What is the Zika virus?
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 believed to be common across Africa and Asia, it did not begin spreading widely in the Western Hemisphere until recently — perhaps sometime in 2013, although its presence was not confirmed until May 2015, when it was identified as the “mystery disease” sweeping across northeast Brazil.
How does a mosquito transmit Zika?
Only female mosquitoes bite people: they need blood to lay eggs, while males drink plant nectar. In the female, the virus travels from the gut to the salivary glands and is injected into the next human victim. When a mosquito bites, it first injects an anti-coagulant saliva so blood does not clog its strawlike proboscis.
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 in daytime. The yellow fever mosquito, Aedes aegypti, takes several bites for each blood meal and prefers biting people; it accounts for most Zika infections. This mosquito is common in the United States typically only in Florida and along the Gulf Coast, although it has been found as far north as Connecticut in hot weather.
The Asian tiger mosquito, Aedes albopictus, also can transmit the virus, but not as effectively. It bites many kinds of animals and is found as far north as Maine and Minnesota in summer, and also in Hawaii.
Can the Zika virus be sexually transmitted?
Yes. Although experts believe that the vast majority of Zika infections are transmitted by mosquitoes, sexual transmission has been reported in 10 countries, including the United States, France, Germany, Italy and New Zealand.
In all known cases as of late June, transmission has been from a man to a woman or to another man, not from a woman to anyone else. The Zika virus has clearly been transmitted by vaginal and anal sex, and possibly by oral sex.
Viral RNA has been found in semen more than two months after symptoms disappeared. Scientists believe the prostate or testes serve as a reservoir, sheltering the Zika virus from the immune system. In at least one case, a man who never had Zika symptoms transmitted it sexually.
Health authorities now recommend that women who are pregnant or trying to become pregnant avoid contact with semen from men who have visited areas where the virus is transmitted. Pregnant women should abstain until they give birth, or should have sex only with partners using a condom.
To reduce the risk of sexual transmission, health authorities recommend that men who have visited areas in which the Zika virus is transmitted by mosquitos and who have had symptoms avoid unprotected sex for six months. Men who have had no symptoms should wait eight weeks.
How might Zika cause brain damage in infants?
Experts aren’t yet certain. Although it is known that the virus causes brain damage, the mechanism is still being studied. The virus can cross the placenta and attack fetal nerve cells, including some that develop into the brain. The radial glial cells, which form the initial “scaffolding” that guides other fetal brain cells into place, appear particularly vulnerable.
What is microcephaly?
An unusually small head, often accompanied by brain damage.
Babies with microcephaly have unusually small heads. Normally, some degree of microcephaly occurs in 1 in 5,000 to 1 in 10,000 of all births. In roughly 15 percent of cases, a small head is just a small head, with no effect on the infant.
But infection with the Zika virus causes a severe form of microcephaly. The brain may stop growing and be small and smooth, lacking the normal indentations. The long nerves connecting the eyes and ears to the brain may be damaged. Children may suffer from constant seizures or be born with permanently rigid limbs.
Microcephaly can also be caused by other infections of the fetus, including German measles (also known as rubella), toxoplasmosis (a disease caused by a parasite found in undercooked meat and cat feces) and cytomegalovirus.
Microcephaly may also result from alcoholism, drug use or some industrial toxins during pregnancy, or from severe malnutrition of the mother. It can be caused by complications of diabetes and by the genes like those that cause Down syndrome.
There is no treatment for the brain damage.
Is there a treatment?
No. 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.