What is being tested?

Parvovirus B19 is a virus that causes a common childhood illness, also called "fifth disease" or "erythema infectiosum." The virus is found in respiratory droplets during an infection and is easily transmitted to others through close physical contact. The infection typically has an incubation period of several days to two to three weeks, is active for a short period of time, and then begins to resolve.

For most people, parvovirus B19 infection is indistinguishable from other mild illnesses that develop and go away within a short time period. Many who are infected have no symptoms or have mild flu-like symptoms such as fatigue, a slight fever, headache, or an upset stomach, and many may not know that they have had a parvovirus B19 infection. The majority of people do not experience any significant symptoms or health problems and once the initial infection resolves, the person becomes immune and will not get the infection again.

Some children with the infection develop a characteristic and distinctive bright red "slapped-cheek" rash on both cheeks and a raised lacy rash on the trunk and extremities. The rashes may come and go for several weeks, reappearing and/or intensifying with exposure to heat and sunlight and with stress. By the time the rash appears, the child is no longer considered infectious. Parvovirus is sometimes called fifth disease because it is the fifth of six common childhood illnesses that can cause rashes.

Less commonly, some adults become infected and may develop "gloves and socks syndrome" with painful swelling of joints and reddening of the hands and feet that typically ends abruptly at the wrists and ankles. This condition usually resolves within a few weeks.

Parvovirus can also be passed from a pregnant woman to her foetus and transmitted through blood and blood products. In certain instances, the infection can lead to more serious disorders.

Parvovirus B19 can cause major health problems in three types of patients:

• People with iron deficiency anaemia or a condition that affects or shortens the life of red blood cells (RBCs), such as sickle cell anaemia or thalassaemia, may develop severe acute anaemia during a parvovirus B19
• infection. Parvovirus B19 targets cells in the bone marrow that become RBCs and disrupts the production of new RBCs, which may affect those with underlying blood disorders more severely.
• Women who are infected during pregnancy can pass the infection to their baby. Most fetuses will be fine, but a small percentage will develop severe anaemia and a few may have an inflammation and infection of the heart muscle (myocarditis). These conditions can cause congestive heart failure in the fetus, miscarriage, hydrops fetalis - associated with fluid accumulation, and sometimes stillbirth. Parvovirus B19 is one of the most common causes of hydrops fetalis in Australia. The greatest risk for fetal complications is during the second trimester of pregnancy.
• In those with compromised immune systems, a parvovirus B19 infection may cause chronic anaemia and be challenging to resolve. This includes people with HIV/AIDS, those who have had organ or bone marrow transplants, and those undergoing chemotherapy treatment for cancer.

Parvovirus B19 testing is not typically used to screen the general population. It is usually ordered to determine whether someone is currently, or has recently been, infected with parvovirus if they are at risk of complications. It may also sometimes be ordered to determine whether someone has ever been exposed to parvovirus. Testing involves either a measurement of parvovirus antibodies, immune proteins produced in response to parvovirus B19 exposure, or the detection of the genetic material of the virus itself (its DNA) during an active infection.

How is it used?

Parvovirus B19 testing is generally not used to test those with mild and uncomplicated infections. It is typically used to detect a current infection or determine immunity in those who are at an increased risk of complications, such as those with sickle cell anaemia. There are several methods of detecting a parvovirus B19 infection:

Antibody testing

Two types of parvovirus B19 antibodies may be produced in response to an infection: IgM and IgG. IgM antibodies are the first to be produced by the body in response to a parvovirus infection. They are present in most individuals within a week or two after the initial exposure. IgM antibody production rises for a short time period and declines. Eventually, the level of parvovirus IgM antibody usually falls below detectable levels. IgG antibodies are produced by the body a few weeks after the initial infection to provide long-term protection. Levels of IgG rise during the active infection, then stabilize as the parvovirus B19 infection resolves. Once a person has been exposed to parvovirus B19, they will have some measurable amount of IgG antibody in their blood for the rest of their life.

Parvovirus B19 antibody testing may be performed to determine immunity to parvovirus in pregnant women who have been exposed to someone with parvovirus B19 or who have symptoms suggestive of parvovirus infection. It may also be ordered on people who have acute or chronic anaemia or persistent joint pain that may be due to a parvovirus B19 infection. By comparing the absence or presence of both IgG and IgM in the same sample, the doctor can distinguish between current, recent, and previous infections. Antibody testing is usually not performed on children who have the characteristic fifth disease rashes and, since parvovirus B19 infection is widespread and causes few problems to those with healthy immune systems, general population screening is rarely done.

Viral detection

Viral detection involves finding parvovirus B19 genetic material using PCR or other nucleic acid testing (NAT) in a blood sample or, less commonly, in a bone marrow sample. Parvovirus B19 NAT is performed primarily to detect active parvovirus infection in immune-compromised people who have acute or persistent anaemia. These people will frequently not produce a sufficient amount of antibodies to detect or to resolve the infection.

When is it requested?

Parvovirus B19 testing is usually not required when a child has the characteristic "slapped cheek" and lacy rashes. The distinctive rashes are sufficient evidence for the doctor to be able to diagnose the infection.

Parvovirus B19 IgG and IgM antibody tests may be ordered when a pregnant female has flu-like symptoms and/or has been exposed to someone with a parvovirus B19 infection to determine if she has an active infection, had a recent infection, or has been exposed in the past. Antibody testing and/or DNA testing may be ordered when someone has acute or persistent anaemia or joint pain that the doctor suspects may be due to a parvovirus B19 infection. Parvovirus B19 NAT is usually ordered when the affected person is immune-compromised.

One or more parvovirus B19 tests may be repeated if they are initially negative, but the doctor still suspects that a parvovirus infection is present, or to evaluate changes in concentrations of antibody over time.

What does the result mean?

Antibody testing

If both parvovirus B19 IgG and IgM are present, then it is likely that the person tested has an active, or had a recent, parvovirus infection. This can be confirmed by measuring IgG levels again 2 or 3 weeks later. A high level of IgG is not as important as an increasing concentration of antibody. If there is a 4-fold increase in IgG between the first and second sample, then the patient has an active, or had a recent, infection.

If only IgM is present, then the patient may have very recently been infected. If only parvovirus B19 IgG is present, then the person had a parvovirus infection at some time in the past and has protection against the virus.

If antibody tests are negative, then the person tested has not had the infection and is not immune. If a pregnant woman is not immune and has been exposed to someone with the disease, she will typically be closely monitored by her doctor.

If a person is symptomatic but has low or undetectable levels of IgG and/or IgM, it may mean that they either have a condition other than parvovirus B19 or that their immune system is not responding normally – not producing an adequate amount of antibody even if parvovirus is present. This may be true for immunocompromised people, and DNA testing may need to be done.

Viral detection

If a parvovirus B19 NAT is positive, then the person is currently infected with parvovirus B19. A negative result does not rule out the infection. The virus may not be present in sufficient amount in the sample to be detected.

Is there anything else I should know?

Sometimes a reticulocyte test may be performed along with parvovirus B19 testing to evaluate red blood cell (RBC) production. This test measures new immature RBCs in the blood that still contain genetic material. Since parvovirus B19 disrupts RBC production, the number of reticulocytes will decrease during an active infection.

Parvovirus B19 infections are usually self-limiting in otherwise healthy people. Treatment may be given to relieve symptoms and, when necessary, to address anaemia.

Pregnant women who pass parvovirus B19 infection to their foetus will be closely monitored for several weeks with periodic ultrasounds. In most cases, the foetus will be healthy. When hydrops fetalis or severe anaemia develops, it can sometimes be addressed by giving the foetus an exchange blood transfusion while still in the womb. The parvovirus B19 infection is not known to cause birth defects.

Common questions

Is this the same parvovirus that effects dogs? Can I catch it from my dog?

No. Different types of parvovirus infect humans and animals. You cannot get parvovirus from your pet, and they cannot catch it from you.

• Is there any way to prevent getting a parvovirus B19 infection?

There is no vaccine available for parvovirus. The virus is passed through close contact, and people may be infectious even when no symptoms are present. However, careful hygiene can help prevent transmission of the virus. Most people are infected when they are children.

• Can my test be performed in my doctor's office?

No, parvovirus B19 testing requires specialised equipment to perform. Your sample will usually be sent to a reference laboratory.

More information

RCPA Manual: Parvovirus B19 antibody

Healthdirect Australia: Fifth disease (Slapped cheek disease)