics and can carry the viruses that cause dengue fever and Zika fever, among others. Photo Credit: James Gathany, Centers for Disease Control

How Zika and Dengue Exploit the Mother-Child Relationship

The viruses that cause <a href="https://mass.pbslearningmedia.org/resource/den08.sci.life.stru.dengue/dengue-virus-invades-a-cell/">dengue</a> fever and <a href="https://www.pbs.org/wgbh/nova/video/zika-the-untold-story">Zika</a> fever have established themselves as formidable forces of devastation. About <a href="http://www.who.int/news-room/fact-sheets/detail/dengue-and-severe-dengue">390 million</a> dengue infections occur globally each year, and 3.9 billion people worldwide—over half the human population—are at risk of infection. Zika is newer to the scene, but infections have already been reported in <a href="http://www.who.int/news-room/fact-sheets/detail/zika-virus">at least 86 countries</a>.Though neither disease carries a particularly high fatality rate, it appears that these stealthy viruses are capable of leaving their mark on a person long after they’ve vacated the body—in a way that could affect the health of generations to come.Today, in the journal Cell Host &amp; Microbe, two new studies report that dengue and Zika viruses may be exploiting a loophole in the human immune system that enhances the severity of both diseases. In the <a href="https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(18)30499-2">first article</a>, a group at the La Jolla Institute for Immunology shows that having a mother who survived Zika may inadvertently worsen the symptoms of dengue in young offspring; in the <a href="https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(18)30546-8">second</a>, researchers from Emory University indicate that fetuses with mothers previously infected with dengue virus may be at a higher risk of acquiring Zika through the placenta.
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The viruses that cause dengue and Zika are closely related, making it easy for the human body to confuse one for the other. This results in a bait-and-switch: When one of these viruses is misidentified, it’s able to conscript the body’s own immune molecules into ushering the pathogen into a susceptible cell. Because dengue and Zika viruses often run rampant in the same geographic regions, especially in the Americas, the new findings underscore the lasting effects of outbreaks, but could also inform the development of future <a href="https://www.pbs.org/wgbh/nova/video/vaccinescalling-the-shots/">vaccines</a>.“These are both really great studies,” says <a href="http://labs.icahn.mssm.edu/lim-lab/dr-lim/">Jean Lim</a>, an immunologist at the Icahn School of Medicine at Mount Sinai who was not involved in the research. “They’re really adding important clues about Zika and dengue, and how preexisting immunity impacts another virus.”Normally, when a virus infects a human, the <a href="https://www.pbs.org/wgbh/nova/video/immunity-and-vaccines-explained">immune system</a> dispatches a cavalry of neutralizing antibodies—Y-shaped molecules that recognize a specific feature of the pathogen, latch onto its surface, and block it from infecting cells. The virus, now coated with a cluster of antibodies, is then swallowed whole and destroyed by white blood cells. If the same virus ever tries to set up shop in the body again, the immune system will recall previous infections and rouse the same set of antibodies to nullify the threat before it can do any significant damage.
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<figcaption>Many of the world's tropical regions are home to the viruses that cause dengue and Zika, increasing the likelihood that individuals may be infected with both pathogens in a single lifetime, or pass compromising antibodies from mother to child. Photo Credit: Jenna Hambrick, La Jolla Institute for Immunology
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But dengue has a bad reputation for refusing to play by the rules. Many different subtypes of this virus exist, and surviving infection with one subtype doesn’t guarantee protection from another. In fact, these subsequent viral encounters can often be far worse than the first. When neutralizing antibodies that have been trained on one dengue subtype run up against another many years later, two issues may arise. First, the virus is just different enough that antibodies will bind poorly to its surface. Second, antibody levels eventually wane in the body, and there are fewer around to put the virus out of commission.As a result of these dips in quality or quantity, the antibodies that confront the new virus fail to disarm it—and when white blood cells arrive on scene to clean up the mess, dengue is gobbled up in a still-active form. From here, the virus will replicate and destroy the immune cell, perpetuating the cycle of infection. This phenomenon, in which antibodies become a composite Trojan horse that unwittingly chauffeurs viruses into these vulnerable cells, is called <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3185591/">antibody-dependent enhancement</a>.To make matters worse, antibody-dependent enhancement doesn’t just cause problems for <a href="http://science.sciencemag.org/content/358/6365/929">individuals infected with multiple strains of dengue</a>. As they develop, fetuses inherit antibodies from their mothers through the placenta. These antibodies stick around throughout the first year or so of life as the offspring develop their own immune systems. But if their mothers happened to have even a glancing blow with dengue, <a href="https://academic.oup.com/femspd/article/59/2/119/647508">babies can be at high risk</a> for <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4340501/">developing severe dengue</a> if they’re infected with a different subtype in the months after birth, while their bodies still contain maternal antibodies.
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So when a very similar-looking virus called Zika began causing serious outbreaks a few years ago, researchers like <a href="https://www.lji.org/faculty-research/labs/shresta/">Sujan Shresta</a>, an immunologist at the La Jolla Institute for Immunology and senior author of the <a href="https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(18)30499-2">first study</a>, began to worry. Zika belongs to the same group of viruses as dengue—flaviviruses—which also include the nasty pathogens that cause West Nile fever, yellow fever, and Japanese encephalitis. If Zika was popping up in the same areas as its infectious cousin, Shresta reasoned, the same-same-but-different concept of antibody-dependent enhancement could spell trouble for the countless individuals and their children exposed to both viruses.Shresta and her colleagues, led by immunologist <a href="https://www.lji.org/careers/postdoctoral-association/#executive-committee">Angela Fowler</a>, decided to test if a mouse mother’s previous Zika infection could worsen dengue symptoms in her offspring. The researchers infected two groups of mouse pups with Zika: one set born to mothers who had been previously infected with Zika virus, and the other born to mothers who hadn’t. They found that the first group of pups had acquired Zika antibodies from their mothers, and were consequently protected from Zika infection, showing that the antibodies were active against their intended target. But when the team repeated their experiment, this time exposing the pups to dengue, the antibodies were instead weaponized by the new virus: The pups who had inherited Zika antibodies suffered more severe dengue symptoms and were more likely to die from the infection.When the researchers next assessed the ability of these maternal Zika antibodies to subdue dengue, unsurprisingly, they weren’t up to snuff. While the antibodies were able to attach to dengue, they did nothing to neutralize it. Instead, they seemed to do the opposite, ferrying the virus into the arms of unsuspecting cells.
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<figcaption>Placental immune cells infected with Zika virus. Zika may more easily infect placental cells by hitching a ride on maternal antibodies to dengue virus. Photo Credit: Mehul Suthar &amp; Matt Zimmerman, Emory University
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“The antibody responses to these viruses are highly complex,” Shresta explains. “One infection can affect subsequent ones, and in this case, the antibody response to Zika has an enhancing effect on dengue.”And the problem of mistaken identity between dengue and Zika could be a two way street. In the <a href="https://www.cell.com/cell-host-microbe/fulltext/S1931-3128(18)30546-8">second study</a>, <a href="http://vaccines.emory.edu/faculty-evc/primary-faculty/suthar_mehul.html">Mehul Suthar</a> and <a href="https://med.emory.edu/MDPHD/trainees/current_students/zimmerman_matthew.html">Matthew Zimmerman</a>, immunologists at Emory University, led a team of researchers in testing this possibility.One of the most dangerous aspects of a Zika infection is the virus’ ability to cross a mother’s placenta into her fetus, often leading to severe birth <a href="https://www.pbs.org/wgbh/nova/article/birth-defects-20-times-higher-for-women-infected-with-zika/">defects</a> like microcephaly. If dengue antibodies have a nasty tendency to guide similar-looking viruses into cells, it’s possible a placental Zika infection could be exacerbated by this inappropriate immune response, endangering a child before it’s even born.
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Unlike the previous study, which used mice, Suthar and his team conducted their experiments on human placental immune cells. When the researchers introduced only Zika to the cells, very few viruses took hold—but if they added dengue antibodies to the mix, the antibodies stuck to the viruses and shepherded them into the cells, where Zika quickly multiplied. This could be a clear-cut way for Zika to pass easily from mother to child, explains Zimmerman, “as if the virus is hitching a ride on a normal, physiological system.”A preexisting dengue infection is by no means necessary for Zika to have devastating effects on a developing fetus, Suthar says, and antibody-dependent enhancement is unlikely to be the only way Zika traverses the placental barrier. However, the researchers’ experiments highlight a subtle way Zika can hijack the body to some potentially devastating ends. Millions of women worldwide harbor low levels of dengue antibodies in their blood from prior exposures—and because infections are often asymptomatic, many of those interested in having children may be unaware of just how vulnerable their fetuses could be to Zika.Of course, neither mice nor donated placental tissue is a perfect stand-in for a human. For instance, mice often have to be genetically manipulated to become susceptible to dengue virus, explains <a href="http://facultypages.vetmed.lsu.edu/faculty/rcarri1">Rebecca Christofferson</a>, an infectious disease expert at Louisiana State University who did not contribute to the new findings. Additionally, antibody-dependent enhancement is often easier to show in cells growing in a dish than in the body of a living animal. But simply raising the possibility of one virus feeding off the dregs of the response to another could make a huge difference in preventing future outbreaks.
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<figcaption>Future dengue and Zika vaccines may need to be designed carefully to avoid antibody-dependent enhancement of other flaviviruses. Photo Credit: Sanofi Pasteur, Gabriel Lehto, Novas Imagens Cine Video
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The new findings may also affect how researchers counteract these diseases in the future. Infections aren’t the only ways a body can generate antibodies—vaccines for either dengue or Zika would induce their production as well. Vaccine safety may need to be re-evaluated in areas where both viruses are prevalent, explains <a href="https://pediatrics.duke.edu/faculty/sallie-robey-permar-md-phd">Sallie Permar</a>, an immunologist at the Duke University School of Medicine who did not participate in the research, so that protection against one disease doesn’t accidentally exacerbate another. Still, this doesn’t invalidate vaccination in the fight against these pathogens: Rather, with the issue of antibody-dependent enhancement in mind, some of the most effective vaccines of the future might be carefully concocted mixtures of strong, neutralizing antibodies active against multiple flaviviruses.And far more than the potential synergy between dengue and Zika may be at play, Christofferson points out. There’s limited evidence that other flaviviruses like <a href="https://journals.plos.org/plosntds/article?id=10.1371/journal.pntd.0001311">Japanese encephalitis virus</a> might also augment the severity of future dengue infections, but it’s still unclear how widespread antibody-dependent enhancement really is outside the lab. More research is needed—and now that more and more countries harbor multiple flaviviruses, Shresta explains, the urgency of understanding these threats is greater than ever.The critical next experiments will be epidemiological studies in humans, says <a href="https://www2.fgcu.edu/CAS/879.asp">Sharon Isern</a>, a virologist at Florida Gulf Coast University who was not involved in the studies. Such future work will be challenging, requiring large numbers of people and extensive clinical history, but reports like these underscore the importance of human-centric follow-up.“Animal models provide good information,” Isern adds, “but the answers are in large human populations.”

Due to the similarities between these closely-related viruses, mothers who have experienced Zika infections may have babies with a higher risk of suffering severe dengue, and vice versa.

Katherine J. Wu

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