Liz Szabo

USA TODAY

The mosquito-borne Zika virus is causing international concern as a result of its link to microcephaly, a rare birth defect in which babies are born with unusually small heads and incomplete brain development.

Here's a closer look at the condition:

Q: What happens to babies with microcephaly?

A: When a baby's head is small, there is less room for the brain, which can lead to many problems, said Paul Lee, a pediatric infectious disease specialist at Winthrop University Hospital in Mineola, N.Y

Babies born with the condition may develop convulsions and suffer physical and learning disabilities as they grow older, according to the World Health Organization (WHO).

Children with microcephaly are at risk for a range of issues: facial distortions, short stature, difficulties with balance and coordination, speech problems and seizures, cerebral palsy, learning disabilities, hearing loss and vision problems, according to the WHO.

There is no treatment for the condition. In Brazil, where the link between microcephaly and Zika first came to light, at least 38 babies with microcephaly have died. "If there is enough brain damage, it just might not be compatible with life," Lee said.

Some children with microcephaly do develop normal intelligence, although their heads remain small, according to the Mayo Clinic.

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Q. Why do doctors think microcephaly could be caused by the Zika virus?

A. Brazil, which reported its first Zika virus diagnosis in May, announced it had received an unusually large number of reports of microcephaly last fall. Brazil usually has 150 cases of microcephaly a year; now, it's investigating thousands of cases. Even if microcephaly was undercounted in the past, there clearly seems to have been an increase.

Zika hasn't previously been linked to this birth defect. But health officials in French Polynesia have gone back to look at rates of microcephaly after their country's Zika outbreak in 2013-2014. That analysis found that microcephaly cases increased there, as well.

Tests have found the Zika virus in the brains of two full-term infants with microcephaly who died just after birth; genetic analyses show the virus was the same as the Zika virus now circulating in Brazil, according to the Centers for Disease Control and Prevention.

Q. What else causes microcephaly?

A. In many cases, the cause of a child's microcephaly is unknown, according to the WHO. But women who suffer from certain infections while pregnant are at higher risk. These infections include toxoplasmosis, caused by a parasite found in undercooked meat; rubella, also known as German measles; genital herpes; syphilis; cytomegalovirus, a type of herpes virus; and HIV.

Women are also at higher risk if they are exposed to toxic chemicals, such as the heavy metals arsenic and mercury, as well as the chemicals in alcohol or tobacco smoke. Radiation exposure also increases the risk, as do genetic abnormalities, such as Down syndrome, and severe malnutrition during fetal life, according to the WHO. Women can protect themselves against rubella by getting vaccinated with the measles-mumps-rubella vaccine, or MMR.

Q. How common is microcephaly?

A. It's normally very rare. According to the WHO, microcephaly affects one baby in several thousand. In the USA, about 25,000 babies a year are born with the condition, according to a 2009 article in Neurology.

Q. How is microcephaly diagnosed?

A. Ultrasounds around the 28th week of gestation can find microcephaly, according to the WHO. A typical pregnancy lasts 40 weeks.

The most reliable way to diagnose the condition is to measure a baby's head circumference within 24 hours of birth, then compare it to growth standards of other babies of the same gestational age and sex, according to the WHO. Doctors also factor in the baby's weight and length when assessing whether an infant has microcephaly. Doctors continue to measure a baby's head throughout infancy, because microcephaly can develop after birth.

The head of a baby with microcephaly will be smaller than that of 97% to 98% other babies, Lee said.

Q. Why are we seeing these birth defects now?

A. The Zika virus has been known for decades, but it was only this fall that doctors began to suspect a link between the virus and microcephaly. In general, Zika virus causes mild illness, with symptoms that include fever, rash, pink eye, headache and joint pain. Four of five people infected with Zika virus have no symptoms at all, according to the WHO.

Doctors don't know why the virus is only now associated with birth defects. It's possible that a second factor, beyond Zika virus, influences the risk of microcephaly, according to the WHO. For example, it's possible that babies only develop microcephaly if their mothers are simultaneously infected with another mosquito-borne illness, such as dengue fever, or if the mother was infected in the past. Both dengue and Zika are spread by the same species of mosquito, called the Aedes aegypti. Dengue is common in Brazil.

Doctors are suspicious of dengue partly because that disease is known to be worse if people catch it a second time.

Q. Could American women experience the same birth defects if Zika comes here?

A. No one knows for sure. Doctors at the Centers for Disease Control and Prevention say that Zika is not likely to spread as quickly in the continental USA as it has in Brazil and the Americas, because Americans are more likely to have window screens and air conditioning, which reduce Americans' exposure to mosquitoes.