Zika: Ten things to know about a new public health emergency
The Zika virus is a global health threat. Despite renewed urgency because of the evidence suggesting that Zika causes birth defects, science has known of the virus for some time. It's a deadly and debilitating virus for some newborns, so it's important to have an accurate picture of the science behind it, the risks of infection and how it affects developing brains. Here are the ten most important things to know about Zika and the danger it represents:
- It originated in Africa: We’re hearing a lot about the effects of the Zika virus in Central and South America right now - but in fact, it was first detected in 1947 in the Zika Forest of Uganda, which gave it its name. The virus evolved in rhesus monkeys before spreading. The virus may have come to the Americas around 2013, possibly by way of Asia.
- It's mosquito borne. It’s thought to be carried by the bite of aedes aegypti, a mosquito common to tropical areas in central and South America, and subtropical areas of the U.S. However, there's still some uncertainty whether aegypti is the sole carrier, and other mosquitoes may prove to be able to carry the virus. If they can, if could be a bigger problem in the U.S. because the range of some mosquitoes exceeds that of aedes aegypti. It could also affect strategies to suppress mosquito populations.
- The structure of the virus is now known. It’s a flavivirus, similar (but not identical) to dengue, yellow fever and West Nile viruses. Understanding the structure may be important for devising treatments. Carbohydrate groups that are part of the Zika virus coating may be used for cell attachment to the target cells, so understanding the molecular mechanisms of this process may eventually help devise treatments and cures, and reduce the severity of the infection.
- Brazil has the most Zika cases. However, the true prevalence is unclear. Brazil’s health ministry has estimated that as many as 1.5 million people may have been infected so far, with over 6,000 cases of microcephaly suspected. The reliability of these numbers is suspect. More stringent criteria indicate a rate of 14.62 cases per 10,000 births in northeastern Brazil. Cases in North America - so far - are from individuals who have contracted it from traveling abroad, or through sexual contact, but because North America is part of the habitable range of the mosquito, there’s worry that carrier mosquitoes will also make it to the US.
- Zika infects 1 in 5 people? Zika can be transmitted through sexual contact. It can be transmitted from an infected pregnant mom to her fetus, or via blood transfusions from an infected donor. It's been stated that only about 20% of infected adults show symptoms of infection, but this widely cited statistic is based on one outbreak in the Yap islands. Experts say that it is impossible to know this rate of infection with precision. Infection during pregnancy still has many unknown factors - for example it isn't known how likely it is that a simple exposure to the virus will produce infection in a pregnant woman. While many do not show symptoms, those that do may experience fever, rash, joint pain, headache, vomiting and/or red eyes. Based on estimates from an outbreak in French Polynesia, an infected mother has about a 1 in 100 chance of giving birth to a baby with microcephaly. While any case is troubling, it also means the chances of a normal birth are quite good.
- Zika appears to cause abnormal brain development. Microcephaly may have many different causes, some more severe than others. Zika produces severe microcephaly, a smooth cerebral cortex, and deposits called calcifications. Babies born of infected mothers may have microcephaly, resulting in severe retardation and developmental delays, seizures and even death. Scientists are hypothesizing that the virus that made its way to the Americas may have mutated.
- Zika creates virus factories in the developing brain. During normal brain development, developing neurons use radial glial as a bridge to get to their final positions in the brain. It’s this growth that’s responsible for the wrinkles in brains that have developed normally. The picture is still developing, but Zika may enter these developing cells by targeting a protein called AXL, then it kills or converts developing cells into virus-making factories – increasing the damage. Since it’s attacking the cells that will make up the cerebral cortex, it reduces the overall size of the cortex and causes the characteristic microcephaly. But, it seems unique from other forms of microcephaly in that it produces smoothing of the cortex, and is associated with calcium deposits in the brains of developing fetuses.
- It’s an emergency. The World health Organization has declared Zika a "public health emergency of international concern" – right now, there’s no treatment or cure. There are no vaccines to build immunity against the virus, nor are there quick reliable tests to diagnose a viral infection or to reliably assess the consequences of the infection to a fetus.
- What to do? Public health officials suggest using protection against sexual transmission. The Centers for Disease Control and Prevention advises men who travel to Zika-affected areas to use condoms with their pregnant partners or avoid sex until the baby’s born. Mosquito control efforts in affected areas may help stop the spread of the virus and suppress the mosquito population that carries it.
- GMO to the rescue? Biologists are discussing whether to use genetically modified mosquitos to interrupt the mosquito carrier’s life cycle. Genetically modified mosquitos have been used to suppress mosquito populations in areas prone to mosquito borne viruses. It’s a controversial practice but potentially very effective.
Despite what we do know about Zika, we still don’t understand nearly enough. The link between Zika and microcephaly is still technically a hypothesis, but increasingly the hypothesis is supported by observations from public health systems and direct scientific evidence. [update 4/7/2016: The WHO has declared a scientific consensus that Zika virus is a cause of microcephaly and Guillain-Barré syndrome]. The uncertainty about whether one is infected, the severe birth defects resulting from an infection, and the possibility of it spreading throughout widespread areas that harbor mosquitoes (and the question of which mosquitoes might carry it) make it important to work together strategically. Such efforts must involve: elected leaders to provide resources to research and control it; public health officials to accurately document the true prevalence of the infection and associated birth defects; pediatricians to effectively diagnose the disease in their patients and provide up-to-date information; scientists to develop effective treatments and strategies; and the public to protect themselves from infection.
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- Centers for Disease Control web page on Zika basics: http://www.cdc.gov/zika/about/index.html
- World Health Organization fact sheet on Zika: http://www.who.int/mediacentre/factsheets/zika/en/
- Tang H, Hammack C, Ogden SC, Wen Z, Qian X, Li Y, Yao B, Shin J, Zhang F, Lee EM, Christian KM, Didier RA, Jin P, Song H, Ming GL. (2016) "Zika Virus Infects Human Cortical Neural Progenitors and Attenuates Their Growth." Cell Stem Cell. S1934-5909(16)00106-5. doi: 10.1016/j.stem.2016.02.016.
- Driggers RW, Ho CY, Korhonen EM, Kuivanen S, Jääskeläinen AJ, Smura T, Rosenberg A, Hill DA, DeBiasi RL, Vezina G, Timofeev J, Rodriguez FJ,Levanov L, Razak J, Iyengar P, Hennenfent A, Kennedy R, Lanciotti R, du Plessis A, Vapalahti O. (2016) "Zika Virus Infection with Prolonged Maternal Viremia and Fetal Brain Abnormalities." N Engl J Med. 2016 Mar 30. [Epub ahead of print]
- Nowakowski T, Pollen A, Di Lullo E, Sandoval-Espinosa C, Bershteyn M, Kriegstein A. (2016) Expression Analysis Highlights AXL as a Candidate Zika Virus Entry Receptor. Cell Stem Cell. in press (proof).
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