Photo/IllutrationGroup A streptococcus dubbed “flesh-eating bacterium” cause streptococcal toxic shock syndrome, which could lead to death in only hours once the bacteria spread throughout the patient’s body. (Provided by the National Institute of Infectious Diseases)

A mysterious and potentially fatal "flesh-eating bacterium" has infected more than 500 people in Japan in 2017, the highest number since record-keeping began in 1999, according to the National Institute of Infectious Diseases.

A total of 525 patients with streptococcal toxic shock syndrome (STSS) were reported as of Dec. 10 include 66 in Tokyo, the highest of any prefecture, 40 in Kanagawa, 32 in Aichi, 31 in Fukuoka and 28 in Hyogo, according to institute data.

The number of patients with STSS was 203 in 2013, but the figure has been rising each year. However, the cause for the increase remains unknown.

STSS is mainly caused by a bacterium called streptococcus pyogenes, better known as group A streptococcus, which is known to bring on strep throat mainly among children. However, part of the bacterium can develop into a particularly severe and often deadly strain.

Patients could die within dozens of hours of onset from multiple organ failure and other symptoms after their bodies pass into a critical stage in a short period of time, the institute warns.

The initial symptoms of the disease are swelling and pain in the hands and feet, fever and other signs. Such swelling spreads over the course of several hours, causing necrosis of a band of tissue that surrounds the muscles of patients.

Once the bacteria that necrotizes muscle tissues spreads throughout the body, it could lead to multiple organ failure and death in dozens of hours with a fatality rate of 30 percent.

Most of the patients are over 30 or older, including many elderly.

The symptoms can appear even if patients have no chronic health problems.

It is believed that STSS begins in patients’ wounds that have become infected, but there are many cases in which the path of the infection remains a mystery.

Apart from group A streptococcus, several other bacteria such as group G streptococcus also cause serious and even life-threatening forms of the disease.

“It is conceivable that a growing number of patients with STSS are infected with bacteria other than group A streptococcus,” said Ken Kikuchi, a professor of infectious diseases at Tokyo Women’s Medical University.

Penicillin-based antibiotics have proven to offer a therapeutic treatment of such infectious diseases. However, it is necessary to give the drug to patients at an early stage before bacteria spreads across the entire body.

Kikuchi said the elderly need to be particularly careful about contracting the deadly disease.

“The signs of an STSS-infected area likely appear from the feet,” Kikuchi said. “The elderly should be careful about swelling of their feet and go see a doctor immediately when swelling appears.”