One baby died and a further three were admitted to hospital in an “unusual” cluster of heart infections in south Wales and southwest England, according to the World Health Organization.

Late on Tuesday night, the UN agency said there had been a rise in severe myocarditis – a potentially deadly inflammation of the heart – in newborns and infants between June 2022 and March 2023.

A total of 15 babies – 10 in Wales, 5 in England – were identified with sepsis, a symptom of myocarditis, the WHO said. Of these, nine tested positive for an enterovirus – a common cold-like pathogen which can cause respiratory illnesses or hand, foot and mouth disease. In very rare instances, young babies can develop myocarditis.

“The reported incident represents an increase in both the number and severity of enterovirus infections in infants under the age of one month,” the WHO said, adding that only one case was identified in Wales in the six years prior to 2022.

“Although enterovirus infections are common in neonates and young infants, the reported increase in myocarditis with severe outcomes in neonates and infants associated with enterovirus infection is unusual,” the agency said.

‘An extremely rare occurrence’

Of the myocarditis patients infected with an enterovirus, one died, three were hospitalised and four managed as outpatients. The prevalence of cases peaked in November, the WHO said, adding that the risk to public health remains low.

The UK Health Security Agency (UKHSA) said a “higher than average number of cases” of enterovirus had been observed in “very young babies” in Wales over the autumn and winter months.

As a result, said Dr Shamez Ladhani, a consultant paediatrician at UKHSA, the agency “is investigating the situation in England to see if any similar cases have been observed here and whether there are any factors driving the increase in cases.”

The Telegraph understands that paediatricians across England have been asked to review the babies in intensive care to determine whether there is a broader outbreak, as the UK does not have routine surveillance in place for enterovirus-linked myocarditis.

Professor Paul Hunter, an expert in medicine at the University of Anglia, suggested there was no cause for concern.

“The big problem with these types of cluster is knowing whether this represents a real change or a random clustering with little long term importance,” he said. “Most but not all apparent clusters do not go onto pose a consequential threat to public health.

“Nevertheless, you cannot afford to assume that they will not progress and you have to investigate them.”

Meanwhile, Welsh health officials are investigating whether the cases in Wales and England are linked, as they have emerged in two geographical clusters.

At the start of May, the country’s health minister, Eluned Morgan, said doctors have been asked to consider whether babies presenting with sepsis – a life-threatening reaction to an infection – could have myocarditis, but stressed “this is still an extremely rare occurrence”.

Dr Ladhani added: “Enterovirus is a common infection of childhood, causing a range of symptoms including respiratory disease; hand, foot and mouth, and viral meningitis.

“In very young babies, enterovirus can, on rare occasions, lead to a severe complication called myocarditis – which causes inflammation of the heart. Most babies and children recover completely from this.”

The current investigation is similar to the UKHSA’s enquiries into an unusual hepatitis outbreak in children last year, which was eventually traced to an adenovirus.

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