Rising Human Parvovirus Cases: What to Know

On August 13, 2024, the Centers for Disease Control and Prevention (CDC) issued an advisory through the Health Alert Network (HAN) about an increase in human parvovirus B19 activity in the United States. Across all ages, the proportion of people tested in a national commercial laboratory with detectable immunoglobulin M antibodies — which indicates recent infection — rose from under 3% in 2022-2024 to 10% in June 2024. Positive tests in children aged 5-9 years have more than doubled, rising from 15% during 2022-2024 to 40% as of June 2024. Although the virus generally causes mild illness, pregnant people and those with blood disorders are at higher risk for complications.
Medscape spoke with Alfonso Hernández-Romieu, MD, with the CDC’s Coronavirus and Other Respiratory Viruses Division, about what healthcare providers should know about human parvovirus B19, what to look for, and how they can advise patients to prevent transmission.
Human parvovirus B19 generally causes mild illness in healthy children and adults, but it can cause severe outcomes, especially among pregnant persons, people with weakened immune systems and those with hematologic disorders such as sickle cell and thalassemia. Like other respiratory viruses, it spreads through respiratory particles. It can also spread during pregnancy. If the mother gets infected with parvovirus B19, she can pass it on to the fetus as well. Very rarely, it can also be transmitted through blood products.
Parvovirus is a big family of viruses, and most parvoviruses infect cats and dogs. Parvovirus B19 is a specific virus that only infects humans and does not infect dogs and cats.
The data that we have show that about 20%-50% of people who are susceptible become infected during a school outbreak, whereas in households, about half of those exposed without prior immunity can get infected. This is pretty similar to what we have seen with COVID-19 and influenza. 
When you become infected with parvovirus B19, it essentially gives you lifelong immunity. I think the reason we’re seeing more infections among kids is because they just haven’t been exposed to the virus. For reference, typically about 40% of people aged 21 will have immunity against parvovirus B19, and by age 40, 70% of adults will have immunity. 
For kids and adults that have normal immune systems, it usually presents with a couple of days of a flu-like illness, including fever, muscle aches, and fatigue. Following that, patients can develop either a rash or can develop some joint pain. The rashes are usually more common in children, and joint pain is more common in adults. About 10 days into the illness, after the fever has gone down, you can see a typical “slapped cheek” rash that’s very common in children, also known as fifth disease (or erythema infectiosum). This is how most pediatricians know that a kid may have had parvovirus B19. What’s important to note is that when this rash appears for kids, it’s usually an immune reaction, so they’re no longer infectious. People are most infectious during the initial part of the disease, when it presents as an undifferentiated respiratory illness.
Initial symptoms will typically develop anywhere between 5 and 10 days after exposure. (It’s important to note that about one quarter of cases are thought to be asymptomatic). After 10 days, when the fever improves and other symptoms such as joint pain or rash appear, someone is considered no longer infectious.
Pregnant people, patients with blood disorders, and patients with weakened immune systems are at the most risk for complications. For pregnant people, the infection will usually not impact the pregnancy. We know that about half of pregnant people have some immunity to parvovirus, and transmission among those who get infected and are not immune is around 30%. The virus can cause fetal anemia, which will need to be monitored closely. Fetal loss or a miscarriage is a rare outcome that occurs in less than 10% of those with acute infection during pregnancy; the highest risk is when the infection occurs before 20 weeks of gestation. If a pregnant person is diagnosed with parvovirus B19, they’ll need more monitoring by their ob/gyn specialists, who may do additional ultrasounds and monitor the fetus’s red blood cell count.
When people who have chronic blood disorders — specifically hemolytic blood disorders such as sickle cell, thalassemia, and hereditary spherocytosis — get infected with parvovirus B19, they can develop transient aplastic anemia, which can be quite severe. Usually, they are treated with additional red blood cell transfusions. 
Patients with weakened immune systems are also at higher risk, specifically people with leukemias or other blood cancers. These patients can become persistently infected and have chronic anemia. 
Our goal with the health advisory was to raise awareness and have healthcare providers consider parvovirus B19 in their differential among patients who present with fever, rash, unexplained anemia, or joint pain. They should consider testing those who are increased risk, and when they see a child that may have parvovirus B19, they should inform caregivers to see if there any people in the same household with high-risk conditions. If the child is in contact with anyone in these high-risk groups, they should be informed of the potential exposure.
Patients in these high-risk groups should also learn how to take preventive actions to avoid transmission. That may be wearing a mask if you’re in a setting where you might have a higher chance of being exposed to parvovirus B19. For example, a pregnant teacher at a daycare may choose to wear a mask when working to minimize that exposure. It aligns with the CDC recommendations to prevent respiratory illness. The core interventions are handwashing, optimizing for cleaner air, staying at home when sick, and adding some additional layers of protection — such as masking — if you are in a high-risk group. 
The tricky part of a parvovirus is that someone may not know they have the disease until they have the typical rash on their face. So, what we’re recommending is that children really should just follow general respiratory guidance for them to stay at home when they’re acutely ill with a fever or other respiratory symptoms and stay home until 24 hours after that fever resolves and other symptoms that are improving. That minimizes the chances that there’s active virus in their respiratory secretions that can transmit to others. We’re not recommending that students with the “slapped cheek” rash be kept out of school because they’re no longer considered contagious.
If they are not pregnant or part of another high-risk group, then most likely they already have some sort of immunity against the disease. If they have a fever or are having respiratory symptoms, then they should also follow the K-12 school guidance that the CDC put out, which is to stay home until the fever resolves and the other symptoms are improving as well. If there are school or day workers that are pregnant and know of an exposure, they should talk to their healthcare provider to get a sense of their risk and to also get tested. This also applies to people with chronic hemolytic blood disorders and weakened immune systems. 
We’re continuing to monitor the situation and will give more information as it becomes available. We have pretty clear data that shows that there’s been an increase in transmission in 2024, so the next steps will be looking into any type of adverse outcomes among pregnant persons and trying to understand if there’s also been an increase in fetal anemia, for example, or other parvovirus B19–related complications.
We are also making sure that we’re communicating with medical associations and patient-provider groups to ensure information is being spread as wide as possible, and that people who are in these high-risk groups can know what they have to do to protect themselves.
The HAN advisory gives a comprehensive background on human parvovirus B19 and recommendations for healthcare providers and the general public. The core CDC respiratory virus guidance is also very important to check, and we have specific K-12 recommendations as well that could be very useful.
Read the HAN advisory on the CDC’s website: https://emergency.cdc.gov/han/2024/han00514.asp
Visit the CDC Parvovirus B19 webpages: https://www.cdc.gov/parvovirus-b19/index.html
Read CDC’s respiratory virus guidance: https://www.cdc.gov/respiratory-viruses/prevention/index.html
Read about school preparedness: Preventing Spread of Infections in K-12 Schools | CDC
 
Public Information from the CDC and Medscape