The number of people catching scarlet fever in the Republic is unknown amid a re-emergence of the highly infectious disease in England.

Cases of scarlet fever have reached a 50-year-high in England, but in Ireland healthcare professionals do not have to report cases to the HSE National Surveillance Centre.

Therefore, the number of cases in Ireland is “not known” according to an infections diseases tracker on the National Surveillance Centre website that records numbers of new cases of infectious diseases notified to the centre each week.

The website notes that in England between 2,000 and 4,000 cases of scarlet fever are reported each year.

However, according to a report in the latest edition of The Lancet Infectious Diseases Journal, cases of scarlet fever have soared in England, baffling government scientists who admit they do not know why the Victorian-era disease has made a comeback.

The number of cases reached a 50-year high last year, a seven-fold increase in five years. According to new research there were 620 outbreaks of scarlet fever in 2016, with more than 19,000 cases, mostly in schools and nurseries. The average age of a patient was four.

The disease was a common cause of death in childhood during the 1800s, but it has become less common and milder over the past century. However, the bacteria involved can still cause severe illnesses including pneumonia, sepsis and liver and kidney damage.

Research is under way to try and find a possible cause and assess prevention strategies. Molecular genetic testing has ruled out a newly emerged strain of the infection with increased ability to spread between individuals.

Scientists described the resurgence as “unexplained and ongoing”. They had suspected a new strain of the disease might have evolved to spread more easily, but the genetic testing ruled that out.

Reports from several countries in east Asia, including China and South Korea, have noted similar findings.

“Whilst current rates are nowhere near those seen in the early 1900s, the magnitude of the recent upsurge is greater than any documented in the last century,” said Dr Theresa Lamagni, head of streptococcal surveillance at Public Health England, who led the study.

She said that the number of cases reported to authorities in 2017 appeared to show a slight decrease, and they were still monitoring the situation.

Symptoms include a sore throat, headache and fever accompanied by a pink-red rash that feels like sandpaper. The disease is not usually serious, but anyone affected should see a GP promptly.

The guidance for schools and creches emphasises regular handwashing, and the need for children or staff with symptoms to stay at home for at least 24 hours after starting treatment with antibiotics.

About one in 40 people with the disease will be admitted to hospital. Just over half are discharged the same day.

A comment on the prevalence of scarlet fever from the HSE press office has been requested.