An epidemic of Zika virus infection, first reported last year in Brazil, has spread to all but the southernmost countries in South America, to all of Central America and to most islands in the Caribbean.

Thus far, residents of the United States who have been diagnosed with Zika acquired the disease while traveling abroad. But the eventual spread of the virus to the United States is certain; the extent or severity of this spread cannot be predicted at this time.

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Clif Cleaveland

Zika was first isolated from rhesus monkeys in the Ugandan jungle in 1947. Human infections subsequently were documented in Central African nations. The virus is transmitted primarily by Aedes mosquitoes, two species of which are common in the U.S., especially Southern and Atlantic Coastal areas.

Through the years since Zika's discovery, human infections have been rare and generally mild. In 2007, a widespread outbreak was documented on Yap Island in Micronesia in the South Pacific. Several instances of Guillain-Barre syndrome, a progressive paralysis which may be temporary, were associated with an outbreak of Zika in French Polynesia in 2013-14.

The great majority of those infected with Zika experience fever, joint pain, conjunctivitis (red eyes) and rash, which subside over several days with no complications. The rash, characterized by small, pink bumps and rough spots on the skin, spreads from the face to the entire body.

Intense international attention followed reports from Brazil in 2015 of severe skull and brain defects in children born to women who had had been infected with Zika during pregnancy. Evidence that supports Zika as the cause of the birth defects includes recovery of the virus from amniotic fluid, placental tissue and brain tissue from affected fetuses that did not survive.

But many questions remain regarding the link between Zika and these birth defects. Are there other factors present in Brazil that facilitate fetal injury? Are Brazilian infections due to a strain of Zika that attacks the developing nervous system?

By December 2014, Brazilian public health officials estimated that 1.3 million cases of Zika had occurred in that country. Prenatal scans of Brazilian women who had Zika infections following conception show brain abnormalities in more than 20 percent of fetuses.

In jungle areas, Zika may be transmitted from infected rhesus monkeys to humans. In urban and suburban areas, Zika spreads by a mosquito biting an infected person, then transmitting the virus to all subsequent individuals who are bitten.

Based on extensive epidemiologic data related to Zika infection, on Feb. 1 the World Health Organization declared a Public Health Emergency of International Concern, which places public health agencies on high alert to detect infection and to promote prevention.

Last month, a case was reported of encephalitis (brain infection) related to Zika in an elderly French man who had visited the South Pacific islands. He suffered coma and left-sided paralysis before partial recovery, which took several weeks. Zika was recovered from his spinal fluid.

Zika also may be transmitted sexually and through blood transfusions. It is unclear how long Zika remains in blood and body fluids of someone who has been infected.

As of late April, 683 cases of Zika infection had been confirmed in Puerto Rico following an outbreak that began in December 2015. Seven cases of Zika-related Guillain-Barre syndrome also were reported. Of 89 pregnant women infected, 14 had delivered healthy babies.

There is no treatment or vaccine available for Zika. Preventive strategies include:

* Women who are pregnant or plan to become pregnant should not travel to areas where Zika outbreaks are underway. They should avoid unprotected sexual intercourse with partners who have recently traveled to Zika-impacted areas.

* Control of mosquitoes is important and involves spraying, reducing breeding sites such as standing water, checking household screens for holes and using insect repellents or clothing treated with permethrin in mosquito-infested areas. In tropical areas where screens are lacking, mosquito netting should be used to protect sleeping sites.

At, a detailed review of Zika can be found in the April 24 issue of the New England Journal of Medicine, pages 1,552-1,563. The Centers for Disease Control and Prevention website at also offers valuable updates and travel alerts.

Contact Clif Cleaveland at