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Continuing Education for Health Professionals

Zika Virus (ZIKV)

Module 1

Overview of the Zika Virus

Zika virus (ZIKV) was first isolated in 1947 by researchers studying Yellow Fever in the Zika Forest in Uganda. For more than 50 years it was little-studied, mainly because it was observed to cause only mild flu-like symptoms and rash in infected individuals. Zika virus was thought to be limited mostly to the continent of Africa, although some cases were reported in southeast Asia, the Phillipines, and India in the decades following its discovery in Uganda. With the increase in air travel in the second half of the 20th century, the virus has rapidly spread to other parts of the world.

ZIKV is transmitted by several different species of Aedes mosquitoes, including the Aedes aegypti and Aedes albopictus. These mosquitos transmit two other important mosquito-borne diseases—Dengue Fever and Chikungunya. In addition, Aedes aegypti transmits Yellow Fever and is sometimes referred to as the "yellow fever mosquito."


image: photo of aedes aegypti mosquito

Aedes aegypti, one of the transmitters Zika virus. The mosquito is black with white bands on its legs and body. Source: Wikimedia Commons.


Aedes aegypti mosquitos are widely distributed throughout the world and thrive in urban environments. Because of this, the potential exists for ZIKV to spread to areas where Aedes aegypti is already present. This is already occuring in warmer and wetter parts of the Western Hemisphere as local mosquitoes pick up the virus from infected travelers and then spread the virus to other people.

As of July 12, 2017, CDC has reported 224 locally-acquired cases in the United States including 1 laboratory-acquired case. There have been 5,109 travel-associated cases in travelers returning from affected areas to the United States, and 46 cases in people infected through other routes, including sexual contact. As of July 12, 2017, almost 40,000 locally-acquired cases have been reported in the U.S. Territories, the vast majority in Puerto Rico (CDC, July 12, 2017). The locally-aquired cases are presumed to be from mosquito-borne transmission.

As of June 27, 2017, laboratory evidence of possible Zika virus infection has been reported in 1,997 pregnant women within the United States and the District of Columbia as well as in 4,175 pregnant women in U.S. territories (CDC, July 6, 2017a). Within the U.S. and District of Columbia, 88 infants were born with birth defects and 8 pregnancy losses with birth defects were reported. In the U.S. Territories, 122 infants were born with birth defects and 6 pregnancy losses with birth defects were reported (CDC, July 6, 2017b). 

In the mid-summer of 2016 the Florida Department of Health identified an area in the Wynwood neighborhood of Miami where Zika was being spread by mosquitoes. The Department of Health issued guidance for people who live in or have traveled to this area any time after June 15, 2016 (based on the earliest time symptoms can start and the maximum 2-week incubation period for Zika virus). On September 19, 2016, the Wynwood neighborhood was declared Zika-free.

In August of 2016, the Florida Department of Health identified two areas of Miami-Dade County where Zika was spreading, including a section of Miami Beach. At that time, CDC took the unprecedented step of advising pregnant women not to travel to this part of Miami. As of June 30, 2017, CDC is recommending that pregnant women who lived in or traveled to this area between August 1, 2016 and June 2, 2017 be tested for Zika virus (CDC, June 30, 2017). 

CDC is continuing to work with Florida health officials to investigate any new cases of locally transmitted Zika virus infection in Miami-Dade County. Although sporadic cases may still occur, the risk of Zika virus transmission in the Miami-Dade County is expected to remain low. On June 2, 2017, CDC removed the designation "Zika cautionary area" from Miami-Dade County, also removing any travel restrictions to the county. People living in, or traveling to, Miami-Dade County are urged to continue to protect themselves from mosquito bites (CDC, June 30, 2017).

In December of 2016, CDC issued guidance related to Zika for people living in or traveling to Brownsville, Cameron County, TX. A month earlier, the Texas Department of State Health Services reported the state’s first case of local mosquito-borne Zika virus infection in Brownsville. Additional cases of mosquito-borne Zika have been identified in the area, suggesting that there is a risk of continued spread of Zika virus in Brownsville. As a result, CDC has designated Brownsville as a Zika cautionary area.

On May 5, 2017, CDC designated Brownsville, Texas a "Zika cautionary area" meaning (1) pregnant women should consider postponing travel to the area, (2) people who live in or travel to Brownsville, TX, should remain aware of Zika virus transmission and strictly follow steps to prevent mosquito bites, (3) pregnant women and their partners who live in or travel to Brownsville, TX, should consistently and correctly use condoms each time they have sex, and (4) pregnant women who live in, traveled to, or had sex without a condom with someone who lives in or traveled to Brownsville on or after October 29, 2016, should be tested for the Zika virus (CDC, May 5, 2017).

Global Aedes aegypti Distribution

image: world map showing aedes aegypti distribution

Source: Moritz UG Kraemer, Marianne E Sinka, Kirsten A Duda, et al. Licensed under Creative Commons CC0 1.0 Universal Public Domain Dedication.


Zika First Identified in Uganda

About seven miles northeast of the Virus Research Institute, there is a forested area called Zika. This area of forest consists of a narrow, dense belt of high but unbroken canopy growth with clumps of large trees. It lies along the edge of a long arm of Lake Victoria from which it is separated by a papyrus swamp.


The greater part of the forest runs parallel with the Entebbe-Kampala road; there is a narrow stretch of grassland between the forest and the road. The forest at no place is more than 500 yards wide. It is about one mile in length and is almost continuous with scattered forest, which in turn joins the forest at Bujuko on the Kampala–Fort Portal road.

Dick, Kitchen & Haddow, 1952
Transactions, Royal Society of Tropical Medicine and Hygiene



image: map of Africa showing location of Uganda

Location of Uganda in Africa (in green) just above Lake Victoria (in white). Source: Marcos Elias de Oliveira Júnior. Source: Wikimedia Commons.

Zika virus was unknown prior to 1947, when it was found in the blood of a rhesus monkey during a Yellow Fever study in the Zika forest of Uganda. In 1948, the virus was isolated from a pool of Aedes africanus mosquitoes collected from the same region of the Zika forest. Although the Zika virus appeared to be largely confined to forest environments, a serologic* survey conducted at that time showed that 6.1% of the residents in nearby regions of Uganda had specific antibodies to ZIKV (Lanciotti et al., 2008).

*Serologic testing: the testing of body fluids, usually blood serum, to detect the presence of antibodies against a particular organism.

Over the next twenty years, ZIKV were isolated from Aedes mosquitoes in Africa (Aedes africanus) and Malaysia (Aedes aegypti), implicating these species as likely epidemic or enzootic vectors. Several ZIKV human isolates were also obtained in the 1960s and 1970s from East and West Africa during routine arbovirus surveillance studies in the absence of epidemics (Lanciotti et al., 2008).

Additional serologic studies in the 1950s and 1960s detected Zika virus infections among humans in Egypt, Nigeria, Uganda, India, Malaysia, Indonesia, Pakistan, Thailand, North Vietnam, and the Philippines. This strongly suggests widespread occurrence of Zika viral infections from Africa to Southeast Asia west and north of the Wallace line* (Lanciotti et al., 2008).

*Wallace line: a boundary line drawn in 1859 that represents a transitional ecosystem zone between Asia and Australia. Asiatic species are found west of the line while a mixture of Asiatic and Australian species are found east of the line.


image: map of Uganda showing Zika forest location

Map of Uganda showing the location of the Zika Forest near Lake Victoria. Source: M. A. Kaddumukasa, Department of Arbovirology, Uganda Virus Research Institute.


Zika Spreads to Indonesia and the South Pacific

In 1977 Zika viral infection was confirmed among seven patients in central Java, Indonesia, during an acute fever study. Clinical characteristics of infection included fever, headache, malaise, stomach ache, dizziness, anorexia, and maculopapular rash; in all cases infection appeared relatively mild, self-limiting, and nonlethal (Lanciotti et al., 2008).

In April 2007 an epidemic of rash, conjunctivitis, and arthralgia was noted by physicians in Yap State, Federated States of Micronesia. Laboratory testing with a rapid assay suggested that a dengue virus was the causative agent. Samples were sent for confirmatory testing to the Arbovirus Diagnostic Laboratory at the CDC, where serologic testing confirmed recent infection in several patients. Additional testing generated DNA fragments, which demonstrated about 90% nucleotide identity with ZIKV. These findings indicated that ZIKV was the causative agent of the Yap epidemic (Lanciotti et al., 2008).

The Yap Island outbreak was estimated to have infected approximately three-quarters of the island’s population (total population was 7,391 according to the 2000 census). About 900 people had mild illness attributed to Zika infection lasting several days (Duffy et al., 2009). The more severe Zika disease symptoms were not observed during the 2007 Yap Island, Micronesia, Zika outbreak, although approximately 5,000 people were infected (Malone et al., 2016).

image: map showing location of Micronesia

Map of Micronesia, a cultural and geographical area in the Pacific. Yap is part of the Caroline Islands. Source: Wikipedia, public domain.


In October 2013 French Polynesia reported its first outbreak (at the time, the largest outbreak ever reported), which was estimated to affect 28,000 people (about 11% of the population) (Besnard et al., 2014).

Zika Arrives in Latin America

In 2014 Zika turned up in Latin America, believed to be transmitted by tourists attending the Va’a World Sprint Championship canoe race held in Rio de Janeiro, Brazil (Musso, 2015). In May 2015 the Pan American Health Organization (PAHO) issued an alert regarding the first confirmed Zika virus infection in Brazil (CDC, 2016a).


Reported Active Transmission of Zika Virus, 2016

image: world map showing active transmission of Zika

Countries and territories with active Zika virus transmission include Mexico, Central America, South America, Samoa (Oceania/Pacific Islands), and Cape Verde (Africa). Source: CDC, 2016a.


ZIKV is expected to continue to spread to Central and North America. Infectious disease modelers estimate that about 200 million Americans—more than 60% of the U.S. population—reside in areas that might be conducive to the spread of ZIKV during warmer months, including areas along the East and West Coasts and much of the Midwest. In addition, another 22.7 million people live in humid, subtropical parts of the United States that might support the spread of ZIKV year round, including southern Texas and Florida. Already there are reports of local spread of the virus within Puerto Rico and of travelers returning to mainland United States with the Zika infection (Collins, 2016).


image: US maps showing distribution of two mosquito types

Countries and territories with active Zika virus transmission include Mexico, Central America, South America, Samoa (Oceania/Pacific Islands), and Cape Verde (Africa). Source: CDC, 2016a.

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