1. What is the Zika virus?
A tropical infection new to the Western Hemisphere.
The Zika virus is a mosquito-transmitted infection related to dengue, yellow fever and West Nile virus. Although it was discovered in the Zika forest in Uganda in 1947 and is common in Africa and Asia, it did not begin spreading widely in the Western Hemisphere until last May, when an outbreak occurred in Brazil.
Until now, almost no one on this side of the world had been infected. Few people here have immune defenses against the virus, so it is spreading rapidly. Millions of people in tropical regions of the Americas may now have been infected.
Yet for most, the infection causes no symptoms and leads to no lasting harm. Scientific concern is focused on women who become infected while pregnant and those who develop a temporary form of paralysis after exposure to the Zika virus.
2 Infected blood
travels to the midgut.
1 Mosquito feeds on
3 Virus enters the
circulatory system …
4 … and travels to
the salivary glands.
5 Mosquito bites again, injecting virus-
infected saliva into victim before feeding.
By Sarah Almukhtar and Mika Gröndahl | Sources: Dr. W. Augustine Dunn; Oxitec; The Anatomical Life of the Mosquito, R. E. Snodgrass
2. How does a mosquito transmit Zika?
The virus moves from its gut to its salivary glands.
Only female mosquitoes bite people: they need blood in order to lay eggs. They pick up the virus in the blood. It travels from their gut through their circulatory system to their salivary glands and is injected into its next human victim. Mosquito saliva contains proteins that keeps blood from clotting. When a mosquito bites it first injects saliva so that its prey’s blood does not clog its straw-like proboscis.
3. What areas is Zika likely to reach?
Zika is spread by mosquitoes of the Aedes genus, which can breed in a pool of water as small as a bottle cap and usually bite during the day. The aggressive yellow fever mosquito, Aedes aegypti, has spread most Zika cases, but that mosquito is common in the United States only in Florida, along the Gulf Coast, and in Hawaii – although it has been found as far north as Washington, D.C., in hot weather.
The Asian tiger mosquito, Aedes albopictus, is also known to transmit the virus, but it is not clear how efficiently. That mosquito ranges as far north as New York and Chicago in summer.
4. Can the Zika virus be sexually transmitted?
Yes, but experts suspect that it rarely happens.
Experts believe that the vast majority of all Zika infections are transmitted by mosquitoes, not sex. As of Feb. 2, there have only been three reports suggesting sexual transmission.
In 2008, a scientist studying malaria in Africa returned to Colorado and apparently infected his wife before developing symptoms himself. Both had fever, rash and headaches. A year later, tests on their blood, which had been frozen, revealed that both had had Zika.
In 2013, live virus was found in the semen of a 44-year-old Tahitian man; whether he infected anyone is unknown.
In early February, health officials in Texas announced that a traveler who had returned to Dallas from Venezuela apparently had infected a sexual partner.
In the first two cases, the men had genital pain and blood in their semen, suggesting that their testes or prostates had been infected. Details regarding the third case were not released.
Based on these reports, the C.D.C. issued tentative new guidelines suggesting that pregnant women avoid contact with semen from men who have recently returned from areas with Zika transmission. Men returning from these regions should consider using condoms, the agency said.
Officials did not provide guidance on how long to do so. British health authorities have suggested using condoms for at least 28 days.
There are still many unknowns, including:
Can a woman pass the virus to a man through sex? Can it be passed through anal, oral or any other form of sex?
Does a man have to have blood in his semen to be infectious? Is he infectious before the blood appears?
If there is no blood, does he have to have had Zika symptoms like fever and rash to transmit the virus? How long does a man remain infectious?
5. How might Zika cause brain damage in infants?
Experts aren’t certain how it happens, or even whether the virus is to blame.
The possibility that the Zika virus causes microcephaly – unusually small heads and often damaged brains – emerged only in October, when doctors in northern Brazil noticed a surge in babies with the condition.
It may be that other factors, such as simultaneous infection with other viruses, are contributing to the rise; investigators may even find that Zika virus is not the main cause, although right now circumstantial evidence suggests that it is.
It is not known how common microcephaly has become in Brazil’s outbreak. About three million babies are born in Brazil each year. Normally, about 150 cases of microcephaly are reported, and Brazil says it is investigating nearly 4,000 cases. Yet reported cases usually increase when people are alerted to a potential health crisis.
6. What is microcephaly?
Babies with microcephaly have unusually small heads. In roughly 15 percent of cases, a small head is just a small head, and there is no effect on the infant, according to Dr. Constantine Stratakis, a pediatric geneticist and a scientific director at the National Institute of Child Health and Human Development.
But in the remainder of cases, the infant’s brain may not have developed properly during pregnancy or may have stopped growing in the first years of life. These children may experience a range of problems, like developmental delays, intellectual deficits or hearing loss.
The consequences can vary widely from child to child. Pinpointing an underlying cause helps clinicians to advise parents about their newborn’s prognosis.
Genetic abnormalities are a common cause. Microcephaly can also be triggered by infections of the fetus, including German measles (also known as rubella), toxoplasmosis (a disease caused by a parasite found in undercooked contaminated meat and cat feces) and cytomegalovirus.
Microcephaly may also result if a pregnant woman consumes alcohol, is severely malnourished or has diabetes. If the defect occurs in a child’s first years, it may be a result of a brain injury during labor.
There is no treatment for an unusually small head.
“There is no way to fix the problem, just therapies to deal with the downstream consequences,” said Dr. Hannah M. Tully, a neurologist at Seattle Children’s Hospital who specializes in brain malformations.
Note: American Samoa, Cape Verde, Tonga and the Marshall Islands should also be avoided.
7. What countries should pregnant women avoid?
About two dozen destinations mostly in the Caribbean, Central America and South America.
The Pan American Health Organization believes that the virus will spread locally in every country in the Americas except Canada and Chile. Here is the C.D.C.’s current list of countries and territories in which Zika virus is circulating. The latest updates are here.
8. How do I know if I’ve been infected? Is there a test?
It’s often a silent infection, and is hard to diagnose.
Until recently, Zika was not considered a major threat because its symptoms are relatively mild. Only one of five people infected with the virus develop symptoms, which can include fever, rash, joint pain and red eyes. Those infected usually do not have to be hospitalized.
There is no widely available test for Zika infection. Because it is closely related to dengue and yellow fever, it may cross-react with antibody tests for those viruses. To detect Zika, a blood or tissue sample from the first week in the infection must be sent to an advanced laboratory so the virus can be detected through sophisticated molecular testing.
The C.D.C. testing algorithm for pregnant women who have visited countries in which the Zika virus is spreading. Credit Centers for Disease Control and Prevention
9. I’m pregnant and I recently visited a country with Zika virus. What do I do?
Some women should get blood tests, and just about all should get ultrasound scans.
On Jan. 19, the C.D.C. issued interim guidelines for women in that situation and for their doctors. The guidelines are complex — and may change.
In general, they say that pregnant women who have visited any area with Zika transmission should consult a doctor. Those who have had symptoms of infection like fever, rash, joint pain and bloodshot eyes during their trip or within two weeks of returning should have a blood test for the virus.
That recommendation is controversial, because even women with no symptoms may have been infected — 80 percent of those who get the virus do not feel ill — and there is no evidence that babies are hurt only when the mother has been visibly ill. But at the time the guidelines were issued, the C.D.C. and state health departments simply did not have the laboratory capacity to test every pregnant woman who visited Latin America and the Caribbean in the last nine months, as well as every pregnant woman in Puerto Rico. On Feb. 5, the agency added a recommendation that pregnant woman who do not show any Zika symptoms should also receive testing two to 12 weeks after returning from a Zika-affected country.
Even for women who get blood tests, the news is not entirely reassuring. Tests for the virus itself only work in the first week or so after infection. Tests for antibodies can be done later, but they may yield false positives if the woman has had dengue, yellow fever or even a yellow fever vaccine.
Under the C.D.C’s testing algorithm, pregnant women who have been to affected regions – whether they have symptoms or not, and whether they have negative or positive blood tests – should eventually have an ultrasound scan to see if their fetuses are developing microcephaly or calcification of the skull.
Unfortunately, an ultrasound usually cannot detect microcephaly before the end of the second trimester.
Some women also should have amniocentesis to test the fluid around the fetus for Zika virus. But amniocentesis involves piercing the amniotic sac with a long needle through the abdomen; it is slightly risky for the fetus and is not recommended before 15 weeks gestation.
Several companies are working on rapid tests for Zika infection. The C.D.C. also usually distributes test kits and training materials to state health departments during outbreaks, which should increase testing capacity.
10. I’m of childbearing age, but not pregnant and not planning to get pregnant. Should I go to an affected country?
Only if you use birth control consistently.
Half of pregnancies are unintended. If you want to visit a country where Zika transmission has been reported, Dr. Laura E. Riley, a specialist who works with high-risk pregnancies and infectious disease at Massachusetts General Hospital, advises strict use of birth control to ensure you don’t get pregnant.
Women who become unexpectedly pregnant while traveling or shortly afterward will face blood tests, monthly ultrasounds and a great deal of anxiety.
“Why would you ever sign yourself up for that?” said Dr. Riley. “There’s enough in life to worry about. I wouldn’t add that to my list.”
11. I’m pregnant now, but wasn’t when I visited one of the affected countries. What’s the risk?
Very low, experts say.
With rare exceptions, the virus does not appear to linger in the body, and people who recover from the infection are immune.
“Our understanding, thus far, is that the risk is very, very low if you were in that place prior to conception,” said Dr. Laura E. Riley of Massachusetts General Hospital.
“I wouldn’t be worried about if you conceived after you got back to the U.S.”
12. I’ve just returned from a place where Zika virus is spreading but experienced no symptoms. How long until I can be sure I can’t pass on an asymptomatic infection?
At least a month.
There are two types of transmission to consider. Once home, you might be bitten by a mosquito that could carry the virus to someone else. Or, in rare cases, you might pass the virus to a partner through sex.
For mosquitoes, you should worry for perhaps 20 days, max.
Health agencies hold to a variety of estimates for the Zika virus’s incubation period in the body, but all are between two and 12 days after the mosquito bite. Most people start feeling sick – if they get symptoms at all – between the third and seventh day.
People who get sick usually recover within seven days, meaning that the immune system is clearing or has cleared the virus from the blood. So if you were infected on your last day in the tropics and experienced no symptoms, for about 20 days, in theory, you might pass the virus to a mosquito back home.
Sexual transmission has too many unknowns attached to it to say anything definitive. But if the Zika virus has to invade a man’s testes in order to be passed through semen, it may linger there for weeks or even months. The testes are somewhat shielded from the immune system, so it can be harder for the body to get rid of an infection there.
British health authorities have urged men returning from areas in which the virus is circulating to use condoms for 28 days to be reasonably sure that they cannot infect a partner.
The C.D.C. has said that pregnant women whose male partners have spent time in these areas should abstain from sex or use condoms during intercourse for the duration of their pregnancy. Guidelines may change once more is known.
13. Does it matter when in her pregnancy a woman is infected with Zika virus?
Earlier in pregnancy seems to be more dangerous.
The most dangerous time is thought to be during the first trimester – when some women do not realize they are pregnant. Experts do not know how the virus enters the placenta and damages the growing brain of the fetus.
Closely related viruses, including yellow fever, dengue and West Nile, do not normally do so. Viruses from other families, including rubella (German measles) and cytomegalovirus, sometimes do.
14. Should infants be tested?
Other birth defects may be linked to the virus.
Federal health officials say that newborns should be tested for infection with the Zika virus if their mothers have visited or lived in any country experiencing an outbreak and if the mothers’ own tests are positive or inconclusive.
The reason, officials said in interviews, is that infection with the virus could be linked to defects in vision and hearing, among other abnormalities, even if the child does not suffer microcephaly. The other defects may require further assessments and testing.
The new guidance applies only to infants of mothers who reported symptoms of Zika virus infection — a rash, joint pain, red eyes or fever — while living abroad in an affected country or within two weeks of travel to such a destination.
15. Is there a treatment?
The C.D.C. does not recommend a particular antiviral medication for people infected with the Zika virus. The symptoms are mild – when they appear at all – and usually require only rest, nourishment and other supportive care.