In 2024, several European countries suddenly reported many cases of the fifth disease, including in pregnant women. In Germany, the number of infections peaked in April 2024. The disease is caused by the human parvovirus B19 (B19V).

Usually Harmless, But not Without Risk

The fifth disease is usually harmless, especially in children. It often presents with fever and a red rash on the face that spreads to the arms and upper body. "Once you've had it, you're immune for life. However, infection during pregnancy can have serious consequences, as it can lead to complications in the fetus, including miscarriage. In children, however, serious complications are very rare," explains Franziska Seidel, a pediatrician and specialist in congenital heart defects at the Charité Heart Centre. In extremely rare cases, myocarditis, an inflammation of the heart muscle tissue, can develop.

Babies and Young Children are at Risk

Shortly after the increased incidence of the fifth disease, an increase in myocarditis cases in children has been observed since the end of 2023, as Teresa Nygren from the Robert Koch Institute (RKI) reported at a conference in Stockholm at the end of November. A joint study by the RKI and MYKKE revealed that more than half of those affected by this rare complication are babies and young children under the age of two.

Data from the MYKKE registry of 29 cardiology centres in Germany, Austria and Switzerland, as well as reporting data and laboratory data on B19V infections from the RKI-designated consultant laboratories, formed the basis. Under the direction of first author Teresa Nygren, the researchers investigated whether the myocarditis cases were an outbreak caused by B19V: "Our suspicion was confirmed. The increased occurrence of this otherwise extremely rare complication seems to be related to the outbreak of the fifth disease," says MYKKE researcher Franziska Seidel.

Increase After the COVID-19 Pandemic

The researchers explain the outbreak wave by the low infection rates of various infectious diseases during the previous COVID-19 pandemic and the increase in infection rates afterwards. Since the summer of 2023, the Multicenter Register for Children and Adolescents with Suspected Myocarditis (MYKKE) has recorded an increase in myocarditis caused by the fifth disease.

From August 2023 to mid-December 2024, MYKKE recorded 61 children with parvovirus B19 myocarditis. This compares to an average of 11 cases per year in the previous decade.

In addition, the researchers expect there to be several unrecorded cases of myocarditis. And it's not over yet, as Franziska Seidel explains: "Because of the delay between B19V infection and myocarditis, more cases are expected. It was only in December that another young patient had to be hospitalized."

The Severity of the Disease is Particularly Striking

From the researchers' point of view, the severe course of the disease is particularly striking: "Most of the young patients, more than two thirds, have been seriously ill. We have rarely seen such severe inflammation of the heart muscle. The children have a significantly reduced left ventricular function. Due to acute pump failure, fourteen of them had to be put on mechanical circulatory support. Two infants, aged 1 1/2 years, did not survive the myocarditis. There were also deaths among patients who were not enrolled in MYKKE," says Franziska Seidel.

Genome Sequencing to Provide Clarity

Researchers at the RKI are therefore currently trying to isolate the genome of the parvovirus B19 responsible for the fifth disease from tissue and blood samples of patients registered in the MYKKE registry.

The sequencing of the genome should clarify whether the genome of the virus has possibly changed. "This would explain the striking severity of the disease progression and is an important clue for further investigations as well as for the best possible medical care," says Franziska Seidel.

Early Detection of Myocarditis

The researchers recommend that their pediatric colleagues be particularly vigilant for signs of a fifth disease. Last summer, MYKKE also involved the German Pediatric Society for Cardiology and Congenital Heart Defects (DGPK), which informed other medical societies.

"Myocarditis can be easily overlooked. The symptoms are often attributed to the infection that triggered the disease, " says Franziska Seidel. Parents should also be alert to symptoms such as fatigue that lasts longer than usual in their children after a fifth disease. "If the physical weakness persists, medical advice should be sought. Children with shortness of breath, chest pain and tachycardia should be seen by a cardiologist."

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