Q&A: What is "fifth disease" and how do I ensure that my baby is safe if I've been exposed?

"Fifth disease" is a common children's viral illness caused by what's called a parvovirus (a single-stranded DNA virus). The illness is actually called Erythema Infectiosum and has sometimes been called "slapped-face" disease because of a rash on the face that makes it look like it's been freshly slapped. A fever accompanies the rash.

Unfortunately, as mild an illness as it is with children, it can be devastating to a developing baby if the virus crosses the placenta in expectant mothers who have been exposed. Spontaneous miscarriage, fetal anemia and swelling ("hydrops"), as well as fetal death have been reported. Fetal transfusions correcting the fetal anemia have been shown to reverse the abnormalities with normal infants born near term.

The complications may occur weeks after maternal exposure, so ultrasounds every two weeks to look for signs of fetal anemia (hydrops) should be done for two months. If signs of hydrops are seen, a fetal transfusion may save the baby's life.

If the expectant mother were to get fifth disease, symptoms may include joint pain (arthralgia), fevers, and rash. But there may be no symptoms at all, so you are right to be addressing mere exposure.

There are two types of antibodies the exposed mother will develop if she contracts this illness. IgM antibodies develop first in about three weeks, last about four months, and then disappear. IgG antibodies develop in three to four weeks, but last for years.

The difference between the two antibodies can be helpful in determining whether a pregnancy has been recently exposed to the disease. For instance, if the IgM is positive, but the IgG is absent, this is a very recent exposure. If both antibodies are present, this also indicates a recent infection. If the IgM is absent, but the IgG is present, this means that the patient had fifth disease possibly several years ago and her current pregnancy is not at risk. If both antibodies are absent, this patient really hasn't been exposed to the disease.

The general rule here, as with many infections and antibody responses to fight infection, is first M, then G (IgM is the acute reaction, IgG is the chronic reaction). Antibody determinations attempted too soon before the antibody response has had a chance to develop will yield a falsely reassuring negative result, so the first thing an expectant mother should do is get an IgM and IgG blood determination about four weeks after exposure. (The IgG is of academic interest only, because the IgM is the deal-breaker, implying exposure to the current pregnancy.)

One particular point concerns those pregnant patients at high risk for exposure: school teachers, day care workers, pediatric medical staff. Although you can't live getting IgM determinations every week, these at-risk pregnant women should still throw this test in with any other blood work that needs to be done. Might as well.

No vaccine is available for fifth disease. Expectant mothers who already have blood problems (like Sickle-cell disease) may experience very dangerous complications with the otherwise mild fifth disease.