The mysterious return of scarlet fever

Scarlet fever, a leading killer of children in the 19th and early 20th centuries, is suddenly making a comeback in many parts of the world, and no one knows why.

The bacterial infection brings on a red, sandpapery rash all over the body, a high fever, and sore throat, and can cause serious health complications, including heart and kidney damage. The advent of antibiotics in the mid-20th century made the disease less deadly.

But scarlet fever, which spreads from person to person through coughing, comes and goes throughout the decades for reasons not well known to science. The number of cases actually began to decline during in the 19th century, before effective treatments were widely used to fight it.

Now researchers are finding the disease’s trajectory has begun to shift once again.

According to a new paper in the Lancet Journal of Infectious Diseases, starting in 2014 there was a sharp uptick in scarlet fever cases in England and Wales — a trend that’s continuing to climb. The rate of scarlet fever cases tripled in 2014 compared to the year before. With one in 500 children under the age of 10 diagnosed with the infection, the UK is seeing its highest rates of the disease in 50 years. (Scarlet fever typically affects children ages 5 to 15, and the median age of cases in England in 2014 was 4 years old.)

But it’s not just England. A number of regions in Asia — Vietnam, South Korea, Hong Kong, and mainland China — have also recorded dramatic increases in cases since 2009.

The best-recorded epidemic has been happening in Hong Kong, according to a Lancet commentary that accompanied the paper. There, cases have risen tenfold between 2000 and 2016.

“The scarlet fever outbreak in Hong Kong has not yet receded,” wrote Mark Walker and Stephan Brouwer from the University of Queensland in Australia, “which does not bode well for a timely resolution to outbreaks occurring elsewhere.”

What all these outbreaks have in common is that no one knows why they’re happening.

Among the potential explanations: There may be changes in the immune systems of people now that have made them more susceptible to the bacterium that causes scarlet fever, Streptococcus pyogenes, which also causes strep throat and impetigo. Or there could be another pathogen people are being “co-infected” with that is predisposing them to the disease.

In the US, scarlet fever is not one of the diseases public health officials require states to report and track, and there have been no signs of an uptick in cases here yet, the Centers for Disease Control and Prevention told Vox.

But the Lancet study authors warned that — with the UK and Asia outbreaks still ongoing — “heightened global surveillance for the dissemination of scarlet fever is warranted.”

There is some good news in the mix. Researchers are finding that the type of scarlet fever that’s circulating now appears to be less deadly than in the outbreaks of the past. Whether that’s because of variations in the strains of the bacteria going around now or because we have better diagnosis and care is also unclear.