Parvovirus is a virus which lives within red blood cells and was first discovered in 1975 in healthy adults when donor blood was being screened for the transfusion service. It was identified as a possible problem for unborn infants in 1984, when the first case of congenital infection was reported.

Childhood Infections

In 1983 it was discovered that parvovirus is responsible for the common chilhood condition called erythema infectiosum, otherwise known as Fifth disease or the slapped cheek syndrome.
Child with parvovirus infection

The incubation period is between four and 20 days. Once symptoms appear, the infectious period is usually over. The antibody prevalence in the general population varies with age: about 21% of those under 5 year are immune, as are 60% of adults.

Outbreaks of Fifth disease in schools and day care settings occurs usually in winter or spring. In the UK, April and May are the peak months, though it may occur at any time. A pattern is emerging of two years of parvovirus epidemic followed by 2 years of low incidence.

Adult Infection

Although in children parvovirus infection commonly passes with nothing more than a short-lived febrile illness, in adults and especially women, it is often accompanied by an acute tenderness of the joints, or arthritis, affecting the hands, wrists and knees. This usually only lasts a couple of weeks, but in about one in 10 women can last as long as a couple of months. There may be a finer rash over the trunk and extremeties, which may wax and wane over a period of several months in response to stress, exercise, sunlight or bathing.

Infection During Pregnancy

The risk of aquiring parvovirus during pregnancy, averaged over epidemic and non-epidemic years, is about 1/400. The risk of infection for a non-immune mother with a child who has Fifth disease is between 50% and 90%, whereas for a child in a day care with another who is infected, the risk is lower at about 20% to 30%.

Parvovirus is a danger to the unborn child in two ways. Firstly there is an increased risk of miscarriage if the infection is aquired during the first 20 weeks of pregnancy. The overall risk of miscarriage following parvovirus infection is around 15%, the infection representing an increase of 9%, and being most marked between 9 and 16 weeks gestation.

The other problem parvovirus can cause is an anaemia, or low blood count, in the baby. This leads to a condition known as hydrops fetalis, where the baby, amongst other things, carries a lot of excess water. A fetus is most likely to develop hydrops when a mother has a parvovirus infection between 9 and 20 weeks gestation. Even if it is acquired at this time, there is overall only a 3% chance of hydrops developing. Hydrops doesn't usually develop immediately.

A pregnant woman who has confirmed parvovirus infection would be offered scans, which can detect if the baby is becoming anaemic. The interval between infection with parvovirus and development of hydrops varies between two and 17 weeks. It seems likely that an early blood transfusion will improve the chances for an affected baby's survival. Thus, a woman who is pregnant, not knowing her immune status and who is exposed to parvovirus infection between 9-20 weeks has a 0.6% risk of hydrops developing and a 4% increase in risk of miscarriage.

Close contact placing a woman at risk of catching parvovirus includes the following:

living in a houshold setting with an infected person
sharing a meal with an infected person
being in the same room for over half an hour as an infected person

Effects on Baby

Follow-up of infants who were exposed to parvovirus in the womb has now been reported up to the age of 10 years. It seems that there are no long-term effects from an infection which doesn't cause miscarriage or hydrops (as most do not).

Once a woman has had parvovirus infection, she is not at risk of getting it again, whether it was acquired during pregnancy or not. It is not a cause of recurring miscarriage.