What is it?

– Zika has gone from a scientific curiosity to a global crisis in just a few years. Scientists have known the virus since 1947, but they long thought is was rare and caused only mild disease. Only a handful of cases, all in Africa, were described in the scientific literature.

– Since 2007, the virus has started spreading rapidly, and in 2015-2016 it caused a massive epidemic in the Americas. Scientists learned it wasn’t so harmless, but that it can cause birth defects, including a smaller head size than usual. On 1 February 2016, WHO declared the virus a Public Health Emergency of International Concern.

– Zika virus is a cousin of dengue, yellow fever, and Japanese encephalitis.


What are the symptoms?

– In the vast majority of people, Zika only causes mild illness, with symptoms like mild fever, skin rash, muscle and join pains, red eyes, malaise, and headache, usually lasting between 2 and 7 days. Many become infected with no symptoms at all.

– In rare cases, Zika can trigger Guillain-Barré syndrome, a condition in which the immune system attacks nerve cells, leading to partial or almost complete paralysis. Most people eventually recover from Guillain-Barré.

– An increasing number of epidemiological and animal studies in 2016 showed that Zika can also harm the fetus when a pregnant woman becomes infected, and lead to birth defects. A tiny head (microcephaly) is the most commonly seen problem, but a range of other conditions have been reported, including involuntary movements, seizures, irritability, swallowing problems, and problems with hearing or vision. Together, scientists call this Zika congenital syndrome.

– It’s still unclear how high the risk to the fetus is from Zika. Northeast Brazil has seen thousands of microcephaly cases in 2015-2016, but rates have been much lower in other countries with Zika epidemics. One possible explanation is that something else in that part of Brazil, such as a toxin or infection with another virus, increases the damage. Scientists are still studying this.

Babies with microcephaly have a smaller head circumference than normal. Credit: CDC


How does it spread?

– Zika is spread by mosquitoes. The most important one is a species called Aedes aegypti, also known as the yellow fever mosquito, which is extremely widespread in the world’s tropical regions . (It also transmits yellow fever, dengue, and chikungunya.) The Asian tiger mosquito (Aedes albopictus) can transmit Zika as well.

– Mosquitoes don’t fly very far. Zika hops from country to country because infected people travel. Also, an infected mosquito may occasionally hitch a ride on a vehicle or airplane.

– Zika can also be transmitted sexually. This was first discovered after a U.S. scientist who returned from Senegal infected with Zika in 2008 accidentally infected his wife in the U.S. After Zika exploded in 2015/2016, more cases of sexual transmission have been reported, including in a gay couple.


How is it treated?

There are no drugs to fight the symptoms of Zika or to prevent the virus from harming the fetus.

Zika spreads primarily via bites by female Aedes aegypti mosquitoes. Credit: CDC


How can it be prevented?

– Controlling Aedes mosquito populations is one important way to control Zika. There are a range of measures countries can take to fight the mosquito in all of its life stages. They include spraying insecticides against mosquitoes around houses and removing small reservoirs of water (such as pots, buckets, and old tires) where mosquitoes find a place to breed. These measures take the involvement of the community, however, and past experience has shown that they can be difficult to keep up.

– People can reduce their risk of being bitten by using insect repellents, wearing clothes that cover the skin, sleeping under bed nets, and screening off windows to keep mosquitoes out. These measures are especially important for women who are pregnant or planning to become pregnant.

– Condoms can be used to reduce the risk of sexual transmission of Zika. Pregnant women in areas with Zika are advised to practice safer sex or abstain from sex.

– There is no vaccine against Zika. Scientists are working on candidate vaccines, but it will take at least until 2020 before one becomes available.


What’s the outlook?

– On 18 November 2016, WHO declared that Zika was no longer a Public Health Emergency of International Concern. Not because the epidemic was over, but because it’s now a firmly entrenched disease that requires a permanent battle rather than an emergency response. In other words: Zika is here to stay, and is likely to cause periodic outbreaks, just like with dengue and chikungunya, two diseases transmitted by the same mosquitoes.

– Researchers are trying hard to understand how exactly Zika causes disease, and whether it has an accomplice that increases the risk of birth defects. Meanwhile, the hope is that a vaccine will soon be ready to stop the virus from doing harm to unborn babies.



Information about Zika from WHO.

The Pan American Health Organization has detailed Zika information for every country in the Americas.

How to prevent sexual transmission of Zika.