Experts warn that a rare but dangerous bacterial infection is spreading at a record rate in Japan, with officials struggling to identify the cause.

The number of cases in 2024 is expected to exceed last year’s record numbers, while concern is growing that the harshest and potentially deadly form of group A streptococcal disease – streptococcal toxic shock syndrome (STSS) – will continue to spread, after the presence of highly virulent and infectious strains were confirmed in Japan.

The National Institute of Infectious Diseases (NIID) said: “There are still many unknown factors regarding the mechanisms behind fulminant (severe and sudden) forms of streptococcus, and we are not at the stage where we can explain them.”

Provisional figures released by the NIID recorded 941 cases of STSS were reported last year. In the first two months of 2024, 378 cases have already been recorded, with infections identified in all but two of Japan’s 47 prefectures.

While older people are considered at greater risk, the group A strain is leading to more deaths among patients under 50, according to NIID. Of the 65 people under 50 who were diagnosed with STSS between July and December in 2023, about a third, or 21, died, the Asahi Shimbun newspaper reported.

Most cases of STSS are caused by a bacterium called streptococcus pyogenes. More commonly known as strep A – it can cause sore throats, mainly in children, and lots of people have it without knowing it and do not become ill.

But the highly contagious bacteria that cause the infection can, in some cases, cause serious illnesses, health complications and death, particularly in adults over 30. About 30% of STSS cases are fatal.

Older people can experience cold-like symptoms but in rare cases, the symptoms can worsen to include strep throat, tonsillitis, pneumonia and meningitis. In the most serious cases it can lead to organ failure and necrosis.

Some experts believe the rapid rise in cases last year were connected to the lifting of restrictions imposed during the coronavirus pandemic.

In May 2023, the government downgraded Covid-19’s status from class two – which includes tuberculosis and Sars – to class five, placing it on a legal par with seasonal flu. The change meant local authorities were no longer able to order infected people to stay away from work or to recommend hospitalisation.

The move also prompted people to lower their guard, in a country where widespread mask wearing, hand sanitising and avoiding the “three Cs” were credited with keeping Covid-19 deaths comparatively low. About 73,000 Covid-19 deaths were recorded compared with more than 220,000 in Britain, which has a population just over half that of Japan.

Ken Kikuchi, a professor of infectious diseases at Tokyo Women’s Medical University, says he is “very concerned” about the dramatic rise this year in the number of patients with severe invasive streptococcal infections.

He believes the reclassification of Covid-19 was the most important factor behind the increase in streptococcus pyogenes infections. This, he added, had led more people to abandon basic measures to prevent infections, such as regular hand disinfection.

“In my opinion, over 50% Japanese people have been infected by Sars-CoV-2 [the virus that causes Covid-19],” Kikuchi tells the Guardian. “People’s immunological status after recovering from Covid-19 might alter their susceptibility to some microorganisms. We need to clarify the infection cycle of severe invasive streptococcal pyogenes diseases and get them under control immediately.”

Streptococcal infections, like those of Covid-19, are spread through droplets and physical contact. The bacterium can also infect patients through wounds on the hands and feet.

Strep A infections are treated with antibiotics, but patients with the more severe invasive group A streptococcal disease are likely to need a combination of antibiotics and other drugs, along with intensive medical attention.

Japan’s health ministry recommends that people take the same basic hygiene precautions against strep A that became a part of everyday life during the coronavirus pandemic.

“We want people to take preventive steps such as keeping your fingers and hands clean, and exercising cough etiquette,” the health minister, Keizo Takemi, told reporters earlier this year, according to the Japan Times.