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Fruit of the Womb
Fruit of the Womb

Cytomegalovirus (CMV): Common and Confusing

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Let's take a break from the PTB saga which I promise to continue soon. The other day I got a call from a physician in Louisiana who had tracked me down to where I currently live in South Carolina. The call was regarding a woman who had lost a baby 4 months ago as the result of a congenital cytomegalovirus (CMV) infection. Seems she had conceived again within 3 months of losing that first baby and was very worried about the effects CMV might have on the new pregnancy in such close proximity to the last. If you ask the average person on the street, they have never heard of CMV, and if you ask the average health care provider, they would not be able to come up with a satisfactory answer to this couple's inquiry. And, as it turns out, the answer is not entirely straightforward, so it is worth a few posts on CMV to help you gain an appreciation for this virus and its natural history before we attempt to respond to the question ourselves...

CMV is the MOST common congenitally (fetal) and perinatally (newborn) acquired virus disease in humans and the single most important infectious cause of mental retardation and congenital deafness in the U.S. and other industrialized nations. CMV is a member of the herpes family and human CMV is restricted to humans with no known animal reservoir. At least 80-90% of individuals are infected with CMV during their lives, but infection may occur in the absence of overt disease or the recognition that an illness is the result of CMV. As with other herpes viruses, once an individual is infected with CMV, periodic 'recurrences' can result from reactivation of virus replication at various sites in which the virus is latently harbored in the body. Although probably not that common, secondary infections can also occur with other strains of CMV. No cross-reactive immunity is afforded by previous infections with other herpes viruses such as herpes simplex, varicella-zoster (chickenpox), or Epstein-Barr (mononucleosis) viruses.

Transmission of the virus can occur from exposure to just about any body fluid, most commonly via saliva, respiratory, and venereal routes or by contact with infected urine or breast milk. Exposure tends to occur at earlier ages in lower socioeconomic groups, promiscuous individuals, and children at day care centers. Serologic evidence (seropositivity) of infection in women during the common childbearing years (18-35) is about 50% for those in middle and upper, and 90% in lower, socioeconomic groups. Among seronegative women, the chance of infection (seroconversion) is about 1-3% per year, however, this is as high 10-20% in women who work in day care settings and 50% in women with infected children under two years of age!

Congenital infections with CMV occur in 1-2% of ALL pregnancies meaning the virus can cross the placenta with relative ease compared to other herpes and most other common virus infections. But, as we shall see later on, congenital infection rarely results in the poor outcome noted above unless the congenital infection is the result of a primary (first time) infection in the mother during the pregnancy. Neonatal infections are commonly acquired by exposure to an infected genitourinary tract during delivery, breast milk, and saliva of family members. Infection of the newborn rarely has the serious consequences that are seen with congenital infections resulting from primary maternal disease during the pregnancy. Serious but, generally not life-threatening, neonatal infections can result under the rare situation in which primary maternal infection occurs late in the pregnancy and delivery occurs before the mother has developed immunity to the virus that could be passed along to the fetus in utero.

As a baseline, a relatively constant percentage (10-15%) of seropositive women is found to be excreting CMV from the urinary tract at any given time. The incidence of excretion tends to increase throughout pregnancy with cervical rates in the third trimester as high as 35-40%! Overall, among seronegative women, there is about a 0.5-1% risk of a primary CMV infection in each pregnancy, although this is much higher in 'at risk' groups such as day care workers, health care providers, and women with young children. Pregnancy does not appear to increase the risk of contracting CMV, nor does it place the woman at greater risk for more severe primary infections. There is some evidence though that women who do develop more severe infections (CMV mononucleosis) and those women with asymptomatic infections who are at greatest risk for transmitting the virus to their babies have qualitatively different antibody responses to CMV than women who do not.

In the next post we will look at the spectrum of maternal and fetal disease caused by CMV...
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