Scarlet fever, a disease that struck fear into the heart of parents when cases surged in the days of yore, appears to be making an unexpected and puzzling comeback in parts of the world.

England and Wales have seen a substantial rise in scarlet fever cases starting in 2014. The number of cases tripled from 2013 and continued to increase in 2015 and 2016, with England and Wales last year recording the highest number of cases there in a half-century, British scientists reported Monday in the journal Lancet Infectious Diseases.

Similar and in some cases even larger surges of scarlet fever have been reported in recent years in South Korea, Vietnam, China, and Hong Kong. Hong Kong, which saw a tenfold rise in cases, continues to report increased annual counts five years after the resurgence was first noticed.

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The reason for the sudden and surprising increase is a mystery. And the authors of a commentary that accompanied the article urge other countries to be on the lookout for similar spikes in cases.

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“Scarlet fever epidemics have yet to abate in the U.K. and northeast Asia. Thus, heightened global surveillance for the dissemination of scarlet fever is warranted,” wrote Mark Walker and Stephan Brouwer, of the University of Queensland in Australia.

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Scarlet fever is not a reportable disease in the U.S., and the Centers for Disease Control and Prevention does not track the condition. Scientists there are aware of the spike in cases in some jurisdictions, but a spokeswoman said officials have not heard of an increase in the United States.

Scarlet fever is one of a diverse array of conditions caused by infection with a bacterium called Streptococcus pyogenes, better known as group A Strep. The bug can cause strep throat and impetigo — crust-forming skin sores that are highly contagious. It can also cause pneumonia and necrotizing fasciitis — flesh-eating disease.

Sometimes known as scarlatina, the condition gets its name from the diffuse red rash that is characteristic of the infection; the rash generally fades after about a week. It is accompanied by a high fever and often by strep throat. Most commonly diagnosed in children, it spreads in the saliva droplets coughed and sneezed out by infected individuals.

In the 1800s and well into the 1900s, scarlet fever was commonplace. And even into the early years of the 20th century, deaths from the infection were common. Readers of the children’s novel “Little Women” will remember the tragic death of Beth March, who succumbed to scarlet fever — a fate she shared with author Louisa May Alcott’s real-life sister, Elizabeth.

Complications of the infection could be serious as well. Some children went on to develop rheumatic fever, a serious infection that causes heart damage. Other complications affect kidneys and joints.

Scarlet fever is now treated with antibiotics, though even before these drugs were widely available the death toll of the infection fell markedly. By the 1950s deaths from scarlet fever were rare and by the 1980s cases of the disease were as well.

But it never went away entirely. “It’s always been with us,” said Theresa Lamagni, an epidemiologist with Public Health England and first author of the paper. Lamagni noted that in 2013 scarlet fever cases in England and Wales were seen at a rate of about 8 per 100,000 children. That soared to 27 per 100,000 in 2014 and 33 per 100,000 in 2016.

The increase in cases has not led to fatalities, though about 3 percent of infected children have been admitted to hospitals. Most of the stays were short, and in some cases may have been a reflection of parental or physician concern in the face of a previously rare condition, Lamagni said.

In an effort to try to figure out what is causing the sudden rise in cases, she and her co-authors studied bacterial samples from 303 infected patients in 2014. Their theory was that a new and more efficient strain might explain the increase. But they found — as did scientists in South Korea and Hong Kong — that multiple strains had caused the infections.

“The strains didn’t give us the answer. We were really pinning our hopes on those, because that’s the most obvious answer,” she noted. “We’re left thinking what on earth it could be. We don’t have an answer at the moment.”

Even though scarlet fever does not have to be reported to the CDC, Lamagni said a surge in the United States would be hard to miss.

“If they were seeing what we’re seeing, they would know about it. It is unusual,” she said.