Study identifies main source of Strep A transmission

Bacteria found in the throat of asymptomatic people were the ‘likely source’ of group A Streptococcus infection in 63% of cases, according to newly published research in The Lancet Microbe.
 
The study suggests that among communities with high rates of group A Streptococcus infections, transmission is mainly from asymptomatic presence in the throat, questioning theories that skin-to-skin contact is the primary mode of transmission.
 
Group A Streptococcus infections continue to cause a high burden of disease for Aboriginal and Torres Strait Islander peoples, who have the highest rates of rheumatic heart fever in the world and some of the highest reported global rates of rheumatic heart disease, one of the sequelae of the infection, alongside skin infections such as impetigo.
 
For this reason, the research conducted household surveys across three remote Aboriginal communities in the Northern Territory between 2003 and 2005, and identified the relative contributions of impetigo and asymptomatic throat carriage to group A Streptococcus transmission.
 
Whole-genome sequence analysis was applied to 320 group A Streptococcus isolates from the survey, including 203 (63%) from asymptomatic throat swabs and 117 (37%) from impetigo lesions.
 
Findings revealed that asymptomatic throat carriage is a group A Streptococcus ‘reservoir’ in associated skin infections, capable of causing impetigo in endemic disease settings.
 
‘Our analysis revealed direct connections between isolates recovered from the skin and throat, challenging common thinking that skin-to-skin contact is the primary mode of transmission,’ lead author Dr Jake Lacey from the University of Melbourne’s Doherty Institute said.
 
‘In fact, we found that bacteria found in the throat of people who are not showing any symptoms of infections were the likely source of infection in 63% of cases.’
 
According to the authors, if group A Streptococcus transmission is predominantly skin-to-skin, then targeted interventions would ‘likely have’ a significant impact on reducing skin infections, but if asymptomatic throat carriage is ‘an important reservoir of infection’, then strategies focusing entirely on interrupting skin-to-skin transmission might not be as effective.
 
The study’s findings may also inform public health initiatives, the authors say, with the shift in understanding regarding how the bacterium is spread potentially influencing group A Streptococcus vaccine development and future research.
 
While currently in development, Australia has no approved vaccine for group A Streptococcus, and the authors recommend that any vaccination targeting transmission of the bacterium should include consideration of asymptomatic throat carriage.
 
‘Understanding the transmission dynamics of pathogens in local settings is an important aspect for disease surveillance and control and mitigation programs,’ they write.
 
As well as informing public health approaches, the authors hope their findings will provide better insight to transmission in settings where the bacteria are prevalent to inform the development of more effective and targeted strategies for prevention and control, identifying that controlling skin infections in Aboriginal and Torres Strait Islander populations ‘has proven difficult’ and the dynamics of transmission are ‘poorly understood’.
 
‘Our research suggests that public health approaches should not solely focus on skin infections but also consider the role of throat carriage in [group A Streptococcus] transmission,’ co-author and infectious diseases physician at the Doherty Institute, Professor Steven Tong, said.
 
‘For instance, vaccines targeting bacteria in the throat may offer greater efficacy in preventing the spread of infection compared to those that only target disease.
 
‘The burden of infection documented in this study also reiterates the fundamental role of primordial prevention in First Nations health initiatives. Importantly, this work also prompts researchers to now consider the importance of throat carriage when assessing the transmission dynamics of [group A Streptococcus].’
 
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group A Streptococcus impetigo rheumatic heart disease throat carriage