Each year, around late winter to spring, we see an increase in the number of serologically-confirmed infections with parvovirus B19. These infections are usually trivial in nature and benign in outcome, but there are important exceptions to this rule. This article will review the typical presentation and course of infection with parvovirus B19, discuss its potential adverse outcomes and in whom that potential is greatest. Parvovirus B19 was discovered and named in 1975 by virologists working at the University of Sydney. It is the predominant genotype (of three) which are pathogenic for humans. Infection is common, occurring sporadically and in clusters, it has a clear seasonality (late winter through to spring) and also has an epidemic cycle with a 4–5 year periodicity. While 50–80% of adults have parvovirus IgG and are regarded as immune, there remains a significant proportion of the adult population who are susceptible to infection.

Infection and its complications

Humans are the only known host for parvovirus B19. The anaemia and thrombocytopenia which are usually subclinical in a normal individual may, in those with increased red blood cell turnover (for example, sickle-cell disease, haemoglobinopathies), lead to significant falls in haemoglobin and, potentially, aplastic crisis. Because B19 is cytotoxic to fetal red blood cell precursors, fetal infection may cause severe anaemia, high cardiac output failure and non-immune hydrops. Unlike rubella, which has a similar presentation and with which it can cross-react in serological assays, B19 has no association with congenital malformations.

Clinical presentation

The clinical presentation of infection is highly variable; Fifth disease, slapped cheek disease and erythema infectiosum all refer to the same febrile exanthem, without significant sequelae, occurring in young children, while an adult frequently presents with fever and arthralgia/arthritis but with no rash at all. However, the same adult with sickle-cell disease may present in aplastic crisis and, in pregnancy, there is a risk of hydrops fetalis, myocarditis and fetal death. In general, the typical presentation of B19 infection in children and its benign outcome require laboratory confirmation relatively infrequent. By contrast, the more variable and dramatic clinical presentation in adults, the absence of any rash rather than the presence of a typical one and, in women, the threat of adverse pregnancy outcomes lead to a much greater reliance on laboratory diagnosis.

Laboratory diagnosis

Generally, diagnosis of parvovirus B19 infection is serological. IgM is usually detectable from just before the onset of symptoms and present in >90% of people by the time of onset of the rash. Detectable IgM is suggestive of infection but not conclusive, unless an IgG seroconversion is also demonstrated or (if IgG was also present at the time IgM was detected) there has been a significant rise when testing is repeated after two weeks. When infection has been diagnosed in a pregnant woman, there is little reason to attempt definitive diagnosis in the fetus. Parvovirus PCR can provide that confirmation however it requires amniocentesis to obtain the required specimen.

Erythema infectiosum (Fifth disease, slapped cheek disease)

These terms all refer to the same presentation of parvovirus B19 infection in childhood. After an incubation period of 4–14 days, and a non-specific prodrome of fever, malaise and rhinorrhoea, a red, macular rash appears on the cheeks, fading to become more lacy and erythematous after a few days. There is no such typical presentation in an adult (see above), with rash being variable or absent. Joint pain and swelling, however, are almost as typical of adult infection as a slapped-cheek rash is in childhood.

Parvovirus B19 infection in pregnancy

Around 40% of women of child-bearing age are susceptible to parvovirus infection. The highest infection rates are seen in school teachers, day-care workers and women with school-aged children in the home. The obvious common factor is their greater likelihood of being exposed to children with erythema infectiosum and that exposure being sustained for longer. Transmission is thought to be through respiratory droplets, with infectivity lasting from one week prior to the rash until the time of onset of the rash. Between 25 and 50% of susceptible household contacts of a case will acquire infection, of whom up to 50% will do so asymptomatically. Therefore, unless women are aware of their potential exposure there is a significant risk of acquisition going undetected. The incidence of parvovirus infection in pregnancy is approximately 1–2% and vertical transmission occurs in about 50%. The risk of hydrops is low (estimated incidence, 3–6%) but there is an overall excess fetal loss of 10% for infection acquired in the first 20 weeks of pregnancy. The fetus is particularly susceptible to hydrops in the second trimester when haematopoiesis is occurring in the liver. During this time, there is a 34-fold increase in red cell mass and a reduction in the life span of the red blood cells. In pregnant women with proven recent infection, the overall fetal death rate of hydrops or its treatment is 0.6% according to ASID guideline.

Management of proven parvovirus B19 infection in pregnancy

When maternal infection is proven or is highly likely, it is not necessary to prove that vertical transmission has occurred, but the fetus should be monitored by frequent ultrasonography. This allows the early detection and assessment of both myocardial dysfunction and fetal hydrops, but more importantly, it makes possible the early detection of fetal anaemia, prior to the development of hydrops. The peak systolic velocity (PSV) of the waveform in the middle cerebral artery can detect moderate to severe fetal anaemia with a sensitivity of 100%, followed by intra-uterine transfusion.   General Practice Pathology is a regular column each authored by an Australian expert pathologist on a topic of particular relevance and interest to practising GPs. The authors provide this editorial free of charge as part of an educational initiative developed and coordinated by Sonic Pathology.