Zika Virus Symptoms, Transmission and Diagnosis

Understanding Zika and why it rapidly spreads

Aedes aegypti mosquitos are seen in a lab at the Fiocruz institute on January 26, 2016 in Recife, Pernambuco state, Brazil. The mosquito transmits the Zika virus and is being studied at the institute.
Mario Tama/Getty Images News/Getty Images

As Zika circled the globe in 2015 and 2016, causing some painful symptoms and placing pregnant women at risk for having babies born with microcephaly, a condition that causes small and underdeveloped brains, many were confused about the virus and where it came from. 

History of the Zika Virus

Zika was first found in a monkey in Uganda. In 1947, a febrile monkey in a Yellow Fever research project in the Zika forest in Uganda was found to have a new virus.

This became known as Zika virus but had only been seen in monkeys. The first known human infection was recognized after a lab accident in 1964.

With hindsight, it was realized there were likely human cases elsewhere previously. It was found that many in different parts of Africa had antibodies to Zika, indicating past (and common) infection. Zika antibodies were found in up to 40 percent of blood sampled in Nigeria in the late 1960s and early 1970s. The virus was also found in Uganda, Tanzania, Egypt, Central African Republic, Sierra Leone, and Gabon. As such, many parts of West, Central, and East Africa are affected.

However, antibody tests can corss react with other similar viruses and may not be reliable. Moreover, the virus may be present, but not in people. In Uganda, it's in monkeys, which do not have much contact with people. Even the mosquitoes of the monkeys do not have much contact with people.

So the virus is present, but not necessarily affecting people there.

Now, cases of Zika have been seen in humans in multiple countries around the globe. Parts of Brazil—initially Bahia, in the northeast—first noticed Zika in 2015. Zika arrived in the region and began to spread to other states in Brazil.

Any cases of Zika were likely imported by a traveler, whether tourist, business person, or sports fan, from regions where it usually found. Brazil has the mosquitoes that can propagate the continued spread of Zika yet had not previously had any infections.

Later in 2015, it reached Paraguay, Colombia, and Venezuela. It also reached Central America (Mexico, El Salvador, Guatemala, Panama, and Honduras). Later, it hit the Caribbean (Puerto Rico, Haiti, Barbados, and Martinique). 

Zika hopscotched around the world on planes. However, if a traveler's viral load (amount of virus in their blood) is too low or the country does not have the right mosquitoes, the virus will not spread. As a result, the virus did not seed outbreaks in all countries.

The disease is often not recognized. It's mistaken for its better known relatives that cause more severe illnesses. These relatives—Dengue, West Nile, Japanese Encephalitis, and Yellow Fever—also cause fevers after mosquito bites. But Zika is more like Dengue-lite in that it causes similar symptoms and less severe outcomes (except for the concerns in pregnancy). Zika can also be misidentified in laboratories as Dengue (and Dengue as Zika) because of antibody cross-reactivity.

Does It Cause Birth Defects?

Zika has been associated with microcephaly in babies. This is a birth defect where babies are born with small heads and incompletely developed brains. There is concern among many, including the CDC, that babies born with microcephaly due to Zika is a risk for pregnant women who live in or travel to affected areas. Brazil has seen a rise in cases to 10 times what it saw before. Over 3000 cases of babies born with small heads have been reported in Brazil, in areas where the virus had quickly spread. 

The virus is not new, having spread in the South Pacific and Africa before.

It may be that these infections were primarily in children, as the virus was common and so caught early in life—and not affecting pregnant women. The viral dynamics, including viral load, may differ in a population that is naive to the virus as well. It may also be that sporadic infection that is often not diagnosed was not connected to Zika. It's hard to recognize the consequences of one, relatively rare infection among many, rather than during an outbreak.

There are many causes of microcephaly. Microcephaly is uncommon. It is also often underdiagnosed, since it is not always looked for. The diagnosis depends on what threshold—how small the head is—meets criteria. It can be due to genes and viruses—like CMV and Rubella—and other infections—like Toxoplasmosis. Microcephaly can be connected to exposure in utero to alcohol and mercury, as well as diseases in the mother, like PKU.

Rates of microcephaly vary around the world based on how common the causes are. It's possible that what was already there is now being seen. However, the rates of this condition in Brazil are very high. Many babies are born each year in Brazil—around 3 million; 3000 is a high rate of microencephaly anywhere in the world. There is reason to be concerned even though the connection is not definitively proven.


Zika causes a less severe, though nonetheless painful, mix of symptoms reminiscent of Dengue: joint aches and swelling, red eyes, headaches (particularly behind the eyes), a rash, and a fever. It can cause conjunctivitis that is exudative (eyes irritated with discharge). The rash, if present, can be all over the body and quite notable; it may have texture.

The fever is often not very high; the thermometer might not register a temperature when someone with Zika feels febrile. Some also lose their appetite, have swollen lymph nodes (especially in the neck), have diarrhea or constipation, become dizzy, have blood in semen ejaculate, or have abdominal or back pain. 

The illness usually develops three to 12 days (often 10) after an infected bite and lasts for four to seven days (often five). Many feel particularly tired even after the other symptoms go away. Rash usually begins three to five days after the fever.

The virus is not thought to cause symptomatic illness in most affected. Most likely have a very mild syndrome that is not recognized or does not begin ill at all. It can be associated with more severe consequences like Guillain–Barré syndrome (GBS), though the amount of risk has not been determined. El Salvador has had a rise, as had French Polynesia before it. Some GBS cases may be related to other concomitant viruses like Dengue, which spread in the same locales with the same vectors.

However, for most who are not pregnant, Zika is generally considered a mild, self-limiting illness that is not usually associated with mortality.

Is the Average American at Risk?

Zika has reached the Americas. There have been cases in Florida and in Texas. The mosquitoes that spread Zika are found in the U.S., particularly in warmer places like Flordia and Texas, as well as Puerto Rico. They are not as common in the U.S., though, as they are in some areas where the virus has spread rapidly.

Even though cases have been reported in Florida and Texas, this does not mean that the average American has a high risk of contracting Zika. Most parts of the country do not provide conditions suitable to the disease-carrying mosquito. The mosquito mainly lives in tropical areas outside of the country's borders.


Aedes mosquitoes spread Zika. Traditionally, the virus spread through Aedes aegypti. It has been found to be in mosquitoes that are even more common in North America, Aedes albopictus.

This means anywhere that has had cases of these viruses, such as the Caribbean, South America, even parts of southern Europe, Asia, Florida, Texas (and, in fact, many other parts of the U.S.) could be vulnerable to outbreaks if the virus is introduced in the right season.

The virus is also sexually transmitted. Mosquito-borne illnesses, thankfully, don't usually double as STDs. It has been known to be transmitted, to date, from sex from man to woman, woman to man, and man to man. The virus is also found in urine. It is not thought that sexual transmission causes as much transmission as mosquitoes, who bite a lot.

There is also the possibility of transmission through blood transfusion. Zika, rare in much of the world, is not standardly screened for. It is possible individuals might donate while they had virus in their blood since the infection can be asymptomatic. A survey in French Polynesia found 3 percent of donors during an outbreak had Zika it in their blood but had no symptoms (or at least none at that time). Thankfully, screening for malaria and other diseases would preclude many of those infected from donating blood in non-malaria and temperate zones.


An antibody test can be performed to see if there has been an immune response to Zika virus. This is not a common test to order and most doctors won't be able to easily order the test. Where there are outbreaks, it is often assumed that illnesses that seem like Zika are Zika without a lab test, but Chikungunya and Dengue can cause similar illnesses and can spread in the same areas through the same mosquitoes.

Zika is similar to other flaviviruses, like Dengue and Yellow Fever. Antibody tests, like IgM, that detect Zika may cross-react with these flaviviruses, diagnosing someone with Zika virus instead of Dengue or vice-versa. There are other tests which can be used to make sure these antibody tests are accurate by showing how strong a response there is - or how large the titer is - through PRNTs (Plaque reduction neutralization test). Zika can also be identified through direct detection of the virus in blood or other samples.

While there isn't a specific treatment, some people require hospitalization if they feel particularly unwell or have other illnesses. There is, however, some interest in the use of azithromycin.

How to Avoid Zika

The best way to avoid Zika is to avoid mosquitoes—in particular Aedes mosquitoes, which may breed and live closer to us than other mosquitoes. Insect repellant and concealing clothing is helpful for preventing bites.

There is no vaccine for Zika, at least not yet. There's work being done to develop one, however.