Zika virus, first identified in 1947 in Uganda, had been thought to produce a rare and mild disease until it suddenly emerged in Brazil in 2015 and spread explosively through South America, Central America, and the Caribbean. The virus arrived in the United States in the summer of 2016.
Zika virus is transmitted primarily by mosquitoes that thrive in tropical climates and urban areas. The virus can cause Zika virus disease. In most cases Zika virus disease is a mild illness, but Zika virus can cause a serious birth defect known as microcephaly, in which babies have abnormally small heads and symptoms that range from mild developmental delays to a fatal condition. Zika virus can also cause a neurologic condition in adults known as Guillain-Barre syndrome that results in muscle weakness or even paralysis in the worst cases.
Zika virus is a member of a family of viruses known as Flaviviruses. This is the same family to which dengue virus, yellow fever virus, and West Nile virus belong. Flaviviruses are enveloped viruses that contain genomes which consist of nonsegmented single-stranded positive-sense RNA. The viruses are transmitted to humans by infected mosquitoes.
Zika virus and the other flaviviruses are part of a larger group of viruses termed arboviruses (for arthropod-borne). The defining characteristic of these viruses is that they are transmitted to humans via the bite of infected arthropods, most commonly mosquitoes and ticks. They are maintained in nature through a complex cycle in which they rotate between a vertebrate animal host and a blood-feeding vector that carries the virus from one host to another. Another recently emergent arbovirus is chikungunya virus, which belongs to the Alphavirus family.
Zika virus is transmitted through the bite of an infected female mosquito. Two types of Aedes mosquitoes are capable of transmitting Zika virus – Aedes aegypti and Aedes albopictus. These are the same species that spread dengue and chikungunya viruses. Aedes aegypti, which is found in tropical and subtropical regions, is the primary transmitter of Zika virus. Initially thought to be present only in 12 states in southern coastal areas of the United States, Aedes aegypti has now been found in about 30 states. Aedes albopictus can survive cooler temperatures and is found in over 30 states, but it may not be playing an important part in spreading Zika virus currently.
Mosquitoes become infected when they bite a person who is infected with the virus. The virus has to replicate and spread within the mosquito for it to be infectious to the next person the infected mosquito bites, and thus the virus spreads.
There is evidence that Zika virus can be transmitted through sexual contact, although this transmission route is much less common than spread via the bite of infected mosquitoes. The first known case of transmission within the United States resulted from sexual contact between a person who had traveled to Venezuela and a female partner in Dallas, Texas who had not traveled to any region where Zika is present. Male-to-male transmission has also been reported.
Additionally, there are reports of person-to-person transmission through an unknown route, although these cases are rare. Zika viral RNA has been detected in the saliva and urine of patients, but it is not clear if the virus can be transmitted through those fluids.
Most people who become infected with Zika virus do not experience any symptoms. About one in five people will become sick, usually with a mild illness whose common symptoms are a mild fever, skin rash, joint pain, and conjunctivitis. The symptoms generally begin a few days after a bite from an infected mosquito and last about two to seven days. Because symptoms may be nonexistent or similar to other viruses (such as dengue virus), many people might never realize they have been infected with Zika virus.
In pregnant women, there is a much graver risk from Zika virus infection. In Brazil, an explosive epidemic of a condition known as microcephaly has been correlated with the outbreak of Zika virus. Microcephaly is an uncommon disorder in which a baby’s head is much smaller than normal due to abnormal brain development while in the womb or shortly after birth. Babies with this condition will often experience developmental delays later in life and some may have vision defects including blindness. The problems affecting babies with microcephaly can range from mild to severe and can sometimes be life-threatening.
Microcephaly can be caused by a number of genetic and environmental factors, including Down syndrome and fetal exposure to a variety of toxins. However, the number of microcephaly cases in Brazil since the Zika virus outbreak began is about twenty times higher than normally would be expected. Such a congenital malformation has not been seen with any other flavivirus or other arthropod-borne virus.
Although experts were initially cautious about ascribing the cause of microcephaly to Zika virus, the link was strengthened by studies that detected the presence of Zika virus in the amniotic fluid and brains of microcephalic babies. Laboratory studies have further shown that Zika virus can infect and kill human fetal brain cells growing in cell culture and also in mice. By April 2016, the body of accumulated evidence was strong enough to arrive at a scientific consensus that Zika virus causes microcephaly. A large study of pregnant women in Columbia, where the viral outbreak peaked in February and where many women are due to give birth in the coming months, is expected to provide additional confirmation.
Another complication of Zika virus is Guillain-Barre syndrome, a rare condition in which the body’s immune system attacks part of the nervous system. Symptoms include muscular weakness and tingling in the arms and legs. In the most severe cases, a person may be almost completely paralyzed and the respiratory muscles affected, and the patient will require hospitalization. Most people recover, but some may continue to experience some degree of weakness. This syndrome can be also be caused by a number of other viruses.
Zika virus disease is usually diagnosed based on symptoms, prior mosquito bites, and travel history. In some cases, infection is confirmed through laboratory testing using a polymerase chain reaction-based test and isolation of the virus from blood samples.
Zika virus was first isolated in 1947 in the Zika Forest of Uganda (named after the word Zika which means "overgrown" in the local language) from a rhesus monkey during a survey to detect yellow fever infections in primates. It was subsequently identified in humans in 1952 in Uganda and Tanzania. Over the next couple of decades, there was evidence of human infections in other African countries, including an outbreak in Nigeria in 1973; human infections were also reported in countries in Asia.
The first known large outbreak of Zika virus occurred on remote Yap Island in the Pacific Ocean in 2007, with an estimated number of 5,000 cases of mild disease. This was followed in 2013-2014 with an outbreak in French Polynesia, with an estimated number of 20,000 cases. It was during this outbreak that an unusual increase in the number of Guillain-Barre syndrome cases was first noted.
The current Zika virus outbreak began in Brazil in 2015 (although the virus was probably introduced into the country some time in 2014), and it spread rapidly to many other countries in South America, Central America, and the Caribbean. Since the current outbreak began, around 60 countries and territories have for the first time reported Zika virus transmission; including prior outbreaks, a total of 73 countries have experienced Zika virus transmission since 2007.
It is estimated that approximately 500,000 cases of Zika virus infection have occurred in Brazil, although exact numbers are not known because of the difficulty in keeping track of the large number of cases and the fact that many infections go unnoticed. After Brazil, Columbia has been the most affected country with around 20,000 estimated cases.
More than 1,500 cases of microcephaly or other fetal abnormalities linked to Zika virus have been reported in Brazil. The first cases of Zika-associated microcephaly were detected in Columbia in April 2016. Altogether, microcephaly and other central nervous system malformations thought to be associated with Zika virus infection have been reported in 31 countries. An increased incidence of Guillain-Barre syndrome has been reported in 23 countries.
Zika Virus in the United States
The first cases of Zika virus disease were reported in the United States in 2015. All 62 cases resulted from travel to areas where Zika virus was circulating within the local mosquito populations. The number of travel-associated cases in the United States rose sharply in 2016 to nearly 4,900. As of June 2018, the cumulative total of symptomatic cases attributed to travel to affected regions is approximately 5,700.
Cases of Zika virus infection by mosquitoes within the United States, that did not involve travel, first came to light in July 2016. The initial cases were limited to one neighborhood in Miami, Florida; several weeks later local transmission was reported in a second neighborhood in the Miami area. Shortly thereafter, local transmission was also reported in Texas.
By the end of 2016, there were 224 laboratory confirmed cases acquired through local mosquito transmission. With the exception of six cases in Texas, all of the remaining cases occurred in Florida. In addition, 45 cases were acquired through sexual transmission.
As of June 2018, there have been 231 symptomatic cases acquired through presumed local mosquito-borne transmission of the virus (in 2017, there were two cases in Florida and five in Texas). To date, there have been 52 cases acquired through sexual transmission since the outbreak began.
More than 2,300 pregnant women in the United States have had laboratory evidence of Zika virus infection, with 116 reports of live-born infants with Zika-associated birth defects and nine pregnancy losses with Zika-associated birth defects. The birth defects include microcephaly and other abnormalities of the brain, as well as abnormal eye development and hearing loss.
Local transmission of Zika virus began in the U.S. territories of Puerto Rico, American Samoa, and the U.S. Virgin Islands in 2016. Puerto Rico has been the most severely impacted with close to 36,000 infections, almost all of them acquired locally. There have been over 4,800 pregnant women presenting with laboratory evidence of infection, with 167 reports of live-born infants with Zika-associated birth defects and eight pregnancy losses with Zika-associated birth defects.
Spread of Zika Virus
Zika virus has emerged in the Americas, without warning, and is spreading rapidly. Cases have been reported in over 55 countries, and it is expected that Zika virus will continue to expand its range into additional regions where the Aedes mosquitoes are found, including areas of the United States. The mosquitoes that transmit Zika virus are present in about 30 states during the warmer months, so much of the United States is potentially at risk with Florida and Texas considered among the highest risk states especially in poor, urban neighborhoods.
Zika virus has been spreading locally in the U.S. territory of Puerto Rico since the beginning of 2016 and began spreading locally in the United States in the summer of 2016. The CDC took the unusual step of advising pregnant women to avoid travel to areas of Miami-Dade county in Florida where Zika virus was transmitted by local mosquitoes.
Widespread infection by Zika virus was worrisome because the virus had not previously existed in the Americas, and therefore virtually no one was immune to the virus; there existed the potential for millions of people to become infected. One study estimated that approximately 90 million people could become infected during the first wave of the epidemic, including about 1.65 million childbearing women and potentially affecting tens of thousands of pregnancies.
As with other mosquito-borne diseases, the outbreak in the United States and Europe has been less extensive than in the developing countries afflicted by Zika virus, at least in part due to better surveillance, mosquito control, and healthcare infrastructure.
Zika Virus Complications
The full range of symptoms and complications that can be brought about by Zika virus is not completely established yet because in the past there has not been an outbreak involving as many people. However, based on the present and growing body of scientific research, experts have confirmed that Zika virus can cause microcephaly in babies born to Zika-infected pregnant women. The risk to the fetuses of infected pregnant women appears to extend throughout pregnancy and not only during the first trimester as initially thought. This is the first time that a birth defect has been attributed to the bite of an virus-infected mosquito.
Although scientists had not formally confirmed a link between Zika virus infection and microcephaly at the time, the strong correlation prompted the World Health Organization to declare a global health emergency Feb. 1, 2016. This is a rare move for the WHO, and it signaled that it considered the Zika virus outbreak to be serious. Pregnant women have been advised not to travel to countries where Zika virus is circulating and in an unprecedented move, health authorities in several countries affected by Zika virus urged women to delay having children. In June 2016, the WHO issued guidelines that advised pregnant women living in areas where the disease is spreading to consider delaying pregnancy to avoid having babies with birth defects.
Treatment and Prevention
There is currently no cure for Zika virus disease and no drugs to treat it. Scientists have begun the process of developing a vaccine, but it is expected to take years to develop one that would be widely available.
For now, mosquito control is considered the best way to prevent infection and combat Zika virus. This includes personal protection measures when outdoors such as minimizing exposed skin and using mosquito repellents that contain DEET and by eliminating places in which the Aedes mosquitoes can breed such as outdoor containers that collect pools of stagnant water. However, this can be a challenging undertaking especially in densely populated areas and less developed countries, and people living in places without window screens and air conditioning are at higher risk.
The realization that Zika can be transmitted not only by mosquitoes but also through sexual contact, even if it is a relatively rare occurrence, further complicates the containment of the virus.
The possibility of transmission through blood transfusions is an additional concern. In August 2016, new guidelines were issued in the United States for the screening of donated blood at blood collection centers. Although there is currently no licensed test to screen potential blood donors, newly available tests have begun to be used in some states.
As with other large viral outbreaks, there is always the concern that the virus could mutate into a form that increases its transmission rate or ability to cause disease.
The spread of Zika virus, as well as other emerging Aedes mosquito-transmitted diseases, such as dengue and chikungunya, is due in part to man-made factors. Urbanization and possibly climate change create additional environments for the Aedes mosquitoes to live and breed, and human migration facilitates the spread of viruses around the globe. Air travelers can easily spread viruses when they become infected in one country and then board a plane to travel to another country and pass on the virus. Because the majority of Zika virus-infected people do not have any symptoms, it is not possible to detect infected people entering a country.
Although Zika virus was discovered in 1947, there is been little research conducted on the virus because until recently it was considered an obscure virus that caused only sporadic and relatively mild illness in Africa and Southern Asia. With the jump into the Americas, the large increase in the number of human cases, and the link to more severe illness, research into Zika virus has taken on a much greater importance and urgency.
Dr. Rebecca Rico-Hesse and her colleagues are involved in a number of experiments to isolate and analyze Zika virus samples to learn more about the virus and how it replicates. Additionally, they would also like to determine the potential for transmission of Zika virus in mosquitoes collected in the Houston area.
The long-term goals of research on Zika virus include developing more accurate tests to detect the virus in infected people (current tests have limitations in that they detect only recent infections or detect infections with other related viruses), obtaining more knowledge about the transmission paths of the virus, understanding how the virus causes disease, and developing a safe and effective vaccine against Zika virus.
Investigation of host cell types and entry factors that mediate Zika virus transmission
In one study, Drs. Rico-Hesse, Jason Kimata, and their colleagues investigated the host cell types and entry factors that aid in mediating the sexual transmission of Zika virus. They infected two types of human prostate cells - stromal cells and epithelial cells - with three isolates of Zika virus that were obtained from persons infected in the Americas. Using a variety of techniques, they found that prostrate cells express several well-characterized factors used by flaviviruses to attach to cells, and that Zika virus, but not dengue virus, can infect and replicate in these cells.
The results indicated that Zika virus can infect prostate stromal mesenchymal stem cells, epithelial cells, and organoids made with a combination of these cells. They further found that Zika virus appears to favor infection of prostrate stromal cells over epithelial cells; this could possibly indicate a preference for infection of stem cells in general. Overall, the results of this study suggest Zika viral replication occurs in the human prostate, and can account for the secretion of Zika virus in semen, thus leading to sexual transmission.
Effect of mosquito saliva alone on the human immune response
In another line of research, Rico-Hesse and coworkers, using a 'humanized mouse' model to study dengue virus infection, found that mosquito saliva alone - in the absence of any virus - can trigger a human immune response which may significantly affect disease development not only by dengue virus, but also by other Aedes mosquito-transmitted viruses including Zika virus and chikungunya virus.
The researchers used a model system that consisted of mice naturally born without their own immune system, but which had received human stem cells which could then give rise to many of the components of the human immune system, so factors involved in disease development could be studied. These animals were injected with dengue virus either through a mosquito bite or through needle injection.
They found that animals receiving the virus through a mosquito bite developed a more human-like disease with more of a rash, more fever and other characteristics that mimic the disease presentation in humans than did the needle-injected mice.
This led the scientists to investigate the role of mosquito saliva on disease development, hypothesizing that the mosquitoes may not just be acting like ‘syringes,’ merely injecting viruses into the animals they feed on, but rather that their saliva may contribute significantly to disease development.
To test this idea, virus-free mosquitoes were allowed to feed on the humanized mice, and then the researchers took blood and a number of other tissue samples six hours, 24 hours and seven days after the mosquitoes bit the mice. They determined the levels of cytokines, as well as the number and activity of different types of immune cells, using highly sensitive techniques and compared these results with those obtained from humanized mice that had not been bitten by mosquitoes.
They discovered that mosquito-delivered saliva induced a varied and complex immune response. Both the immune cell responses and the cytokine levels were affected. At various time points, the levels and activities of other types of immune cells also increased as others decreased.
Overall, the researchers found evidence that mosquito saliva alone can trigger long-lasting immune responses – up to seven days post-bite – in multiple tissue types, including blood, skin and bone marrow. The diversity and complexity of the immune response was a surprise to the investigators.
The researchers would now like to determine which of the more than 100 proteins in mosquito saliva are mediating the effects on the immune system and whether these effects could increase disease development. Identifying these proteins could help in the design of strategies to fight transmission of Zika, dengue, and other viral diseases transmitted by Aedes aegypti mosquitoes.
For More Information
- Information about Zika virus from the Centers for Disease Control and Prevention
- Information about Zika virus from the National Institutes of Health
- Information about Zika virus from the World Health Organization
- Information about Zika virus from the Pan American Health Organization
- Map displaying the geographic range of Zika virus
- Information about microcephaly
- Information about Guillain-Barre syndrome