Zika virus is a flavivirus related to yellow fever, West Nile virus, dengue fever and Japanese encephalitis. It was first discovered in a rhesus monkey in the Zika forest of Uganda in 1947. Until recently, outbreaks have been concentrated in Africa, Asia and the Pacific. Zika virus is a mosquito-borne virus transmitted primarily by the mosquito Aedes aegypti. Another mosquito, Aedes albopictus, is also likely to transmit the virus.
In May 2015, the World Health Organization received reports of the first local transmission of Zika virus in the Western Hemisphere, with locally transmitted cases identified in Brazil. Since then, more than 14 countries or territories in the Americas have been identified as sites of local transmission. Although local transmission of the Zika virus has not been documented in the United States and Canada, a number of imported cases with no local transmission have been described. In Brazil, it is estimated that there have been over 1 million cases, mostly in the northeastern part of the country.
How is Zika virus transmitted?
Zika virus is transmitted by the bite of the day-biting Aedes mosquito. A number of reports have begun to emerge of women becoming infected through sex with infected males recently returned from endemic areas.
Is there a risk of local (authochthonous) transmission in the US and Canada?
Both Aedes aegypti and Aedes albopictus are found in the United States, and cases of locally transmitted dengue fever and chikungunya infections have already been reported in in Florida. Local transmission of Zika in the US is possible, but not in epidemic proportions. The southern-most states are most likely to be affected. Local transmission of Zika virus cannot occur in Canada because the Aedes mosquito is absent.
What are the symptoms of Zika?
Only 20% of those infected with Zika virus become symptomatic. Characteristic clinical findings include acute onset of fever, maculopapular rash, arthralgia, myalgia and conjunctivitis. Clinical illness is usually mild, with symptoms lasting several days to a week.
A marked increase in cases of microcephaly has been reported in Brazil. Zika virus RNA has been identified in the amniotic fluid and tissues of infants born with microcephaly, but a causal association has not been confirmed. There has also been an increase in cases of Guillain-Barré syndrome (GBS), a neurological illness that damages the lower limbs, in countries where Zika is being transmitted. A Zika outbreak in Polynesia in 2013 was accompanied by an uptick in GBS cases, although GBS also occurs with many other viruses, including influenza and dengue fever. It is uncertain whether people infected with Zika virus will have lifetime immunity.
How is Zika infection diagnosed?
Zika virus infection can be confirmed by DNA testing (RT-PCR) on serum specimens collected within the first week of illness. Zika antibodies do not develop until at least four days after the onset of acute illness. However, antibody tests have proved problematic, showing false positive results as a result of the antibodies cross-reacting with related flaviviruses such as dengue and yellow fever. For example, people with silent dengue infection (only one out of three infections is symptomatic) or yellow fever immunization in the past may develop cross-reacting Zika antibodies. Antibodies to Zika virus may linger in the body for many years. Zika virus testing is currently underway at the Centers for Disease Control and Prevention (CDCP) in Atlanta, GA. PCR testing and antibody studies will only be performed by the CDCP on symptomatic individuals.
Treatment of Zika virus infection:
Treatment is generally supportive and can include rest, fluids and use of pain medication and drugs to reduce fever. However, because dengue fever and Zika virus often circulate in the same areas and have similar symptoms, non-steroidal anti-inflammatory drugs (NSAIDs) such as ASA (Aspirin), ibuprofen (Advil) and naproxen (Naprosyn) should be avoided because of the increased risk of hemorrhage in dengue fever patients. Pregnant women who have a fever should be treated with acetaminophen (Tylenol).
What is the relationship between Zika virus and fetal damage during pregnancy?
Zika virus infections have been confirmed in a number of infants born with microcephaly in Brazil. The time frame and geographical location of reports of infants with microcephaly have coincided with the outbreak of Zika virus infections in Brazil. The number of infants with microcephaly currently reported in Brazil is higher than normal.
Microcephaly can be diagnosed during pregnancy with ultrasound, most easily late in the second trimester of pregnancy or early in the third.
Pregnant women with a history of travel to an area with Zika virus transmission and who report symptoms consistent with Zika virus disease (including acute onset of fever, maculopapular rash, arthralgia and conjunctivitis) during their trip or within two weeks after it should be tested. Pregnant women with a history of travel to an area with Zika virus transmission and who have ultrasound findings of fetal microcephaly or intracranial calcifications should also be tested for Zika virus infection. Right now, the CDCP does not recommend laboratory tests on individuals who have travelled to an area of Zika transmission and not shown any Zika virus symptoms, nor will it provide laboratory testing for them. Women who have travelled to an endemic area and have no symptoms of Zika infection should inform their obstetrician or midwife of travel so that their pregnancy can be closely monitored with fetal ultrasound.
Zika virus is transmitted to the developing fetus when infection occurs in pregnancy. The highest risk to the fetus is during the first 24 weeks of pregnancy, especially the first 12.
Zika virus infection can be prevented by avoiding being bitten by the Aedes mosquito, whose peak biting time is the early morning and late afternoon. Long-sleeved shirts and pants may be helpful, weather permitting. Insect repellents containing DEET (25-30%) and Picaridin (20%) are effective for four to six hours and are safe for pregnant women and children over two months of age . In children, these repellents are safe, but should be used sparingly (i.e. no sheet-dipping!). Screened accommodations and air conditioning are helpful. Travellers to vacation resorts are likely to be better protected because of local insecticide spraying (this should be confirmed by communicating with the resort directly). No vaccine is available.
Pregnant women should avoid travelling to areas where Zika virus is transmitted. Where travel plans cannot be changed, scrupulous insect protection measures should be used. Since the virus can only be transmitted to the fetus during the first week of illness, and taking the incubation period into consideration, attempts to conceive a child should wait for at least six weeks after leaving an endemic area. Since male-to-female transmission has been documented, abstinence or condoms should be used for at least one month after returning from an endemic area. No future pregnancies will be affected after a woman, pregnant or otherwise, has been infected with the virus.