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There are a lot of viruses out there that can infect humans, but two things that can
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get really alarming is when a virus spreads quickly and when it causes serious harm.
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Zika virus has the potential to do both of these things, which is why it’s gotten a lot of attention.
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Given this, it makes sense to understand a bit about Zika virus and the disease it causes.
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Zika virus is an arbovirus, meaning it’s transmitted via certain arthropods, specifically
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mosquitos, so it’s a mosquito-borne virus. Mosquito-borne doesn’t mean that the virus
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is “born” in the mosquito, though, but it’s “borne”, with an ‘e’, which
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means carried or transported. Sometimes we call organisms like this “vectors”, what
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all they do is transport the virus. So with Zika virus, just like other mosquito-borne
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viruses like dengue fever, yellow fever, Japanese encephalitis, and West Nile virus, the mosquito
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acts as a vector that transmits the virus from one person to the next. These viruses
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are all in the genus flavivirus.
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In order to mature her offspring, female mosquitoes need a blood-meal, which they get from unsuspecting hosts.
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Mosquitoes find their blood-meals using chemical compounds that we and other organisms
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give off, like carbon dioxide, ammonia, lactic acid, and octenol. So when a mosquito that
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also happens to be carrying the virus finds her meal and digs in, the virus infects the
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human host and starts to multiply or reproduce within the human. With most flaviviruses though,
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the virus isn’t able to replicate enough in the human host to actually be able to reinfect
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another mosquito, and so the human is considered a dead-end-host. However, the Zika virus,
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along with yellow and dengue fever, is well enough adapted to human hosts such that they
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can multiply to a point where it can re-infect another unsuspecting mosquito, which can then
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go on to infect more people. This window lasts for the first week of infection, during which
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the Zika virus can be found in the blood. So if humans with the disease can transmit
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back to mosquitoes then you can imagine that areas where there’re a lot of mosquitoes,
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would be a set-up for spreading the virus super quickly, right?
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Now the Zika virus is transmitted via mosquitos in the Aedes genus. These blood thirsty little
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guys can bite at night, but are mostly active during the daytime. Aedes mosquitoes are also
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the same ones that transmit Chikungunya fever and dengue fever. When Aedes aegypti or Aedes
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albopictus, both species of Aedes mosquito, lands on your skin and sticks in it’s long
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nose—or proboscis, it pierces the epidermis which is the topmost layer, composed almost
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entirely of keratinocytes. Keratinocytes basically serve to protect against foreign pathogens,
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and it’s typically pretty good at that. That proboscis though keeps going into the
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dermis, since the epidermis just gets oxygen from the air and doesn’t have its own blood
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supply, whereas the dermis does, and this is what our mosquito’s after, right, the
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blood meal. Since the proboscis goes through both the epidermal and dermal layers, the
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cells in those layers are susceptible to infection by Zika virus. So in addition to keratinocytes,
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fibroblasts, and dendritic cells have also been found to be permissive to Zika virus,
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meaning they have some sort of receptor or attachment site that basically says," here
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you go man, come on in."
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Now we still don’t know everything about the Zika virus infection, but we do know that
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when it enters the cell, it injects a single-stranded positive RNA strand. "Positive" means that this
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piece of RNA’s a lot like our own mRNA; it’s basically ready to rock and get translated
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into proteins. The virus’s genome is translated by our own cellular machinery into more viruses.
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Eventually those cells-turned-virus-making-factories die, which actually ends up releasing more
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viruses to infect more cells.
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As bleak as all that sounds, our immune system’s actually pretty good at fighting off Zika
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virus, and only 1 in 5 get sick from infection, and the often the others won’t even notice
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they’ve been infected. Common symptoms when patients have them, are mild fever and skin
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rash, but some also experience muscle and joint pain, headaches, and conjunctivitis,
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or red eyes. The incubation period, or time from infection to symptoms isn’t known,
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but it’s thought to be from a few days to a week.
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Treatment usually just involves treating the symptoms, things like getting plenty of rest,
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drinking fluids to prevent dehydration, and taking medicine like acetaminophen to help
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reduce pain and fever.
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Okay so we’ve hit the “spreads quickly” part, which really matters most in places
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with lots of mosquitoes, but what about the “causes serious harm” part? Well, although
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it causes mild symptoms in adults, there’s more to the story. In October 2015, in areas
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of Brazil where Zika virus has been circulated quite a bit, public health officials noticed
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a significant increase in babies born with microcephaly, which is when a child is born
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with an abnormally small head and therefore abnormally small brain size; this has the
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tendency to cause serious neurological and intellectual deficits, seizures, as well as
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vision or hearing problems. It was noticed that there was a huge increase in babies with
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microcephaly—up to a 20-fold increase—among Brazilian states with Zika virus outbreak.
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As of November 2015, the European Center for Disease Control has stated that it’s plausible
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the Zika virus is able to cause microcephaly in the developing fetus or newborn, as the
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Zika virus can be transmitted from mother to baby during pregnancy or around the time
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of birth, although it’s not yet known how often this happens or how exactly the Zika
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virus is linked to microcephaly.
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In addition to being spread mostly by mosquito bites, and in some cases from mother to child,
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Zika virus has also been reported to spread through both blood transfusions and sexual
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contact.
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Currently, there’s no vaccine for Zika virus, so it’s highly advised to take precautions
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when traveling to areas of outbreak, mostly limiting mosquito bites, so doing things like
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wearing bug spray all day, or wearing long-sleeve shirts and pants, especially during the day
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when the Aedes mosquitoes are most active. If infected, it’s especially important to
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avoid mosquitoes to avoid spreading Zika virus to others, especially in that first week of
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symptoms. The World Health Organization currently suggests pregnant women consult their doctor
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or travel clinic for guidance and recommendations.