“Why didn’t anyone tell me about this virus?” is a frequent response I hear from parents upon learning their newborn is infected with cytomegalovirus, or CMV. Although more than half of the U.S. population will be infected with CMV by the age of 40 and the disease is common worldwide, few people have ever heard of it.

CMV belongs to the same virus family as cold sores and chickenpox and, like those viruses, lives in the body for life. Most children and adults experience very mild or even no symptoms with their initial infection. A healthy immune system is typically able to keep CMV under control so people don’t become sick or even know the virus is living in their body.

So if most people are unlikely to get sick from CMV at any age, then why is the virus so important to understand? As an infectious disease and immunology specialist, I have focused on this question for most of my two-decade career. One major reason is that CMV – unlike the other viruses in its family – can pass from mother to fetus during pregnancy.

Congenital CMV, or cCMV, is the most common infection before birth and the leading infectious cause of birth defects. About one in every 200 infants – typically 20,000 to 30,000 infants in the U.S. – are born with cCMV per year, and nearly 20% of them have permanent neurodevelopmental disabilities such as hearing loss or cerebral palsy. Every year, more children are affected by cCMV than several familiar childhood conditions like Down syndrome and fetal alcohol syndrome. Compared to later stages of pregnancy, CMV infection in the first trimester carries the highest risk of stillbirth or severe effects when the immune system and organs like the brain are developing.

Rates of cCMV differ significantly by race, ethnicity and other demographic factors, with Black and multiracial infants twice as likely to have cCMV compared to other groups. Black and Native American infants also have a higher risk of death from cCMV compared to white infants.

Screening for rubella, HIV and syphilis is routine for early prenatal care in the U.S. Counseling to avoid kitty litter to prevent toxoplasmosis is also common. If CMV can infect a fetus and cause birth defects, then why aren’t pregnant people tested and treated for this virus too?

Prenatal CMV screening is not standard of care due to several limitations of the current testing approach. Some available tests can be difficult for health care providers to interpret. Testing provides information about whether the parent has CMV, but it does not sufficiently predict the risk of fetal transmission or severe symptoms.

Prenatal screening for a healthy person with a normal pregnancy does not usually offer useful information. That’s because anyone can have a baby with cCMV regardless of whether they tested positive or negative for it before or earlier in pregnancy. CMV testing may be useful for pregnant people who are experiencing acute illness, such as prolonged fever and fatigue, or who have an abnormal fetal ultrasound.

Even if more accurate tests were available, there are currently no medical interventions approved by the Food and Drug Administration to reduce the risk of fetal CMV infection. Biweekly antibodies against CMV seem to reduce fetal transmission when given around conception or during the first trimester, but CMV is rarely diagnosed that early in pregnancy.

Researchers are currently evaluating the drug valacyclovir as a potential treatment to prevent fetal transmission. Valacyclovir is commonly used to prevent or treat genital herpes during pregnancy. Findings from a recent clinical trial in Israel suggest that valacyclovir may reduce the risk of CMV transmission to the fetus.

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