A novel respiratory pathogen is gaining traction in the United States: the human parvovirus B19, a highly infectious agent that predominantly affects children through respiratory droplets. Following reports from 14 European nations of an unusually high incidence this year, the U.S. Centers for Disease Control and Prevention (CDC) issued an alert earlier this month, noting an upward trend in the detection of parvovirus B19. According to Dr. Stanley Deresinski, a clinical professor of medicine specializing in infectious diseases at Stanford Medicine, the actual increase might surpass the CDC's projections, as there is an absence of systematic monitoring for parvovirus B19 infections within the country. He suggests that the surge could be attributed to an 'immunity gap,' a consequence of reduced human interaction during the COVID-19 pandemic, leaving a significant group of individuals without immunity, as they were not exposed to the virus during that period.

Although the CDC is not actively tracking the number of cases, recent serological tests from commercial labs indicate a rise in infections. The prevalence of individuals with IgM antibodies, indicative of a parvovirus response, has risen across all age groups from a mere 3% between 2022 and 2024 to 10% as of June 2024. The virus is most prevalent among school-aged children, with the latest CDC data indicating a significant increase in children aged 5 to 9. The proportion of these children with IgM antibodies has more than doubled from 15% between 2022 and 2024 to 40% in June 2024. Parvovirus B19, colloquially known as "fifth disease," earned its name as it historically ranked fifth among common childhood illnesses marked by a rash.

While the symptoms of parvovirus are typically mild and self-resolving, they can mimic those of the flu. Dr. Deresinski explains that in those who exhibit symptoms, the illness often presents in two stages. The initial phase, characterized by fever and muscle pain, commences about a week post-exposure and lasts for approximately five days, coinciding with the period of peak transmissibility. Affected individuals may also manifest headaches, coughs, and sore throats. In the subsequent phase, as the body's immune system mounts an antibody response, children may develop the characteristic "slapped cheek" rash on their faces, while adults might experience joint pain. Additionally, parvovirus B19 can provoke a general rash across the chest, back, buttocks, arms, or legs.

Dr. Deresinski notes that children between the ages of 4 and 10 are most susceptible to the disease, with outbreaks often coinciding with the resumption of school. A single parvovirus-infected child has the potential to infect a substantial portion of their classmates. By the age of 20, it is estimated that about half of the U.S. population has been affected by the illness. However, many individuals remain unfamiliar with parvovirus B19, possibly confusing it with the canine parvovirus, a distinct and highly contagious virus among dogs.

The virus poses a higher risk to certain populations. It can be transmitted from a pregnant woman to her fetus via respiratory droplets. The CDC's August advisory highlights an observed increase in parvovirus B19 infections among pregnant individuals, with some cases leading to severe fetal anemia, necessitating fetal transfusions or even pregnancy loss. The virus can also be transmitted through blood transfusions, posing a risk to individuals with sickle cell anemia who may require transfusions. In such cases, parvovirus infection can exacerbate anemia, potentially leading to an aplastic crisis, where the bone marrow abruptly ceases red blood cell production, requiring hospitalization. The CDC further warns that immunocompromised individuals, such as those with leukemia, other cancers, organ transplants, or HIV, are at risk of developing severe anemia.

Treatment for parvovirus B19 is generally symptomatic, as the virus is typically mild and self-limiting. Patients may be advised to take acetaminophen to alleviate headaches and fever, and nonsteroidal anti-inflammatory drugs like ibuprofen to manage joint pain and inflammation. Staying hydrated and getting adequate rest can also help mitigate symptoms.

Currently, there is no vaccine or specific treatment to prevent parvovirus B19 infection. However, the CDC offers general recommendations to curb the spread of respiratory viruses: Cover your mouth and nose with a tissue when coughing or sneezing, and if a tissue is not available, use your elbow instead of your hands. Proper handwashing techniques should be learned and practiced, with frequent handwashing being particularly crucial in childcare and healthcare settings. Regular cleaning of frequently touched surfaces, such as countertops, handrails, and doorknobs, is also advised. Individuals experiencing parvovirus symptoms are encouraged to stay home and avoid contact with others. However, once the characteristic rash appears, the infected person is no longer considered contagious.