- Zika virus infection is acquired through the bite of day-biting mosquitoes, primarily in tropical countries in the Americas, the Caribbean, and the Western Pacific. The majority of Asian and African countries have lower risk. Male-to-female and male-to-male sexual transmission can occur in countries with or without mosquito-borne transmission.
- Risk is high for travelers in populated urban and residential areas of affected regions.
- Symptoms present in only about 20% of cases and include a rash, itching, headache, and muscle and joint pain.
- Consequences of infection include Guillain-Barré syndrome, an autoimmune, neurological complication. Infection during pregnancy is strongly associated with congenital malformations in the fetus.
- Prevention includes wearing long sleeves and long pants as well as using personal protective measures against mosquito bites. Additional measures are necessary for pregnant women or those trying to conceive, including:
- Not traveling to the most severely affected areas
- Abstaining from sex or practicing safer sex while in Zika-affected areas
- Abstaining from sex or practicing safer sex when one or both partners have returned from or reside in a Zika-affected area
- No vaccine or preventive drugs are currently available.
- Returned travelers with symptoms or pregnant partners should seek expert medical assistance.
Zika virus infection is transmitted by Aedes aegypti mosquitoes. It is closely related to dengue, West Nile virus, and Japanese encephalitis. Symptoms appear in approximately 1 of 5 infected persons. When symptomatic, infection usually presents as an influenza-like syndrome and is often mistaken for dengue or chikungunya.
Until 2007, limited reports indicated infrequent, sporadic human infection in at least a dozen countries of Africa and Southeast Asia, which continues to the present. After a large outbreak in Micronesia in 2007, the virus spread to French Polynesia (occurring in 2013-14), a few other Pacific Islands, and in early 2015, Latin America (starting in Brazil), the Caribbean, Cabo Verde, and Singapore. In the U.S., mosquito-borne transmission occurred in focal areas in Miami-Dade County, Florida and Brownsville, Texas in 2016. More than 1 million cases have occurred during the current global outbreak. Mosquito-borne transmission in areas above 2,300 m (7,500 ft) is not believed to occur.
Humans become infected when bitten by Aedes aegypti mosquitoes. Transmission via blood transfusion, during pregnancy or delivery, and during sexual activity (male-to-female and male-to-male) may occur, although fewer than 30 confirmed cases due to sexual transmission have been reported to date. Dead Zika-virus breakdown products have been detected in saliva, urine, and breast milk of ill persons, without proof of any risk of infecting others.
All persons residing in or visiting a Zika-affected area when there is ongoing transmission are at risk of acquiring Zika virus. In nonaffected areas, sex partners of infected males returning from affected areas are at a low but undetermined risk of acquiring infection. Transmission via blood transfusion has not been documented in nonaffected areas.
The incubation period is 3 to 14 days. Most persons have a rash (reddened skin covered with small bumps), headache, malaise, muscle aches, joint pain, eye redness, and occasionally, fever. The illness is usually mild, with symptoms lasting 4 to 7 days. Hemorrhagic fever has not been reported.
In the early stages of the disease, Zika virus infection is indistinguishable from dengue and chikungunya, which often coexist in the same locations.
Consequences of Infection
Severe disease requiring hospitalization is uncommon. Based on available data, Guillain-Barré syndrome, an autoimmune neurological complication that can occur after a viral infection, occurs in an estimated 1 per 5,000 infections. No deaths have been reported.
Zika virus infection during pregnancy is strongly associated with congenital central nervous system malformations, including microcephaly (a condition in which an infant's head is significantly smaller than normal). The frequency of this complication, other outcomes, and the factors that increase risk to the fetus are not yet fully understood. A fetus may be at risk during all trimesters of pregnancy, although the risk of adverse fetal effects is highest during the first trimester. Several cases of adverse pregnancy outcome (microcephaly or fetal loss) have been reported in pregnant women in the U.S.; all women reported previous travel to a Zika-affected country while pregnant.
Need for Medical Assistance
Medical assistance is not normally necessary because serious complications are extremely rare. Most Zika virus infections resolve spontaneously over a few days. Self-medication with paracetamol (acetaminophen) may help relieve some symptoms.
Pregnant women with symptoms consistent with Zika virus infection during or within 2 weeks of travel to risk areas or who have ultrasound findings of fetal microcephaly or calcifications in the head should seek expert medical care and testing as soon as possible upon returning home. Pregnant women without symptoms may be tested from 2 to 12 weeks after returning from a risk area. Pregnant women with possible sexual exposure to Zika virus should be tested.
Testing of any traveler (including pregnant women) returned from non-Zika affected areas is not recommended.
At this time, testing men for the purpose of assessing risk for sexual transmission and testing asymptomatic women contemplating pregnancy are not recommended.
Insect precautions and personal protection measures against day-biting mosquitoes are the main prevention strategy.
Aedes sp. mosquitoes, unlike malaria-transmitting Anopheles sp. mosquitoes, are daytime feeders, with 2 peaks of biting activity during the day: 2 to 3 hours after dawn and the mid-to-late afternoon hours. This pattern, however, turns to one of all-day activity indoors or during overcast days.
In risk areas, travelers (and especially those who are pregnant) should be especially vigilant in applying repellent during daytime hours, particularly during peak biting times. See Insect Precautions.
Containers with stagnant water can serve as breeding sites for mosquitoes and should be removed from the proximity of human habitations whenever possible.
Pregnant women (in any trimester) or those trying to conceive should discuss specific travel recommendations with a travel health provider. The level of risk varies by country, and travel recommendations for pregnant women are tiered by risk level.
- Significant risk (Mexico, Central and South America, and the Caribbean): Do not travel to affected areas.
- Low risk (most of Southeast Asia and a few African countries): Consider postponing nonessential travel after receiving informed advice.
- Negligible risk (most African countries): Use insect precautions.
Many other typical tropical infections pose greater risk than Zika virus infection for pregnant women and their unborn children.
Sexual transmission appears uncommon at present. For couples where one or both partners (symptomatic or asymptomatic) have returned from or reside in a Zika-risk area, practicing abstinence or safer sex (consistent male or female condom use, nonpenetrative sex, and a reduced number of sexual partners) is recommended for, at least, the duration of the pregnancy when the female partner is pregnant. Travelers with symptoms should ideally abstain from sex pending test results and seek expert advice if Zika virus infection is proven.
Current understanding is that, if infected, Zika virus usually remains in the blood of an infected person for about 10 days. The virus will not cause infection in a baby that is conceived after the virus has cleared from the mother’s blood. Men living in areas with no active transmission of Zika virus, but returning from active transmission areas, should abstain from sex or practice safer sex for a period of at least 6 months upon return. Women returning from active transmission areas or after having unprotected sex with an infected partner should wait at least 8 weeks before attempting to conceive. These recommendations apply to both symptomatic and asymptomatic persons. The longest interval from symptom onset in an index case to documented sexual transmission is 32 to 41 days. However, WHO, in contrast to most other authorities, advises couples or women, whether symptomatic or asymptomatic, to wait 6 months after return before attempting to conceive.
Countries are developing their own policies regarding blood donation. In the U.S., the recommendation is that all blood donations be screened and deferred for 120 days following a positive test or symptom resolution, whichever timeframe is longer.