Cytomegalovirus (CMV): Structure, Congenital Disease, and Laboratory Diagnosis

Most people carry cytomegalovirus for life and never know it. It infects silently in childhood, settles into latency, and does nothing noticeable to a healthy immune system. The virus becomes dangerous in only two situations, and both define this article: when it reaches a fetus whose mother has a first infection in pregnancy, and when it emerges in a patient whose immunity has failed.

In the first, it is the commonest congenital viral infection in the world and the leading non-genetic cause of childhood deafness. In the second, it causes sight-threatening and life-threatening disease in transplant recipients and people with advanced HIV. The same silent virus, two very different threats.

What CMV is

Cytomegalovirus is a member of the herpesvirus family and is structurally identical to the other herpesviruses: a double-stranded DNA genome, an icosahedral capsid, and a lipoprotein envelope. It is also called human herpesvirus 5. It has a single serotype, and humans are its only natural host; animal CMV strains do not infect people. The name means "large cell," because infected cells enlarge and develop the characteristic inclusions described below.

CMV is a beta-herpesvirus. This subgroup matters because it predicts the virus's behavior: beta-herpesviruses infect and establish latency in a wide variety of cell types, and they are relatively slow-growing and less cytopathic than the alpha-herpesviruses such as herpes simplex virus. This broad cellular range is why CMV can be transmitted through so many body fluids and can cause disease in so many organs when immunity fails.

Transmission and epidemiology

CMV infection occurs worldwide and is extremely common: in most populations more than half, and often more than 80%, of adults carry antibody to the virus. The overwhelming majority of these infections are and remain asymptomatic.

The virus spreads through many routes, and the route tends to track with the stage of life:

Around birth, CMV passes from mother to child across the placenta, during passage through the birth canal, and through breast milk. In young children, saliva is the main vehicle, and young children shed virus at high levels for long periods, which makes them the principal source of infection for seronegative pregnant women. After puberty, sexual transmission through semen and cervical secretions becomes important. CMV is also transmitted by blood transfusion and organ transplantation, which is why donor and recipient CMV status matters in transplant medicine.

Pathogenesis

The fetus is at risk mainly when the mother acquires a primary CMV infection during pregnancy, because she has no pre-existing immunity to limit the virus before it crosses the placenta. Prior maternal immunity is protective, but, unlike rubella, only partially: non-primary infection, from reactivation of latent virus or reinfection with a new strain, can still cause congenital infection, though at a much lower rate than primary infection. CMV is therefore the one congenital infection where prior immunity does not fully close the door.

When the fetus is infected, the result can be cytomegalic inclusion disease, named for its cellular hallmark: enlarged cells containing prominent intranuclear inclusions. Two features of fetal risk deserve emphasis, and they run in opposite directions:

Transmission risk rises across gestation. The chance that the virus crosses to the fetus after a primary maternal infection increases with gestational age, from roughly one in five in the first trimester to a majority by the third.

Severity falls across gestation. Despite the rising transmission rate, the damage is most severe when infection occurs early. First-trimester infection is the most likely to cause serious congenital abnormalities. This inverse relationship, high severity but low transmission early, low severity but high transmission late, is a recurring pattern across the congenital infections.

Clinical findings

Congenital infection. Only about 10 to 15% of infants infected in the womb are symptomatic at birth. When they are, the picture of cytomegalic inclusion disease includes microcephaly, seizures, sensorineural deafness, jaundice, petechiae or purpura, and hepatosplenomegaly, and it is an important cause of intellectual disability. Infected infants continue to excrete CMV, especially in the urine, for years, which is diagnostically useful and an infection-control consideration.

The remaining 85 to 90% are asymptomatic at birth, but "asymptomatic" is not the same as "unaffected." A significant fraction of these children go on to develop late-onset sensorineural hearing loss in the first years of life, and hearing loss may be the only manifestation of congenital CMV. This is why congenital CMV is the leading non-genetic cause of sensorineural hearing loss in children, and why it cannot be dismissed even when a newborn looks well.

Immunocompetent adults. In a healthy person, CMV occasionally causes a mononucleosis-like illness with fever, lethargy, and atypical lymphocytes on the blood film. It is distinguished from Epstein-Barr virus mononucleosis by being heterophile-antibody negative (a negative Monospot test). This is the classic exam contrast: EBV mononucleosis is heterophile-positive, CMV mononucleosis is heterophile-negative.

Immunosuppressed patients. This is where CMV is most dangerous outside the fetus. In organ and bone marrow transplant recipients it causes systemic disease, especially pneumonitis and hepatitis. In advanced HIV or AIDS it causes CMV retinitis, which can progress to blindness, and gastrointestinal disease including colitis with intractable diarrhea.

Laboratory diagnosis

Several approaches are used, chosen by the clinical setting.

Cytology and histology. Infected cells are enlarged and contain a large intranuclear inclusion with a surrounding clear halo, giving the classic owl's-eye appearance. These inclusion-bearing giant cells can be seen in urine and in tissue, and can be highlighted with fluorescent antibody or histological stains.

Viral culture. The preferred culture method is the shell vial technique, in which the specimen is centrifuged onto a cell monolayer and detected with immunofluorescent antibody, giving a result in about 72 hours, far faster than conventional culture.

Antigen and molecular detection. The pp65 antigenemia assay detects the pp65 protein, a component of the CMV nucleocapsid, within infected blood leukocytes using a fluorescein-labeled monoclonal antibody, and gives a measure of the amount of virus present. PCR detects CMV nucleic acid in blood, cerebrospinal fluid, amniotic fluid, and tissue, and is now central to diagnosis and to monitoring viral load in immunosuppressed patients. Prenatal diagnosis of congenital infection is made by detecting CMV DNA in amniotic fluid.

Serology. A four-fold or greater rise in antibody titer indicates active infection. In the newborn, interpretation follows the general congenital-infection rule: IgG may be passively transferred maternal antibody, whereas maternal IgM cannot cross the placenta, so any IgM in the infant is its own and points to true fetal infection. The wider question of how to read a panel of antibodies across several congenital pathogens at once is covered separately, as optional further reading, in the article on the TORCH panel test.

How to Remember

Owl's eye means CMV. The enlarged cell with a large intranuclear inclusion ringed by a clear halo is the single most recognizable CMV finding. "Owl's eye, big cell, cytomegalo" ties the morphology to the name: cytomegalo means large cell.

Beta means broad and slow*.* As a beta-herpesvirus, CMV infects many cell types, spreads through many fluids, and grows slowly with little cytopathic drama until immunity fails. That one property explains its many transmission routes and its many target organs.

Primary infection in pregnancy is the fetal trigger, but the door is only half closed. The fetus is at greatest risk from a mother's first infection in pregnancy. Unlike rubella, prior immunity is only partly protective, so CMV is the exception where a previously immune mother can still, rarely, transmit.

Transmission up, severity down. As pregnancy advances, the virus is more likely to reach the fetus but less likely to cause severe damage. First trimester: least likely to transmit, most likely to devastate.

Asymptomatic at birth is not the all-clear. Most congenitally infected babies look normal at birth, yet many later develop hearing loss. CMV is the leading non-genetic cause of childhood sensorineural deafness for exactly this reason.

EBV positive, CMV negative*.* Both cause a mononucleosis syndrome, but the heterophile antibody (Monospot) is positive in EBV and negative in CMV.

Key exam facts in one table

Where Students Get Confused

CMV mononucleosis and EBV mononucleosis are the same. They look similar, with fever, lethargy, and atypical lymphocytes, but the heterophile antibody separates them: EBV is heterophile-positive (positive Monospot), CMV is heterophile-negative. A mononucleosis picture with a negative Monospot should raise CMV.

A baby who is well at birth does not have congenital CMV, or is safe. Most congenitally infected infants, 85 to 90%, are asymptomatic at birth. A substantial number of them still develop sensorineural hearing loss later in childhood. Normal at birth does not mean unaffected, which is the whole reason CMV is the leading non-genetic cause of childhood deafness.

If the mother was already immune, the fetus is protected*.* This holds for rubella, but CMV is the exception. Prior maternal immunity greatly reduces but does not eliminate the risk; non-primary infection through reactivation or reinfection can still, uncommonly, cause congenital disease. Primary infection remains the far greater danger.

Higher transmission later in pregnancy means later infection is worse. The opposite. Transmission to the fetus becomes more likely as pregnancy advances, but the severity of damage is greatest with first-trimester infection. The two trends run in opposite directions.

The owl's-eye inclusion is diagnostic on its own. It is highly characteristic and a strong clue, but definitive diagnosis rests on culture, antigen detection, or PCR. In the newborn, the standard is detection of CMV DNA or virus in urine or saliva within the first two to three weeks of life, before later postnatal acquisition can confuse the picture.

References

• Riedel S, Hobden JA, Miller S, et al. Jawetz, Melnick & Adelberg's Medical Microbiology. 28th ed. New York: McGraw Hill; 2019.
• Levinson W, Chin-Hong P, Joyce EA, Nussbaum J, Schwartz B. Review of Medical Microbiology and Immunology. 17th ed. New York: McGraw Hill; 2022.
• Leruez-Ville M, Foulon I, Pass R, Ville Y. Cytomegalovirus infection during pregnancy: state of the science. Am J Obstet Gynecol. 2020;223(3):330–349. https://doi.org/10.1016/j.ajog.2020.02.018