Cytomegalovirus (CMV)
CMV is part of the herpesvirus group (herpes simplex, CMV, Epstein-Barr, and varicella-zoster). CMV infection is widespread, since serologic evidence of infection varies from about 30% to over 90% between different geographic areas and population groups. In general, there is lower incidence in Western European nations and many areas of the United States. The two major periods of infection appear to be fetal or early childhood and late adolescence or young adulthood. Certain population subgroups (e.g., male homosexuals, transplant patients, and patients with HIV-1 infection) have increased incidence or risk. Infections are acquired through contact with body secretions or urine, since CMV is present for variable (sometimes long) periods of time in saliva, blood, semen, cervical secretions, breast milk, and urine.
The majority of persons with acute CMV illness remain totally or almost asymptomatic. Those who become symptomatic most often develop a 2-3 week illness resembling Epstein-Barr infectious mononucleosis both in clinical symptoms and signs and in laboratory test results (with the exception that the heterophil antibody test and specific tests for EBV are negative (see discussion of EBV in this chapter). Some patients, mostly immunocompromised, develop more severe disease.
After infection there is an incubation period of 40-60 days (range, 4-12 weeks). During this time circulating CMV antigen can be detected at about day 30-60 and viremia can be demonstrated by sensitive culture methods during a restricted period from approximately days 55-85. Incubation leads to acute illness, manifested by shedding of virus into body secretions, a process that can last for months or years. IgM-type antibody rises early in the acute phase of illness, followed in about one week by IgG antibody.
After the acute infection stage, there is usually a latent period during which viral shedding may continue but at reduced levels. The latent stage may last throughout life or there may be one or more episodes of reactivation.
About 1%-2% (range, 0.7%-4.0%) of pregnant women develop primary CMV infection; of these, fetal infection is thought to occur in 30%-50% of cases, of which about 20% develop symptomatic disease. About 5%-15% of mothers have CMV reactivation during pregnancy, with fetal infection occurring in about 10%. There are some reports that congenital (in utero) infection is more likely to occur in the second and third trimesters but that severe injury to the fetus more likely (but not exclusively) occurs when infection takes place in the first or second trimester. Primary CMV maternal infection is much more dangerous to the fetus than a reactivated infection during pregnancy. Overall, congenital intrauterine CMV infection is reported in about 1% (range, 0.2%-2.2%) of infants, of which only about 5%-10% develop clinical symptoms. In the newborn, CMV disease may appear in two forms:
1. A subacute form with predominantly cerebral symptoms, manifested by the picture of cerebral palsy or mental retardation. This is the classic form acquired in utero.
2. An acute form with various combinations of hepatosplenomegaly, thrombocytopenia, hepatitis with jaundice, and cerebral symptoms such as convulsions. There usually is anemia, and there may be nucleated RBCs and an increase in immature neutrophils (predominantly bands) on peripheral blood smear.
Noncongenital infection in the newborn may take place during or after birth. It has been reported that 3%-28% of pregnant women have cervical infection by CMV, this presumably being the source of infection during birth. The infant can also become infected through breast milk. The great majority of these infants are asymptomatic, but a few develop some aspects of the acute congenital CMV syndrome, which may include pneumonia. Infants, especially when premature or seriously ill, who acquire CMV infection through blood transfusion are more likely to have severe disease. In young nontransfused children less than age 6 months, some may develop pneumonia as the predominating or only manifestation of CMV infection. In young children in general, infection is common, with reported infection rates in the United States varying from 10%-15% by age 1 year and about 35% (range 20%-80%) by age 10 years in some populations and 36%-56% by age 1 year in other populations. After the neonatal period, infection is most commonly acquired from other children through contact with saliva or urine. Infection is especially common in day-care centers and similar institutions. The great majority of affected children are clinically asymptomatic; but in those with symptoms, probably the most common manifestation is a viruslike febrile illness (often mild), frequently accompanied by mildly abnormal liver function tests. In older children, incidence of infection is much less. In adults, many primary infections are thought to be related to sexual intercourse and many others are due to exposure to infected children. In older children and adults the majority are asymptomatic but those patients with symptoms usually have a 2-3 week illness resembling Epstein-Barr IM, discussed previously in this chapter, except for negative serologic tests for IM. Data from several studies indicate that about 65% (range, 33%-79%) of heterophil-negative IM-like illnesses are due to CMV. CMV infection is unusually frequent in kidney or other organ transplant patients (38%-96% of cases) with symptomatic cases ranging from 8%-39% (least common in renal transplants). Most serious CMV transplant infections occur in previously noninfected recipients who receive infected organs. It is also more frequent in immunosuppressed persons, patients on steroid therapy, and patients with leukemia or lymphoma. In these patients there is predominantly lung or liver involvement that usually is overshadowed by the preexisting nonviral disease. Cytomegalovirus is the predominant cause for the mononucleosis-like postperfusion (posttransfusion) syndrome that may occur 3-7 weeks after multiple-unit blood transfusion or bypass surgery. Studies have estimated that about 7% of single unit transfusions produce CMV infection and about 20% (range 3%-38%) of multiple-unit transfusions. More than 90% of homosexual males are said to have CMV antibody, and severe symptomatic infection occurs with increased frequency in advanced HIV-1 conditions including AIDS.
Laboratory abnormalities in CMV infection. In symptomatic adult infection, splenomegaly is reported in about 35% of cases (range, 22%-40%) and lymphadenopathy in about 15% (range, 5%-28%). Hematologic and biochemical results are summarized in Table 17-4. In general, abnormal enzyme levels display only about one half the degree of elevation seen in patients with IM (which themselves are only mild to moderate), but there is a considerable degree of overlap. Peak elevations are reported to occur about 4-5 days after onset of illness for bilirubin, AST, and ALP and between 7-21 days for GGT. Enzyme elevations usually return to normal by 90 days after onset of clinical illness. GGT abnormality is often the last to disappear and occasionally may persist to some degree for several months.
Table 17-4 Laboratory test results in cytomegalovirus infection
Laboratory diagnosis of cytomegalovirus infection. The most definitive method for diagnosis of CMV infection is virus culture, but serologic tests are the most widely used procedures. In the newborn with congenital CMV brain disease, periventricular cerebral calcification is demonstrable by x-ray film in about 25%; this is highly suggestive, although the same pattern may be found in congenital toxoplasmosis.
Cytomegalovirus inclusion body cytology. In newborns or young children, characteristic CMV inclusion bodies may be demonstrated within renal epithelial cells on stained smears of the urinary sediment in about 60% of cases; this may be an intermittent finding and may require specimens on several days. A fresh specimen is preferable to a 24-hour collection, since the cells tend to disintegrate on standing. Virus culture is unquestionably better than search for urine cells with cytomegalic inclusion bodies. In older children and adults the kidney is not often severely affected, so urine specimens for CMV inclusion bodies usually are not helpful. However, in tissue biopsies, presence of intranuclear cytomegalic inclusion bodies correlates better with CMV actual disease than detection of virus by other means.
Virus culture. Classic culture methods have been replaced by the newer, faster, and more sensitive shell vial technique. Urine, sputum, or mouth swab culture for the virus is the method of choice. Fresh specimens are essential. For urine, an early morning specimen is preferable. For best results, any specimen must reach the virus culture laboratory within 1-2 days. The specimen should not be frozen, because freezing progressively inactivates the virus. This is in contrast to most other viruses, for which quick freezing is the procedure of choice for preserving specimens. The specimen should be refrigerated without actual freezing. In this way it may be preserved up to 1 week. It should be sent to the virus laboratory packed in ordinary ice (not dry ice) and, if possible, in an insulated container. Isolation of the CMV now takes 3-7 days (in contrast to conventional culture, which took several weeks). Both urine and throat swab specimen results may be positive for CMV several weeks or months after the end of acute illness. CMV culture cannot differentiate between active infection, reinfection, or reactivation of latent infection, with three exceptions: a positive culture of peripheral blood lymphocytes demonstrates the short-lived (2-3 weeks) viremic phase of primary acute infection; a positive fetal amniotic fluid culture or positive urine culture from newborns or neonates means congenital CMV infection; and a positive urine culture in previously seronegative transplant patients strongly suggests newly acquired infection.
Detection of CMV antibody. Conversion of a negative to a significantly reactive test or a fourfold rising titer in specimens taken 1-2 weeks apart is one way to demonstrate primary infection. Current methods are immunofluorescence, ELISA, indirect hemagglutination, and LA. Most of these tests detect IgG antibody. Since CMV antibody is common in the general population and since IgG antibody levels persist for years, if only a single specimen is obtained, a negative result cannot guarantee that virus is not present in the latent stage at low titer; while a positive result can only show exposure to CMV with possible partial immunity. Acute and convalescent IgG specimens are necessary to demonstrate acute-stage infection. One difficulty with IgG tests applied to neonatal specimens is maternal IgG antibody to CMV, which may appear in fetal or newborn serum.
Procedures are available that detect IgM antibody alone. IgM antibody persists in the blood for only a relatively short time (1-6 months, occasionally as long as 1 year). In adults, CMV-IgM by EIA has been reported in 90%-100% of patients in symptomatic phase of primary infection and in about 40% of reactivated infections. In maternal CMV primary infections, maternal IgM does not cross the placenta. Theoretically, the presence of IgM antibody should mean primary acute or recent infection. However, besides acute infection, reinfection by another CMV strain and reactivation can also induce a CMV-IgM response. Other sources of IgM such as rheumatoid factor can produce false abnormality in CMV-IgM tests unless there is some way to remove or counteract these interfering substances. Rheumatoid factor has been reported in 27%-60% of patients with CMV infection, both neonates and adults. In addition, acute EBV infection (which resembles CMV infection clinically) also produces IgM antibody that may react in the CMV-IgM tests. Finally, it is reported that 10%-50% of infants and 10%-30% of adults with acute CMV infection have no detectable CMV-IgM antibody. Immunocompromised patients and some patients with AIDS also fail to produce detectable amounts of IgM antibody.
Detection of CMV antigen. Three antigens, called early, intermediate-early, and late have been cloned from the core portion of the CMV and can be detected by monoclonal antibodies using immunofluorescence or ELISA methods, or tissue cell stains on smears or biopsies. The most useful have been immunofluorescent or tissue immunologic stains on bronchioalveolar lavage or biopsy specimens, and on peripheral blood leukocyte preparations to detect early antigens for demonstration of acute infection antigenemia. This is reported to be more sensitive than culture with faster results and earlier detection of acute-phase CMV infection. Nucleic acid (DNA) probe methods (now commercially available) also have been used to detect CMV virus in bronchoalveolar lavage, urine, and peripheral blood leukocytes. When amplified by PCR, the probes have shown greater sensitivity than culture. DNA probes can also be used on biopsy specimens.