Should we worry about the 'flesh-eating killer bug'?

Behind the Headlines

Wednesday July 15 2015

Group A streptococcus is usually not deadly

Streptococcus bacteria can be harmless, but are sometimes deadly

A "terrifying" and "flesh-eating" bug that "kills one in four it infects invasively" is spreading around the world, warns The Daily Telegraph in news that surprisingly didn't make its front page.

So why is everyone in the country not wearing biohazard onesies? Probably because the threat from this kind of infection is extremely low.

The key fact is that while the emm89 strain of group A streptococcus bacteria was reported to kill one in four people it infects invasively, just over a hundred were infected in this way by this strain in 2013.

The case fatality rate reported in this study of 21% (actually closer to one in five than one in four) makes these invasive infections very serious. For comparison, in the latest outbreak of Ebola the case fatality rate was around 50%. Fortunately, it is not common.

In fact, "strep A" bacteria are generally very common and usually harmless or only slightly problematic. They live on our skin and give us sore throats, earache, and the usually self-limiting but very contagious scarlet fever.

The research behind this news brought together data on the increasing prevalence of the emm89 bacteria and genetic changes in the strain over time, and their effects on the bacteria. Researchers were surprised to find its structure was different from other types of invasive bacteria.

Infections are best avoided by maintaining good hygiene, including washing your hands. 

Where did the story come from?

The study was carried out by researchers from Imperial College London and other research centres in the UK.

It was funded by the National Institute for Health Research Biomedical Research Centre and the UK Clinical Research Collaboration.

The study was published in the peer-reviewed scientific journal mBio. This is an open access journal, meaning the study can be accessed online for free.

The news has focused on the spread of these bacteria in the UK and the high death rate in people who have an invasive infection.

But this research doesn't show that the new form of emm89 is any more deadly than other invasive forms of group A streptococcus. In fact, researchers were mainly interested in the genetics of these bacteria, prompted by the new form of emm89 becoming more common.

The Telegraph failed to make it clear that, in general, invasive infections with group A streptococcus are uncommon. There were about 1,500 cases in 2013, and only about 100 caused by emm89. The Telegraph's "terrifying" and "flesh-eating" coverage could therefore be seen as unnecessarily alarmist. 

What kind of research was this?

This laboratory study looked at the DNA of a strain of group A streptococcus bacteria.

Each year, 600 million people worldwide have a group A streptococcus infection. The bacteria are often found on the surface of the skin and in the throat, and can cause minor infections in these areas.

In rare cases, the bacteria go deeper into the body to cause more serious "invasive" infections. This can include pneumonia and the "flesh-eating" skin infection, necrotising fasciitis.

One type, called emm89, has become one of the main group A streptococcal bacteria that causes disease. Over the last 10 years, the researchers have found an increase in invasive disease caused by emm89.

Because the emm89 strain has not been widely studied, researchers wanted to study its genetic makeup and how it has changed over time. They wanted to understand whether these changes might explain why it has become more common. 

What did the research involve?

The researchers used national data on all cases of invasive group A streptococcal disease in England and Wales between 1998 and 2013. They wanted to know how common the emm89 strain was and how many people died from the invasive infection.

The researchers also analysed the DNA of 131 emm89 samples (58 invasive, 73 non-invasive) collected between 2004 and 2009 to see how it had changed.

They used computer analysis to look at how these changes were likely to have arisen over time, and looked at how these changes might impact the biology of the bacteria.

Finally, the researchers investigated how the genetic changes made differences to the properties of the bacteria in a lab. 

What were the basic results?

Prevalence of the streptococcus A emm89 strain

The researchers found an increase in the amount of invasive group A streptococcal disease caused by the emm89 group of bacteria in England and Wales between 1998 and 2013.

Between 1999 and 2005 all forms were increasing, but between 2005 and 2009 emm89 was increasing more than other types of group A streptococcus. Emm89 was responsible for 10% of all invasive A streptococcal disease in 2005, and 18% in 2007.

Between 2003 and 2013 about a fifth of people with invasive A streptococcal disease died within 30 days.

Genetic changes in the streptococcus A emm89 strain

The researchers identified genetic changes in the emm89 group over time in the UK. Analysis suggested a group of emm89 bacteria with one particular set of genetic changes had emerged in the 1990s and taken over as the main form of the bacteria over time.

The researchers found this group (or "clade") had changes in two regions of its DNA known to affect how infectious the bacteria are. This included losing the genes that usually make the outer coating of the bacteria.

This was surprising – without these genes, the bacteria cannot produce this coating, which had been thought to be essential for the bacteria to infect cells and stop the immune system destroying them.

The researchers found bacteria from this clade could stick to and grow on a plastic surface in the lab better than other forms of emm89. All the forms survived and multiplied in human blood in the lab similarly well.  

How did the researchers interpret the results?

The researchers concluded that, "The rise of emm89 iGAS in the United Kingdom coincided with the emergence and increased prevalence of a variant acapsular clade that differed from the rest of the emm89 population." 

Conclusion

Unlike in the BBC 4 drama, "Cordon", it's unlikely that the streets will be barricaded because of an outbreak of this deadly infection featured in today's news.

The study behind the headlines looked at genetic changes over time within the emm89 form of group A streptococcus bacteria. It found a new form emerged that has become more common over time, and identified the genetic changes that may have contributed to this rise.

This type of study is useful for researchers to look at how infectious organisms change over time and become more successful. It can help researchers track the spread of different forms of bacteria, and may help us develop ideas about new ways to treat them.

There are some points to note about this study. The researchers observed there was no evidence this new clade caused more severe invasive disease than other strains.

Also, although the papers call this a "flesh-eating bug", many of the infections caused by group A streptococcus are mild. The term "flesh eating" is news-speak for one form of invasive group A streptococcus infection called necrotising fasciitis, which makes up only some of the invasive cases of group A streptococcus disease.

Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on Twitter. Join the Healthy Evidence forum.

Analysis by Bazian

Edited by NHS Choices

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