The Zika Virus: What Have We Learned So Far?

By Professor Barry Fox, M.D.

“Zika is a major emerging viral infectious disease problem that has no simple or easy solution.”

Image of Male Aedes Aegypti mosquito, the carrier of the Zika virus.
Male Aedes Aegypti mosquito, the carrier of the Zika virus.

Although the Zika virus existed in central Africa decades ago, the sudden and rapid spread of the virus around the world in a couple of years was significant, but not unprecedented. Other viral diseases, including Dengue and Chikungunya virus which have the same mosquito vector of transmission, had a similar spread over just a few years.  In the case of Zika, approximately 50 countries have been affected.

What is it, and How is it Spreading?

Photograph of Zika virus under a microscope
Transmission electron microscope image of negative-stained, Fortaleza-strain Zika virus (red)

Zika is officially classified as a flavivirus and is closely related to Dengue and West Nile viruses. It is spread by Aedes aegypti mosquito species. Unlike most mosquitos in the U.S. which tend to bite at dusk and dawn, Aedes can be aggressive daytime biters. They can also reproduce in a container of water as small as a bottlecap. This characteristic makes mosquito control measures much less likely to be successful.  The best way to prevent Zika is to prevent mosquito bites. A. aegypti mosquito’s have a limited geographic range in the southern U.S. Both southern Florida and Texas have been affected so far in the U.S., but the A. aegypti mosquito also has a worldwide range.

Much of the worldwide attention for the Zika virus is from its link to congenital neurological abnormalities in pregnant women. Microcephaly is the most common abnormality, leading to incomplete brain development.  Children born with this condition not only struggle to survive, but represent an ongoing healthcare burden, often in nations that already have limited healthcare resources. Miscarriage, and/or stillbirths are also attributed to the infection.

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What are The Symptoms of the Virus?

Obviously a great concern to the continental U.S. would be a broader spread of Zika and a larger impact on pregnant women here. Of additional concern is another species of Aedes which could potentially harbor the Zika virus: A. albopictus. This species has a much larger geographic range throughout most of the U.S.

One of the unique features of this virus is that infection in males leads to contamination of sperm for up to 90 days; hence,  Zika has also become a sexually- transmitted disease, estimated to be 5-10 % of total cases. Men who have traveled to or lived in Zika endemic areas need to take special precautions with sexual relations with their partners, especially if they are in the family planning mode.

Illustration of a baby with microcephaly compared to one with typical head size.
A baby with microcephaly (left) compared to a baby with a typical head size

Of those humans bitten by an infected mosquito, only one in five will have symptomatic disease. These include several days of mild symptoms including fever, chills, muscle aches, and a rash. These symptoms are also milder than infections with Chikungunya or Dengue virus, and milder than influenza infections. The asymptomatic nature of most infections complicates the diagnosis and potential identification in women of childbearing age. There have also been reports of Guillain-Barré syndrome, an uncommon neurological disorder with loss of ability to control and sense ones arms and legs,that may be tied to the virus.

During the first week of infection, Zika virus can be found in the bloodstream, and can transfer from an infected person back to another uninfected mosquito via mosquito bites. The newly infected mosquito can then transfer the virus to other people in a vicious cycle. There are reliable blood and urine tests available to establish a diagnosis of Zika infection, but there are no effective anti-viral treatments for infection, nor will there likely be in the near future.

Pregnant women can be tested for Zika infection, and they can also have prenatal care to assess the developing baby’s  risk of congenital infection, including ultrasound exams, and possible amniocentesis, or sampling of the fluid in the amniotic sac.

Where Is The Disease Now?

image of mosquito spreading Zika virus to human
Cases have now been transmitted locally in south Florida and Texas.

Originally Zika infections were confined outside the continental United States, and cases in the US were all “imported”. There is a significant burden of Zika infections in Puerto Rico. Also, cases have now been transmitted locally mosquito to humans in south Florida and Texas. In spite of local community efforts at mosquito control, the prevailing health authority opinion is that Zika will likely remain in these and other regional geographic areas.

Pregnant women have been advised by the Center for Disease Control not to travel to areas of the U.S., or other areas of the world with known Zika virus cases. The local tourist bureau should not be your source of information if you are thinking of traveling abroad; rather, check the CDC site for information.

The World Health Organization has issued a global response plan in 2016, available for review at http://www.who.int/emergencies/zika-virus/en/ .

Map of Zika Virus transmission
Map produced by the US Centers for Disease Control of active Zika Virus transmission as of April 13, 2016

One of the more pressing global needs regarding Zika, since it’s most likely here to stay, is a vaccine for prevention. There are several types of vaccines simultaneously being investigated. A federal ethics panel, however, has recently declined to allow humans to be deliberately infected with the Zika virus — although there are established rules of vaccine development, the rules may need reconsideration under the current world need for a vaccine.

In summary, Zika is a major emerging viral infectious disease problem that has no simple or easy solution. It is hoped that the virus will remain limited in its transmission areas in the United States. Pregnant women need to understand the risk of Zika in local areas, and if they are considering any travel outside the US, avoid Zika infected areas.

Professor Barry C. Fox is is a Clinical Professor of Infectious Disease at the University of Wisconsin School of Medicine and Public Health
His lecture series An Introduction to Infectious Diseases is now available to stream on The Great Courses Plus.
PHOTO By Muhammad Mahdi Karim (Own work) [GFDL 1.2 (http://www.gnu.org/licenses/old-licenses/fdl-1.2.html)], via Wikimedia Commons
PHOTO By NIAID (Zika Virus) [CC BY 2.0 (http://creativecommons.org/licenses/by/2.0)], via Wikimedia Commons

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