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From: Infectious Disease: Congenital and Perinatal Infections: A Concise Guide to Diagnosis Edited by: C. Hutto © Humana Press Inc., Totowa, NJ
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Parvovirus B19
Mobeen H. Rathore
INTRODUCTION
Parvoviruses belong to the family Parvoviridae. They are single-stranded, relatively uniform, isometric, nonenveloped deoxyribonucleic acid (DNA) viruses that infect many animals, including humans. The parvoviruses are extremely resistant to inactiva- tion and can survive in a wide pH range (3.0–9.0) and at 60°C for up to 12 hours.
Parvovirus B19 is the only genus of parvovirus proven to cause human infection. First discovered in 1975, parvovirus was not linked to human disease until 1981 (1,2). In 1983, parvovirus B19 was associated with erythema infectiosum; in 1984, this virus was associated with poor outcome of pregnancy and in 1985 with arthropathy (3–5).
Parvoviruses are 20–25 nm in diameter, and the B19 genome has been completely sequenced (6,7). There is some antigenic variability between strains; however, the sig- nificance of this variability is not known (8). Cultivation of the virus requires red blood cell precursors, but none of the cultivation techniques have practical use for clinical diagnosis. Parvovirus B19 causes a number of diseases in humans. Most parvovirus B19 infections, however, are likely to be asymptomatic or not recognized.
PATHOGENESIS
During the first week after infection, parvovirus B19 causes a viremia that may be associated with fever, malaise, and other constitutional symptoms. During this period of viremia, the bone marrow is also infected, and the virus replicates in and kills the erythroid progenitor cells, resulting in their depletion. The reticulocyte count drops precipitously and is followed by anemia. In most otherwise healthy individuals, these changes are inconsequential with complete recovery of the bone marrow. However, in individuals with red blood cells that have a shorter than normal life span and who have underlying chronic anemia, parvovirus infection can result in serious anemia. This could be a potential problem in pregnant women who are already severely anemic.
Parvovirus B19 can also occasionally infect other cell lines, causing leukopenia and thrombocytopenia (9,10). The immune response to parvovirus is primarily through antibody production and immunity is lifelong.
Parvovirus B19 infection in the animal model suggests that the virus can easily over-
come the placental barrier. Parvovirus B19 is pathogenic throughout pregnancy, and
because it destroys the dividing cells rather than just inhibiting mitotic activity, it is more embryocidal and less teratogenic (11).
TRANSMISSION
Parvovirus B19 infections usually occur in winter and spring but can occur through- out the year (12). Although direct proof of the routes of transmission and spread are lacking, it is most likely that parvovirus B19 is transmitted from person to person by direct contact with the respiratory secretions of infected individuals. Indirect proof of spread by respiratory secretions is provided by studies in which parvovirus B19 was isolated from respiratory secretions of infected individuals and by studies of human volunteers infected by intranasal inoculation of the virus (13–15).
Spread of infection by direct contact rather than airborne spread is supported by the epidemiology of parvovirus B19. Infection spreads slowly, and the virus causes infec- tions in contacts over long periods of time rather than by rapid spread over short peri- ods of time, a pattern that is typical of infections spread by aerosol droplets. Vertical transmission is the only proven natural method for transmission of parvovirus B19.
Contaminated blood products and needles can also transmit the infection. Parvovirus B19 in secretions of infected persons can cause environmental contamination, and chemicals commonly used to clean environmental surfaces may not always be effec- tive in eliminating the virus from surfaces and fomites. As a result, nosocomial trans- mission of parvovirus B19 without direct contact with an infected individual can occur (16,17). Because environmental decontamination and eradication of parvovirus B19 from surfaces is difficult, this virus can be transmitted in many settings in which sur- face contamination by respiratory secretions occurs. In day care centers, homes, and schools where young children live and play, contamination of the environment with saliva occurs, and there is a high risk of transmission of infection to others sharing the space, including susceptible pregnant women.
RISK OF MATERNAL INFECTION DURING PREGNANCY
There are no data that provide direct evidence of the incidence of acute parvovirus B19 infection during pregnancy. Seroprevalence studies in the general population sug- gest that 28–86% of women of reproductive age do not have antibodies to parvovirus B19 and may be at risk for acute parvovirus B19 infection (18–20). Seroprevalence increases with age; therefore, younger women are at a higher risk for acute infection compared with older women (Table 1).
There are few data about the incidence of acute parvovirus B19 infection in pregnant women. One study suggested an annual seroconversion rate of 1.4% in the absence of an outbreak (19). This would give an infection rate of 1.1% for susceptible women over the 40-week period of pregnancy. In another study of 1610 pregnant women, 35% had serologic evidence of prior parvovirus B19 infection at the time of study entry. Of the women, 2% had serologic evidence of acute infection at entry, and another 2% of the women seroconverted prior to delivery. This study had a total infection rate of 4% (60 women) during pregnancy (21). Another study noted a seroconversion rate of 13%
during an epidemic of parvovirus B19 infection compared with 1.5% in nonepidemic
periods (22).
FETAL AND NEONATAL RISK WITH MATERNAL INFECTION
Transplacental parvovirus B19 infection is well documented; however, the incidence of intrauterine parvovirus B19 infection, although considered low, is not known. One study estimated a perinatal infection rate of 33% (23). In another study, 56 infants born to 55 women who were infected during pregnancy showed no in utero ultrasonographic evidence of hydrops fetalis. Six (11%) of these infants had serological evidence of congenital parvovirus B19 infection at birth. All newborns were healthy at birth and were without evidence of congenital abnormalities. Forty-eight of these infants were followed up to 12 months of age and remained clinically well, and all of them had serology for parvovirus B19 performed around 12 months of age. Based on confirmed infection at 12 months of age, it was determined that in mothers infected after 20 weeks of gestation, the risk of infection was more than double that for those infected before 20 weeks of gestation (35 vs 16%) (21).
Most neonates born to women infected with parvovirus B19 during pregnancy will appear normal, and less than one-third of the women who are infected during preg- nancy become symptomatic. Therefore, most infections will often not be recognized in either the pregnant woman or the newborn. However, parvovirus B19 infection during pregnancy can have an adverse outcome. In one study, 5 of the 60 women infected during pregnancy had spontaneous abortions, but evidence of parvovirus B19 infection was present in only one abortus. This study had a fetal death rate directly attributable to parvovirus B19 of fewer than 1 in 1000 pregnancies (21).
Parvovirus B19 intrauterine infection is a known cause of fetal demise (24); how- ever, the frequency of B19 as a cause of fetal death is unknown. Although parvovirus B19 may contribute to only a limited number of fetal deaths, the estimates of 1–3%
may be low (25). In a study that enrolled pregnant women with serologically confirmed parvovirus B19 infection, the overall fetal death rate was 33% (26). However, in a much smaller cohort of women infected during an outbreak of parvovirus B19 infec- tion, the fetal death rate was only 3% (27). Other studies have shown that the risk of fetal death is higher if infection occurs before 22 weeks of gestation (28,29).
Nonimmune fetal hydrops is a well-described result of maternal parvovirus B19 infection, although its frequency is not clear. Reported frequencies varying from less than 1 to 27% have been reported in various studies (30,31). More recent data sug- gested a much lower incidence of nonimmune hydrops associated with parvovirus B19 infection in the fetus.
Table 1
Age-Related Prevalence of Parvovirus B19 Infection
aAge Prevalence
<1 year Unknown
b1–5 years 2–15%
5–19 years 15–60%
>20 years 45–75%
aBy anti-B19 IgG.
bBecause of transplacentally acquired antibodies. (Adapted from refs. 38 and 39.)
Most live neonates with parvovirus B19 infection are either asymptomatic or have nonimmune hydrops. Reports of an association of parvovirus B19 infection with birth defects are growing. There is a possible association of parvovirus B19 infection with ophthalmological, cardiac, and neurological birth defects, but there is no epidemio- logical information to confirm this association (4). The frequency of developmental delay in children with congenital parvovirus B19 infection followed up to 10 years of age is not different from that in the general population (28).
COUNSELING OF PREGNANT WOMEN
Pregnant women with parvovirus infection should be counseled about the limita- tions of current data concerning the risk to the fetus and the long-term outcome for the infant. Routine antenatal screening for parvovirus B19 is not recommended because it is not possible to prevent the risk of infection. It should be stressed that the current state of knowledge indicates that the risk for an adverse pregnancy outcome, although not zero, is low. Women should be further counseled that they should report exposure to parvovirus B19-infected persons to their physician, and that the probability of a good outcome of pregnancy is high even if infection occurs during pregnancy.
Although a large number of women of childbearing age are potentially susceptible to parvovirus B19, the risk of intrauterine infection is low. Because of this, exclusion of pregnant women from work cannot be supported and is not recommended. In addi- tion, the majority of infected individuals are asymptomatic, and those having symp- toms are contagious prior to developing symptoms.
A discussion of risk factors is warranted with all pregnant women, especially those who are around children. Preconceptional counseling of these women should include discussion about the fetal risk of parvovirus B19 infection during pregnancy. Some women may choose to get tested for parvovirus B19 prior to getting pregnant, and serology would be the recommended test in these women. The presence of immunoglo- bulin (Ig) G antibody to parvovirus indicates that a woman is not at risk for parvovirus B19 infection. Currently, exclusion of pregnant women from the workplace (and other high-risk groups) during an outbreak of parvovirus B19 is not recommended. Women at greatest risk are those who work in the health care industry (especially with chil- dren), teachers, day care center workers, and mothers with an infected child in the household (12,32).
Clinicians are frequently faced with the situation of a pregnant woman with work- related exposure to parvovirus B19 infection. In this circumstance, testing using serol- ogy is recommended. Interpretation of the mother’s serology and risk for the fetus are described in Table 2. Exposure during a workplace outbreak may warrant diagnostic testing for the mother (Fig. 1).
Prenatal Evaluation of the Mother and Fetus
The prenatal evaluation and diagnosis of parvovirus B19 infection in the mother is
not difficult, but determination of infection in the fetus may be complicated. Universal
screening for parvovirus B19 is not recommended because of the low risk of maternal
infection and low risk of adverse outcome to the fetus (33). Screening is recommended
for pregnant women exposed to individuals with erythema infectiosum, in an aplastic
crisis, and in an outbreak (because of a high attack rate in some situations) (26–
Table 2
Maternal Parvovirus B19 Serology and Associated Fetal Risk
IgG IgM Maternal infection Fetal risk
+ – Past infection No risk
+ + Recent acute infection At risk
+ Active infection At risk
No infection No risk
+, positive; , negative.
Fig. 1. Algorithm for maternal evaluation for parvovirus B19 infection during pregnancy.
28,30,31,33,34). A pregnant woman presenting with rash and arthropathy should be evaluated for parvovirus B19 infection. If maternal infection is suspected, this should be confirmed by laboratory tests. An algorithm for evaluation of mother and fetus is shown in Fig. 1.
The diagnosis of parvovirus B19 infection in pregnant women is not different from any other individual with suspected parvovirus B19 infection. Clinical presentation and disease caused by parvovirus B19 infection in pregnant women may not be very different from otherwise healthy individuals. The infection presents as a self-limited, mild, viral-like illness in 50% of individuals; 25% remain asymptomatic, and the re- maining 25% may present with a rash (classically slapped-cheek appearance in chil- dren) or joint symptoms (35). Asymptomatic pregnant women may be diagnosed with parvovirus B19 infection because of serologic testing during a community outbreak of the infection or exposure of a pregnant woman to an individual with the infection.
Women who are immunosuppressed or those with an unstable red cell population and short red cell life span (e.g., hemoglobinopathies) can have a severe and prolonged illness. There are no data to suggest that pregnancy has an effect on parvovirus B19 infection, making disease caused by this infection worse.
Both IgG and IgM antibodies to parvovirus B19 should be tested. Parvovirus B19 IgM is detectable 1–3 weeks after infection, and high titers are present during the sec- ond week after infection and can persist for up to 3 months (14). The parvovirus B19 IgM is usually present when the patient is symptomatic with a rash. Parvovirus B19- specific IgG is detectable within 1–2 weeks and at the time of the appearance of rash and arthropathy and can persist for prolonged periods of time (14). The presence of IgM only or IgM and IgG anti-B19-specific antibodies is diagnostic of an acute infec- tion (Table 2). Because of the appearance of both IgG and IgM anti-parvovirus B19 antibodies in such close temporal proximity, it is not unusual for both IgG and IgM antibodies to be present simultaneously in acute infection. The presence of IgG anti- bodies only is evidence of past infection, and the patient is considered immune. The absence of both IgG and IgM anti-parvovirus B19 antibodies is an indication that the patient is susceptible to parvovirus B19 infection.
Because intrauterine parvovirus B19 infection cannot always be diagnosed with se- rology, detection of viral DNA may be needed to make a diagnosis. Parvovirus B19 polymerase chain reaction (PCR) can detect DNA in amniotic fluid and maternal and fetal blood in infected patients. Prenatal diagnosis of fetal parvovirus B19 infection can be confirmed using amniotic fluid, fetal blood, and other fetal tissue specimens.
Because the incidence of intrauterine infection and disease (including hydrops) is low,
the benefits of performing such tests cannot be justified because of the associated risks
of the procedures for obtaining these specimens. The role of maternal F-fetoprotein
levels in diagnosis of fetal parvovirus B19 disease is uncertain. Maternal F-fetoprotein
levels may be elevated in some cases, and some authorities recommend serial levels in
mothers who are infected during pregnancy. Because maternal F-fetoprotein levels
have not been evaluated as an indication of fetal parvovirus B19 disease, their sensitiv-
ity and specificity are not known. It cannot be recommended as a routine test for evalu-
ation of fetal parvovirus B19 disease.
Diagnosis of intrauterine parvovirus B19 disease requires careful follow-up of the fetus once exposure and infection with parvovirus during pregnancy is confirmed or highly suspected. The diagnosis of parvovirus B19 disease in the fetus is primarily based on the diagnosis of hydrops. A baseline fetal ultrasound is recommended. The use of serial ultrasound examinations throughout pregnancy, however, is not univer- sally recommended (34). Because a fetal infection will most likely be manifested within 3 months of maternal infection, serial ultrasounds should at least be considered during this time period. Serial ultrasonography can detect signs of hydrops, such as scalp edema and ascites, as well as pericardial and pleural effusions. The placenta may also appear large in these cases.
If hydrops is diagnosed after a maternal parvovirus B19 infection, the fetus should be followed. A conservative approach with weekly ultrasonographic evaluation is rec- ommended. If the hydrops does not resolve, cordocentesis should be considered to assess the fetal hematocrit. Cordocentesis is not without risk to the fetus, and the risks should be considered when performing this procedure. The ability to perform an in- trauterine transfusion should be available if one is needed.
Clinical Evaluation of the Infant
As with older children and adults, most parvovirus B19 infections in newborn in- fants are asymptomatic (19). Therefore, the diagnosis of perinatally acquired parvovirus B19 infection depends largely on a high index of suspicion, a history of maternal expo- sure to or infection with parvovirus B19, and serological evaluation of the mother and the newborn. However, serological evaluation of an asymptomatic newborn is fraught with difficulties because of an unpredictable immunological response in the baby.
First, the baby may have transplacentally acquired maternal anti-B19 IgG antibod- ies, which may take up to 1 year to become undetectable. The use of anti-B19 IgM antibodies for diagnosis of newborn infection is insensitive. A number of studies have shown a very small percentage of newborn infants infected with parvovirus B19 have detectable anti-B19 IgM antibodies at birth. In addition, in some infants with detect- able anti-B19 IgM antibodies at birth, neither anti-B19 IgM nor IgG antibodies were found later in infancy. The use of anti-B19 IgA antibodies for diagnosis of infection is even less well studied and remains largely a research tool (36).
There is no single test that is diagnostic for congenital parvovirus B19 infection if the neonate is asymptomatic. Although parvovirus B19 infection may be diagnosed by use of electron microscopy to visualize virus in tissue specimens, the need for tissue specimen limits the utility of this test. The use of PCR or DNA hybridization to detect parvovirus B19 viral DNA provides much more promising tests for clinical diagnosis.
Parvovirus B19 PCR has been used to detect the virus in urine and saliva (19).
All newborns with nonimmune hydrops should be evaluated for a possible parvovirus B19 infection. Some authorities have suggested that all abortuses should also be evaluated for parvovirus B19 infection.
The clinical presentations of symptomatic parvovirus B19 infection, including neo-
nates, are shown in Table 3. The current state of knowledge about parvovirus B19
infection in the newborn and the available diagnostic tests dictate a follow-up of an
Table 3
Clinical Presentations of Symptomatic Parvovirus B19 Infection Erythema infectiosum (fifth disease)
Arthropathy
Hematological complications (including congenital anemia
a) Chronic infection in immunocompromised
Hydrops fetalis
aFetal death
aCongenital abnormalities (neurological, cardiac, ophthalmological)
aMyocarditis
aNeurological diseases
aVasculitis
aSpecific for newborns. (From refs. 40–46.)
