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  • Writer's pictureP.K. Peterson

Cytomegalovirus: a Hidden Risk for Pregnant Women

“[Why do we tell them] about seafood (potential mercury and infection risks), soft cheese (potential listeria risks), undercooked meat and pregnant cats or kittens (toxoplasmosis risks), sexually transmitted infections (HIV, syphilis) and not about CMV?”

- Bill Rawlinson, Director of Serology and Virology, Prince of Wales Hospital, Randwick, New South Wales, Australia

“What you don’t know won’t hurt you. A dubious maxim: sometimes what you don’t know can hurt you very much.”

- Margaret Atwood, Canadian poet, novelist, environmental activist

By age 40, more than half of American adults have been infected by cytomegalovirus (CMV), and nearly one in three children is infected with this virus by age five. CMV is the number one infectious cause of birth defects and is the single biggest infectious disease risk in transplant recipients. Yet many people are unaware of the infection or of the threats CMV poses. My main goal in this week’s Germ Gems post is to heighten awareness of the risk of this viral infection to one group of especially vulnerable potential victims—pregnant women—and to discuss strategies for CMV prevention.

About CMV. CMV is a double-stranded DNA virus that belongs to the Herpesviradae family of viruses that includes herpes simplex virus, varicella-zoster virus, and Epstein-Barr virus (EBV). Like its herpes virus relatives, CMV shares the ability to become dormant (latent) after the initial infection but then reactivate and cause secondary infection.

CMV is transmitted by direct contact with infectious body fluids such as urine, saliva, blood, tears, semen, and breast milk. It can also be transmitted sexually or through infected transplanted organs and by blood transfusions.

The outcome of a CMV infection is determined largely by the state of one’s immune system. In those with a healthy immune system, the infection is asymptomatic. (In some cases, however, healthy people develop a mild illness with fever, sore throat, fatigue, and swollen glands—an illness that can be mistaken for infectious mononucleosis caused by EBV.) By and large, two groups with compromised immunity are at the biggest risk of developing serious CMV disease: the unborn fetus, with an immune system that isn’t yet fully developed; and organ or bone marrow transplant recipients who are given anti-rejection drugs that impair the immune system.

Congenital CMV. While latent CMV can reactivate and cause secondary infection, it is primary maternal infection that carries the highest risk of transmission of CMV to the fetus. The mother acquires the infection (maternal infection) through body fluids, including urine, saliva, and sexual contact and then transmits the virus to the fetus in what is called “vertical transmission.” The rate of “vertical transmission” increases as the stage of pregnancy advances. In the first trimester, 30% to 40% of fetuses may become infected, whereas in the last trimester up to 70% to 80% of fetuses are at risk of acquiring infection from their mother.

Among women with primary CMV infection whose fetuses become infected, only 10% to 15% of newborns are symptomatic at birth. In newborns with “symptomatic” or recognizable clinical features of congenital CMV, the babies can manifest their infection in a number of ways, such as having a small head, seizures, rash, or liver, spleen, or lung problems.

The other clinical features of congenital CMV are thrombocytopenia (low platelet count), hepatitis, chorioretinitis (eye disease), and neurological disease, such as sensorineural hearing loss (damage to the auditory nerve in the inner ear) and developmental delay. And of those babies who are asymptomatic at birth, approximately 50% develop long term sequelae (complications) over time.

What can be done? Doctors do not routinely test all pregnant women for CMV infection. This is because there is no laboratory test that can predict which developing babies will become infected with CMV or have long-term health problems. But for symptomatic newborns, an antiviral drug called valganciclovir may improve hearing and neurodevelopmental outcomes. In addition, as was reported in a systematic review in the October 15, 2022 issue of Clinical Infectious Diseases, mounting evidence suggests that another antiviral agent, valacyclovir, given antenatally (before birth) to pregnant mothers prevents the vertical transmission of CMV to the fetus.

Because some of the sequelae of fetal CMV infection, like sensorineural hearing loss and neurodevelopmental delays, aren’t always detectable at birth, better diagnostic tests are needed. The recent addition of a newborn screening tool for congenital CMV is a major diagnostic advance. Pioneered by University of Minnesota pediatrician Dr. Mark Schleiss and implemented in February, 2022 by the Minnesota Department of Health, this test involves looking for CMV in a blood spot that is already taken from the heel of newborns at birth to screen for metabolic diseases. If the blood spot tests positive for CMV, early treatment is then implemented.

The question all pregnant women want answered. The question that all pregnant mothers really want answered is, ”How can I help protect my baby from CMV?” The U.S. Center for Disease Control and Prevention (CDC) publishes the “CMV Fact Sheet for Pregnant Women and Parents.” Because the saliva and urine of children with CMV contain high amounts of the virus, the CDC suggests pregnant women can lessen their risk of getting CMV by reducing contact with saliva and urine from babies and young children. Avoiding getting a child’s saliva in one’s mouth can be facilitated by not sharing food, utensils, or cups with a child. Also, washing your hands thoroughly after changing diapers is a must.

All pregnant women would benefit from this bit of “Hygiene Counseling”:

  • Wash your hands regularly with soap and water;

  • Avoid kissing young children, including tear and saliva contact;

  • Avoid sharing glasses and kitchen utensils;

  • Dispose of diapers and paper handkerchiefs carefully;

  • Use a condom to prevent spread of CMV via vaginal fluid and semen.

In addition, pregnant mothers with older children in group care need to pay special attention to avoiding contact with potentially contaminated saliva or urine. According to pediatric infectious diseases specialist Dr. Jason Brophy, “Almost all the babies that I see who have congenital CMV, there is an older toddler at home who is in daycare.”

CMV vaccine. The impact of CMV on fetal development and on transplant recipients became known in the 1970s. Since that time, scientists have sought to develop a vaccine to protect against this highly destructive virus. This has proven difficult due to the properties of the virus.

The U.S. National Vaccine Program Office has emphasized the development of a CMV vaccine as a priority. A number of candidate vaccines have been developed, including most recently an mRNA-based vaccine produced by Pfizer scientists. Most authorities suggest, however, that it will be five to ten years before an effective CMV vaccine is available. Once this happens, it should be cause for major celebration by all expectant mothers and by patients slated for organ or bone marrow transplantation.

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Main Page images courtesy of Shuxian Hu, MD. Dr. Hu is a scientist in the Neuroimmunology Research Laboratory at the University of Minnesota.

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