The Zika virus skyrocketed to the top of global health concerns back in 2015, but since then, it has fallen off the public radar for most of the world. Is that because the virus is no longer a concern or because we've grown tired of reading headlines about Zika?
To really answer that question, we need to get more familiar with the Zika virus. First discovered in monkeys living in the Zika forest of Uganda, this disease wasn't identified in humans for another 5 years [1]. After its discovery, the disease was extremely rare with only 14 cases between 1952 and 2007, when a large-scale outbreak hit the Pacific island of Yap, in the Federated States of Micronesia affecting around 75% of the residents [2]. Scientists believe Zika was more common this roughly 50 year period, but since its symptoms, a non-itchy rash, were minor and the areas with confirmed cases were home to other, more serious mosquitos-spread illnesses, Zika was rarely reported [2].
It wasn't until the disease made it to the Americas and Caribbean that health officials and news anchors started to take notice. This outbreak was at an unprecedented scale, with over half a million suspected cases between 2015 and 2018 [3,4], and was the first time the disease was associated with microcephaly [2].
Microcephaly is a condition that causes babies to be born with smaller than average heads due to a disruption in brain development. While scientists never like to say anything is certain, they believe Zika causes microcephaly, and that women exposed to Zika early on in their pregnancies are at the highest risk [5].
Zika, like dengue fever and west nile virus, is spread through mosquitos, which bite an infected person and carry the virus over to their next victim. Viruses cannot replicate without a host because they are essentially just little packets of genes that get passed from person to person, so the virus does not affect the mosquito that's carting it around. Once it gets inside a human though, it gets to work tricking human cells into making virus genes. Many viruses will target a specific cell type and cause the cell to make so many viruses, it explodes. Zika does the same thing, but it picks a very important cell type to infect, stem cells that will eventually become glial cells [6].
Stem cells are cells that can become anything later in their lives. They are what let us go from being 1 cell to trillions. However, the farther along we go in development, the less flexible these cells become. The first cell can become everything we will need in our body, but by the time we are born, most of our cells have a pretty fixed path. The cells Zika targets are those located in the brain that should become glial cells later.
Glial cells are extremely important for nerve function. Glial cells surround nerve cells, called neurons, to hold them in place and keep them separate from each other. Plus, glial cells act as a go-between for nerve cells, bring them "food," taking away any waste, and speeding up the signals that go through nerve cells. They are so important that they outnumber nerve cells 3 to 1 in the brain [6], so when Zika tricks these cells into making copies of the virus until they burst and die, it kills off a big part of the brain.
Adults don't have to worry too much about this since most of our brain cells are on a set path, they aren't stem cells, so they are not the target of Zika. However, babies developing in the womb are are a much higher risk and that risk goes up the younger the fetus is. Women who are infected with Zika very early on in their pregnancies may give birth to a child with a head severely smaller than an average child while those infected late into their pregnancy may give birth to a child with a only slightly smaller than normal head. As you can imagine, having a smaller head and a smaller brain causes a lot of developmental issues, and is what sparked an international outcry back in 2015.
Nowadays, you don't hear about large outbreaks Zika or a large number of babies being born with microcephaly on the news, but that doesn't mean that Zika is no longer a threat. When the PBS news hour interviewed Dr. Roberta DeBiasi, chief of Pediatric Infectious Diseases and co-director of the Congenital Zika Program at Children’s National Health System, she said she definitely thinks another outbreak is on the horizon, she just doesn't know when [3] since scientific understanding of Zika is still rather limited.
Scientists are torn between putting Zika in the same box as chickenpox, West Nile, or dengue since each one acts differently. If it acts like chickenpox, and once you get it once you're immune to it, we'd expect to see travelers and children to be the only ones to get Zika. If it works like West Nile, we'd expect cyclical outbreaks every 5 or so years that affect everyone, and if it works like dengue fever, we would expect to see it affecting everyone seasonally.
While we wait for scientists to figure this out and come up with a Zika vaccine, it's best to take precautions when traveling anywhere around the equator in the Americas. That means using bug spray, avoiding the outdoors during dawn and dusk when mosquitos are most active, and, if needed, sleeping in bug nets to prevent bites while you sleep. Protecting yourself against mosquitos is especially important if you are pregnant or thinking of becoming pregnant, even if you are a man since Zika can be spread sexually.
Overall, the number of Zika cases is on the decline, but it has not gone away! Keep it in the back of your mind when planning your next trip, or if you live in areas where Zika was reported, sitting outside this evening.
Sources:
[1] https://www.who.int/emergencies/zika-virus/timeline/en/
[2] https://www.who.int/bulletin/online_first/16-171082/en/
[3] https://www.pbs.org/newshour/science/what-happened-to-zika
[4] https://www.paho.org/hq/index.php?option=com_docman&view=download&category_slug=cumulative-cases-pdf-8865&alias=43296-zika-cumulative-cases-4-january-2018-296&Itemid=270&lang=en
[5] https://www.cdc.gov/pregnancy/zika/testing-follow-up/zika-syndrome-birth-defects.html
[6] https://www.sciencedaily.com/releases/2017/02/170217161023.htm