A new strain of Group A Streptococcus (or Strep A), called M1UK, is behind the spike of scarlet fever cases seen in the last 3 years in the United Kingdom, researchers from Imperial College London reported in The Lancet Infectious Diseases yesterday.
The new strain produces more of one type of streptococcal toxin than do existing strains, which has led to a higher incidence of sore throat, scarlet fever, and rare invasive infections across Britain since 2014.
In 2016, there were more than 19,000 cases of scarlet fever, and by 2018, Public Health England tracked a doubling of scarlet fever cases compared with 2014.
"Scarlet fever is a very visible signal of how much Strep A is circulating in the wider community, and causing sore throats. Strains of Strep A that cause these commoner throat infections and scarlet fever are the same strains that cause rarer invasive diseases - and therefore a rise in these commoner throat infections including scarlet fever could lead to increases in all forms of Strep A infection," said Shiranee Sriskandan, MBBChir, senior author of the study and a professor of infectious disease at Imperial College, in an Imperial press release.
Group A Streptococcus is among the top 10 infectious causes of human mortality, with more than 500,000 deaths annually.
Scarlet fever outbreaks, which occur in some patients with sore throats caused by Strep A, had been most common at the beginning of the 20th century. Scarlet fever most often affects children ages 4 to 6 and occurs seasonally, with peak activity in March, April, and May.
New strain concentrated in London
To conduct the study, a team of researchers analyzed Streptococcus pyogenes emm genotypes, and notifications of scarlet fever and invasive disease in 2014 through 2016 using regional (London) and national (England and Wales) data. A total of 135 non-invasive and 552 invasive emm1 isolates from 2009 through 2016 were compared with 2,800 global emm1 sequences.
"Coincident with national increases in scarlet fever and invasive disease notifications, emm1 S pyogenes upper respiratory tract isolates increased significantly in northwest London in the March to May period, from five (5%) of 96 isolates in 2014, to 28 (19%) of 147 isolates in 2015 (P = 0.0021 vs 2014 values), to 47 (33%) of 144 in 2016 (P = 0.0080 vs 2015 values)," the authors found.
Genetic analysis suggests that M1UK emerged in 2010. As of 2015, around two thirds of non-invasive emm1 isolates from northwest London were in the new M1UK lineage. Among the 2,800 global isolates, M1UK was detected only twice, once each in Denmark and the United States.
"Whether the M1UK lineage will be suited to other environments is unknown; management of streptococcal sore throat differs greatly between countries, as do other important factors such as climate," the authors concluded.
M1UK is susceptible to common antibiotics, and the researchers detected no antimicrobial-resistance genes in their isolates.
Strain not tied to scarlet fever in China
In an accompanying commentary written by Australian and Chinese researchers, the authors emphasize that the study "sends out an important warning for the global public health community: recently emerging scarlet fever group A streptococcus strains have enhanced invasive potential, which might have profound implications for the future global health burden."
They also explain that the M1UK strain has not been detected in a wave of recent scarlet fever outbreaks in Hong Kong and mainland China.
About 500,000 cases of scarlet fever have been recorded in Hong Kong and China since 2011. The cause of the increased pathogenicity is unknown, they write.
Sep 10 Lancet Infect Dis study
Sep 10 Imperial College press release
Sep 10 Lancet Infect Dis commentary