The Link Between Zika and Microcephaly


A birth defect known as microcephaly gained attention in 2016 because of the Zika virus, which spread over Brazil and other parts of Latin America, causing the World Health Organization to declare a public state of emergency. The virus was ultimately found to cause the birth defect.

The Centers for Disease Control and Prevention (CDC) continues to advise pregnant women postpone travel to areas with Zika because of concern that infection of the mother at any stage of pregnancy may increase the risk of microcephaly.

The CDC also recommends those planning a pregnancy to discuss with their doctor if considering travel to affected areas.

What Is Microcephaly?

Microcephaly is where a baby's head is smaller than expected. The word refers to small (micro in Latin) and head (cephaly in Greek). It's a developmental abnormality where a head is too small relative to the body and compared to others of the same age.

This can be noticed at birth. The small head often means that the brain is not just small, but it has developed abnormally. This can mean neurologic delays for affected children. The specific delays will depend on the severity and specifics of the abnormal development.

There is a wide range of sizes of newborn baby heads. Some are too big and some are too small, which may point to different problems that need to be addressed. Babies' heads are usually measured soon after birth and at doctor's visits, which is usually when the condition is diagnosed.

It can also be diagnosed on an ultrasound during pregnancy, which may also show other abnormalities. (This may be defined as two standard deviations below the mean (or average), or it may be three standard deviations below).

What Does Microcephaly Mean for Babies?

It is a lifelong condition. There is no treatment to reverse it if diagnosed.

The underlying cause may need to be treated.

It can be associated with developmental delays (slow to speak, sit, stand or walk), intellectual difficulties (problems learning or functioning in daily life), seizures, problems seeing or hearing, problems eating and swallowing, and difficulty walking or balancing. Developmental services with early intervention and other support and medical services may help children. Different cases have different developmental courses.

Is Microcephaly Just a Small Head?

Microcephaly is often associated with other findings. There can be calcifications in the brain (clumps of calcium that don't belong in the brain); the brain mass can be very small; the ventricles can be large (which is where the cerebrospinal fluid that flows around the brain passes through).

What Causes Microcephaly?

Microcephaly is uncommon. It is often not diagnosed everywhere. It may not be looked for or it may not be recorded everywhere.The diagnosis depends on what criteria is used. There is variation in the definition of microcephaly with different head sizes used.

Microcephaly can be primary, where the brain does not develop from the start, which is generally genetic. In some cases, microcephaly can be due to having one more chromosome than most people.

Microcephaly can also be secondary, where brain development begins normally but is halted or tissue is damaged.

Microcephaly can be connected to exposure in utero to alcohol and even malnutrition or lead or mercury exposure. It can be due to a traumatic injury. It may also be connected to diseases in the mother, like Phenylketonuria (PKU). Sometimes it seems like an exposure is the cause but it likely is not.

What Made Us Think That Zika Causes Microcephaly?

Zika has been known for years, but its role in the development of babies was never recognized. It may have been in areas where it was so common that most were infected as children, well before becoming pregnant.

It may have been rare enough, spreading mostly between monkeys and mosquitoes, that few pregnant women were infected. Or it may have been that so few babies were born in facilities that tracked microcephaly—and Zika did not occur as a sudden epidemic—that the relationship between a mild (or non-existent) symptoms in pregnancy was not correlated with occasional birth defects.

This all changed when Zika arrived and gathered momentum in the Western Hemisphere. No one was immune. The population of mosquitoes and people were vulnerable. The virus spread epidemically.

a. In Brazil, two miscarriages and two babies born with microcephaly were shown to have the Zika virus present, mothers reported having had Zika symptoms, and the two babies born were shown to have Zika present in their underdeveloped brains.

b. A baby born in Hawaii was born with microcephaly and was found to have Zika after the mother had symptoms during pregnancy when she lived in Brazil early in pregnancy.

c. Brazil recognized many more cases than it usually saw of microcephaly. The Zika virus first arrived in May 2015, from October 2015 to the beginning January 2016, over 3500 cases of microcephaly were detected. Some cases were particularly severe; some died. Even if this interest in microcephaly led to more recognition of cases, which otherwise would have been under-reported, this is a notably higher rate. This has reached over 10 times higher than normal.

This also doesn't seem to be simply a case of Brazil suddenly diagnosing microcephaly. The U.S. usually diagnoses microcephaly in two to 12 babies per 10,000 live births. This would about 200 to 1200 cases in three months in the U.S., which sees more births. Brazil has fewer babies born (Brazil has roughly three-quarters the number of births. Brazil has less than two-thirds the population of the U.S. (200 million vs. 318 million) and only a slightly higher birth rate (15/1000 compared with 13/1000)).

d. The cases of microcephaly were most common in areas hardest hit by Zika, near Bahia in the Northeast of Brazil.

f. Ultrasound in Brazil showed microcephaly in two fetuses along with specific signs (like calcifications) similar to other viral brain infections. Both pregnant mothers reported having had Zika-like symptoms (but were now negative); amniocentesis documented Zika in the amniotic fluid, ie. in utero infections.

Given these concerns, the Brazilian Ministry of Health, the US CDC, and many others are worried about the effect this infection, otherwise mild, may have on infants.

Why Are We Just Now Learning This About Zika?

Zika is new to the Western Hemisphere, but it is not new to the world. It has been found in Africa and more recently Asia and the Pacific Islands. It has been recognized for over 50 years. The virus though is often not diagnosed in clinics. It's likely if there were an association, it might not have been found.

There also would not have been as many pregnant women getting infected and so an 'outbreak of microcephaly' would not be present, which would have facilitated the correlation. In clinics with fewer resources, records may not be kept on details like microcephaly, even if recognized. Babies—and even fetuses—with severe microcephaly may also not survive as long, especially if resources are more limited.

Also, in an area where the virus was common, girls would get bit by a mosquito and infected well before they were old enough to become pregnant, and therefore avoid this problem. This is seen with Chickenpox, which usually affected children and not pregnant women, in countries like the US before vaccination was available.

It is also possible that the sudden emergence may affect, even amplify, viral load dynamics.

Are There Other Viruses That Cause Microcephaly?

Yes, there is potential for other viruses to cause this condition.

Varicella: The virus that causes chickenpox can also be pretty nasty in pregnancy. Before vaccination existed, very few children made it to adulthood without being infected. As a result, very few women were at risk of getting chickenpox while they were pregnant. Most had been infected and had become immune before they became pregnant. In those rare cases where a mother developed varicella during pregnancy, there were multiple risks to the baby. One of these was microcephaly.

Chickenpox can also be a serious illness to the mother as well.

Rubella: The virus that is now eliminated in the Western Hemisphere used to cause birth defects if mothers were infected during pregnancy. One of these birth defects was microcephaly. These birth defects—Congenital Rubella Syndrome—was much of the reason for focusing on elimination. Rubella vaccination has been pretty important, preventing a lot of babies and their families from facing these problems.

Cytomegalovirus (CMV): The virus, which is quite common, leads to a mild mono-like illness in the mother, but can have substantial effects on the infant. An infection carries a 30 to 40 percent risk of transmission to the fetus; about one in four to one in five infected fetuses will show signs of the infection after birth. This can lead to a number of impairments—such as hearing loss, as well as visual and development abnormalities. It can also lead to microcephaly.

Toxoplasmosis: This parasitic infection can lead to microcephaly as well. It can also lead to other signs—rash, yellowed eyes or skin (jaundice), and calcifications in the brain, as well as eye problems and water build-up in the brain (hydrocephalus).

HIV: Pregnancies in HIV+ mothers can be quite healthy with good medical care. In those who do not have access to or cannot access HIV medications, there can ​be risks to the baby. Of these risks, microcephaly has been identified occasionally.