Cytomegalovirus CMV

Cytomegalovirus (CMV) can cause serious infections in newborns. In adults, especially pregnant women, CMV infections usually have no symptoms. However, infection during pregnancy can be severe for the fetus—impeding its growth and development, and potentially leading to long-term disability.

CMV is a member of the herpesvirus family. Once infected, the virus remains dormant in the body’s cells, and can reactivate later. The incubation period for CMV infection is around one month. Transmission occurs solely between humans—via saliva, tears, bodily fluids, sexual contact, blood, urine, organ or bone marrow transplants, and breast milk. Reactivation can lead to renewed viral shedding. Children under three years old are especially prominent CMV carriers.

In most cases, CMV infection is asymptomatic. Only about 5% of infected individuals show symptoms similar to mononucleosis—mild fever, sore throat, headache, swollen lymph nodes, and inflamed tonsils. Lab tests may reveal elevated liver enzymes, increased white blood cells, and reductions in white blood cells or platelets. The illness usually resolves on its own within a few weeks.

However, CMV can be life-threatening for fetuses, premature infants, or people with weakened immune systems.

Prevalence

CMV is the most common cause of fetal infection globally, occurring in approximately 0.2–1% of pregnancies. Since the eradication of chickenpox, CMV has become the leading non-genetic cause of hearing loss and developmental delays in children.

Prevalence of CMV varies significantly. In developed countries, 30–50% of women of childbearing age have not had CMV. Infection during pregnancy is most often a new one—about 1–4% contract it for the first time. If infection occurs during the first trimester, the virus is passed to the fetus in about 30–35% of cases. Of those fetuses infected, 10–15% develop congenital CMV infection.

When a pregnant woman’s earlier CMV infection reactivates—or she acquires a new strain—the risk of fetal transmission is much lower (around 1–2%). Among those fetuses infected, the rate of symptomatic infection is similar whether the mother experienced a primary infection or reinfection.

Risks of Congenital Infection

Most infants infected in utero are born without symptoms. Yet, a prenatal CMV infection can lead to miscarriage, preterm birth, growth delays, brain underdevelopment, cognitive impairment, neurological development delays, and progressive hearing loss.

Of the children with congenital CMV infection, approximately 18% show classic symptoms at birth—such as enlarged liver and spleen, thrombocytopenic purpura, brain abnormalities, and sensorineural hearing loss. Some infants die from these complications. In many cases, sensorineural hearing loss is progressive and not detectable at birth.

Hearing or vision impairments and developmental delays may emerge later in childhood or adolescence. In the U.S., an estimated 28,000 children are born annually with congenital CMV; in Finland, the estimate is between 60–120 per year.

Diagnosis and Treatment

Diagnosing primary CMV infection during pregnancy is challenging. A congenital infection may be suspected if prenatal ultrasound shows abnormalities—such as echogenic bowel, enlarged ventricular spaces within the brain, microcephaly, or restricted fetal growth. Detection in the mother uses antibody testing.

Amniocentesis after 21 weeks of gestation is the most reliable method to confirm fetal infection. The absence of CMV DNA in amniotic fluid effectively rules out congenital infection. In newborns, congenital CMV is diagnosed based on symptoms and confirmed via urine PCR testing.

Currently, no effective treatment exists for congenital CMV. Antiviral therapy during pregnancy has not shown benefit. For symptomatic infants, treatment with valganciclovir medication is recommended.

Prevention

CMV is not screened for during pregnancy because effective treatments are lacking. Prevention focuses on good hygiene—hand washing and avoiding contact with bodily fluids—especially during pregnancy and contact with young children.

No CMV vaccine is currently available on the market, though development is underway and promising results have emerged. CMV vaccine candidates have already been studied at FVR research clinics in Finland.